WP4- NA4: Trace gases networking: Volatile organic carbon...

Deliverable WP4 / D4.9 (42)

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WP4- NA4: Trace gases networking: Volatile organic carbon and nitrogen oxides

Deliverable D4.9: Final SOPs for VOCs measurements Summary:

This SOP provides a guideline for good measurement practice for the analysis of volatile organic

compounds (VOCs) under the EU FP7 infrastructure project ACTRIS. Only active sampling is part of this

SOP. For passive sampling respective guidelines from the EU should be used.

The SOP contains the following topics:

1. General introduction ............................................................................................................................... 1 2. Data Quality Objectives ........................................................................................................................... 4 3. VOCS Measurement Setup ...................................................................................................................... 5

3.1 Facility requirements ..................................................................................................................... 5

3.2 Personnel requirements ................................................................................................................ 5

3.3 Occupational health and safety ..................................................................................................... 5

3.4 Instrumentation requirements ...................................................................................................... 5

3.5 Air inlet and sample lines ............................................................................................................... 6

3.6 Associated key measurements ...................................................................................................... 6

3.7 Environmental issues that affect ACTRIS stations and VOCs observations .................................. 7

4. Sampling .................................................................................................................................................. 8 4.1 Location of the inlet ....................................................................................................................... 8

4.2 Off-line sampling ............................................................................................................................ 8

4.2.1 Sampling inlet lines (NMHCs and OVOCs) ................................................................................. 8

4.2.2 Adsorption tubes ........................................................................................................................ 8

4.2.3 Stainless steel canisters ............................................................................................................. 8

4.2.4 OVOCs (DNPH) ........................................................................................................................... 9

4.3 On-line sampling/Quasi continuous observations ........................................................................ 9

5. Measurement techniques ..................................................................................................................... 10 5.1 GC technique ................................................................................................................................ 10

5.1.1 Removal of water/ozone/carbon dioxide/particles ................................................................ 10

5.1.1.1 Water removal/management ............................................................................................. 10

5.1.1.2 Ozone removal ..................................................................................................................... 11

5.1.1.3 Carbon dioxide removal ....................................................................................................... 11

5.1.1.4 Particle filters ....................................................................................................................... 12

5.1.2 Sample Preconcentration ........................................................................................................ 12

5.1.3 Capillary columns for GC analysis of VOCs and OVOCs ........................................................... 13

5.1.4 Detection .................................................................................................................................. 14

5.2 PTR-MS (provided by R. Holzinger, T. Petäjä, S. Dusanter) ......................................................... 16


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6. Reference materials .............................................................................................................................. 18 7. Quality assurance .................................................................................................................................. 19

7.1 Calibration measurements ........................................................................................................... 19

7.1.1 Method for measurements of Laboratory/Working standards and Target gases ................. 20

7.1.2 Zero Gas ................................................................................................................................... 21

7.1.3 Method for Detecting Effects of Ozone on Reactive Compounds .......................................... 24

7.2 Audit procedures .......................................................................................................................... 25

7.3 Measurement protocol ................................................................................................................ 25

7.4 Measurement uncertainties ........................................................................................................ 25

7.4.1 Calculation of mole fractions for linear detection systems..................................................... 25

7.4.2 Determination of Precision ...................................................................................................... 26

7.4.3 Determination of Uncertainty ................................................................................................. 27

7.4.4 Determination of detection limit ............................................................................................. 28

8. Data Management ................................................................................................................................ 29 8.1 Data evaluation ............................................................................................................................ 29

8.1.1 FID: effective carbon number .................................................................................................. 29

8.1.2 Time series of calibration gas measurements ......................................................................... 29

8.1.3 Target gas measurements ....................................................................................................... 30

8.1.4 Results of standard addition measurements .......................................................................... 31

8.1.5 Data checks of final mole fraction data in time series ............................................................ 32

8.1.6 QC in xy-plots (used at Rigi, Switzerland by Empa) ................................................................. 34

8.1.7 QC in repeatability and reproducibility: .................................................................................. 35

8.1.8 Recommended QC and minimum QC, thresholds for flagging the data ................................ 37

8.2 Metadata ...................................................................................................................................... 37

8.3 Ancillary data ................................................................................................................................ 37

8.4 Data archiving at the station or laboratory ................................................................................. 37

8.5 Data submission ........................................................................................................................... 37

9. References ............................................................................................................................................. 40 10. Appendices ....................................................................................................................................... 43


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1. General introduction Volatile organic compounds (VOCs) consist of low-boiling non-methane hydrocarbons (alkanes,

alkenes, alkynes, aromatics, terpenes) and oxygenated hydrocarbons, such as alcohols, ketones, or

aldehydes. When released into the air VOCs play an important role in atmospheric chemistry and in

the oxidizing power of the atmosphere, which then affects climate and air quality. VOCs are emitted

by the biosphere and by anthropogenic activities, such as motor vehicle exhaust and solvent usage. A

complex mixture of several hundred VOCs is emitted with half-lives ranging from several months in

the case of ethane, to hours for the most reactive ones, such as isoprene or anthropogenic alkenes.

VOCs are removed from the atmosphere primarily by their reaction with hydroxyl radicals – a process

which forms intermediate oxygenated organic compounds.

The scientific background for the need of VOCs monitoring in global and regional networks has been

extensively presented (for example in the GAW Reports 111 (WMO, 1995), 171 (WMO, 2007a), and

204 (WMO, 2012), Helmig et al., 2009). In populated areas VOCs and their degradation products are

responsible, together with NOx, for the photochemical formation of ozone (O3) and other photo-

oxidant pollutants including secondary organic aerosol (SOA). Thus, they couple into photochemical

ozone production, aerosol formation, and cloud processes and thus impact air quality and climate.

Therefore, measurements of VOCs are essential and are among the long-term monitoring

parameters in GAW (GAW Report 172 (WMO, 2007b)) and regional programs like EMEP.

Only a few VOCs (e.g. formaldehyde) can be observed by satellite instruments. Therefore long-term

in-situ measurements of these VOCs are essential for deriving trend estimates of globally

representative mole fractions and source contributions. This standard operation procedure (SOP)

covers exclusively ground-based in-situ measurements of VOCs.

Table 1 The list of priority VOCs focused in the GAW report No. 171 (WMO, 2007a) Molecule Lifetime

(OH= 1E6 cm-3)

Importance to GAW Steel flask

Glass flask

Analysis Method

Network Type

1. Ethane 1.5 months

• source of methane • natural sources • biomass burning • fossil fuel • ocean production (S.

hemisphere) • trend in size of seasonal cycle • indicator of halogen chemistry

√ √ GC-FID global

2. Propane 11 days • source of methane • natural sources • biomass burning • fossil fuel • ocean production (S.

hemisphere)


3. Acetylene 15 days • motor vehicle tracer • biomass burning tracer • ratios to other hydrocarbons • trends


4. Isoprene 3 hours • biosphere product • sensitive to temperature/land • use/climate change • O3 precursor • oxidizing capacity • precursor to formaldehyde

? ? GC-FID PTR-MS

Africa S. and N. America Europe

5. Formaldehyde 1 day • indicator of isoprene - - DOAS Small


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oxidation • biomass burning • comparison with satellites • trends

number of sites in Tropics for comparison with satellites

6. Terpenes 1-5 hours • precursors to organic aerosols

- - GC-MS PTR-MS

Selected sites in forested areas

7. Acetonitrile 0.5-1 year • biomass burning indicator • biofuel burning indicator

- ? GC-MS PTR-MS

Global

8. Methanol 12 days • sources in the biosphere (methane oxidation)

• abundant oxidation product

- ? GC-MS PTR-MS

Global

9. Ethanol 4 days • tracer of alternative fuel usage

- ? GC-MS PTR-MS

Global

10. Acetone 1.7 months

• abundant oxidation product • free radical source in the

upper troposphere

? ? GC-MS PTR-MS

Global

11. DMS 2 days • major natural sulphur source

• sulphate aerosol precursor • tracer of marine

bioproductivity

? ? GC-MS PTR-MS

Global

12. Benzene 10 days • tracer of combustion • biomass burning indicator

√ ? GC-FID GC-MS

Global

13. Toluene 2 days • ratio to benzene used for air mass age

• precursor to particulates

- ? GC-FID GC-MS

Global

14. Iso/n-butane 5 days • chemical processing indicator

• lifetime/ozone production

√ √ GC-FID GC-MS

Global

15. Iso/n-pentane

3 days • ratio provides impact on NO3 chemistry

√ √ GC-FID GC-MS

Global

GC-FID is Gas Chromatography – Flame Ionization detection

GC-MS is Gas Chromatography - Mass Spectrometry

DOAS is Differential Optical Absorption Spectroscopy

PTR-MS is Proton Transfer Reaction Mass Spectrometry √ indicates state of current pracGce

There are three groups of VOCs that are distinguished in current literature, GAW Reports and in this

SOP: the non-methane hydrocarbons (NMHCs), the oxygenated volatile organic compounds (OVOCs)

and biogenic VOCs (mainly isoprene and the group of monoterpenes, often summarized as BVOCs).

Priority substances of these groups have been identified in GAW Report 171 (WMO, 2007a) and

detailed guidelines for their measurements are provided in the respective report, following the

general quality assurance (QA) recommendations and the strategic plan by GAW (GAW Report 172

(WMO, 2007b)). As analytical systems for the measurement of the different groups of VOCs are

generally capable of analyzing not only the priority species identified in GAW Report 171 but also a

list of chemically similar compounds, this SOP covers a broader range of compounds than specified in

GAW Report 171 (Table 1). This is in line with the EMEP objectives and addressed in the ACTRIS

Description of Work.

The measurement of atmospheric VOCs is mostly done by gas chromatographic methods. However,

for carbonyl OVOCs DNPH (dinitrophenylhydrazine) cartridge sampling and high-performance liquid

chromatography (HPLC) exist for certain VOCs and the relatively new on-line method proton transfer


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reaction mass spectrometry (PTR-MS). As gas chromatography (GC) is most widely used, we focus on

this method in this SOP, but other techniques are discussed, too.

The measurement of VOCs by GC is generally performed in a series of steps with (1) intake manifold

and sampling line, (2) traps to remove water and ozone, (3) pre-concentration, (4) gas

chromatographic separation, (5) analysis in detector, and (6) data processing and data delivery. A

sample of atmospheric VOCs can be introduced to the analytical system directly from ambient air, a

canister or a preconcentration tube. The sample is normally passed through a moisture and/or ozone

removal system and then concentrated on an adsorbent medium that is cryogenically cooled using

liquid nitrogen, liquid carbon dioxide, or thermoelectric closed-cycle coolers. The sample optionally

can be refocussed cryogenically by a cooled secondary trap to narrow the band width for injection

onto the capillary GC analytical column. The concentrated sample is then thermally desorbed into the

analytical GC column and finally analysed by flame ionization detection (FID) or mass spectrometry

(MS) (or any other suitable detectors).

For the quantification of VOCs a calibration standard should be used, which either contains VOCs in

ambient air, or contains artificially mixed VOCs in nitrogen (N2). Mole fractions should be in the

region of the expected ambient mole fractions. If needed the standard has to be diluted with zero air

or N2 to a mole fraction, which is in the mole fraction range measured at the specific station.


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2. Data Quality Objectives Data quality objectives (DQOs) define qualitatively and quantitatively the type, quality, and quantity of

primary data required and derived parameters to yield information that can be used to support

decisions. In WMO/GAW DQOs were for example introduced in the 2000-2007 strategic plan (WMO,

2001). For VOCS measurements the DQOs have the objectives to (a) detect long-term changes in

background mole fractions and (b) quantify year-to-year variability. This section quantifies what are

tolerable levels of uncertainty for reaching these goals. The following DQOs have been approved by the

GAW-VOCS expert group:

The enhanced ACTRIS DQOs defined in Table 2 are the objective for good performing stations. Within

ACTRIS, generally the GAW DQOs shall be reached. These stated DQOs are valid for individual ambient

air measurements. This is different from the interlaboratory comparability objectives used in the

greenhouse gas community where the objectives refer to uncertainties in measurements of calibration

standards comprising multiple measurements. During the ACTRIS non-methane hydrocarbon

intercomparison most stations reached the GAW and ACTRIS DQOs when analysing NMHCs in nitrogen,

but for compressed air (whole air) more scatter was observed and only a few stations reached the

ACTRIS DQOs (Hoerger et al. 2014). The long-term objective is that all stations measure ambient air

within the ACTRIS DQOs.

Table 2 Data quality objectives for the measurements of VOCs GAW

uncertainty GAW

repeatability ACTRIS

uncertainty ACTRIS

repeatability alkanes

10% 5% 5% 2%

alkenes incl. isoprene monoterpenes

20%

20%

15%

15%

5%

10%

2%

5%

alkynes

15% 5% 5% 2%

aromatics

15% 10% 5% 2%

mole fraction <0.1 nmol/mol (ppb) (monoterpenes)

0.02 ppb

0.015 ppb

0.005 ppb

(0.010 ppb)

0.002 ppb

(0.005 ppb)


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3. VOCS Measurement Setup

3.1 Facility requirements

Facility requirements include 24-hour available electricity and communications, a secure

environmentally conditioned building suitable for the instruments and staff and ease of access. The

facility and equipment should be suitable to sustain long-term observations with greater than 90% data

capture (i.e. <10% missing data). The air sampling should be structured in a way to avoid local

contamination sources. The laboratory building and inlet location on site should be set upwind of any

other buildings, garages, parking lots, generators, other emission sources – any nearby areas where

fossil fuels or biomass may be combusted and where intensive agriculture is undertaken. Station

personnel should also remain downwind of the sampling laboratory and refrain from smoking as

necessary. Within the facility, temperature control and clean lab environment are required.

Instrumentation should not be exposed to sunlight.

3.2 Personnel requirements

Each set of measurements at an ACTRIS station should be conducted under the guidance of a designated

Responsible Investigator (RI). For VOCs, it is recommended that the RI have training in atmospheric

chemistry, meteorology and atmospheric composition monitoring. There are requirements for

technicians with skills in (1) analytical chemistry, particularly atmospheric composition, (2) electrics and

electronics, and (3) IT, particularly instrument control, data acquisition and data processing. It is

recommended that station staff participate in the ACTRIS workshops, GAWTEC training programme and

other GAW specialist activities where appropriate.

Provision should be made for back up staff to cover the periods when regular staff is away at training,

leave etc.

3.3 Occupational health and safety

The VOCs program includes use of equipment that can cause the following occupational health and

safety issues:

• High voltages;

• High-pressure gas lines (for example associated with the zero air generator);

• Noise;

• Heavy and awkward equipment.

Other hazards may occur. Appropriate occupational health and safety information, protective

equipment and training are required.

3.4 Instrumentation requirements

The following instrumentation is required for a reliable long-term VOCs monitoring station in ACTRIS:

• VOCs system and suitable inlet as described in Sections 4 and 5. This system must be

calibrated as recommended in Sections 7 and 8 of this SOP;

• Zero air supply that includes H2O, VOCs, O3 and NOx removal (see Section 7);

• Inlet line and filter both inert to VOCs;

• Instrument control and data acquisition interface;


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• Computer;

• Internet connection/remote computer access;

• Uninterruptable power supply.

Equipment varies in specification and performance. The WCC, existing long-running ACTRIS and GAW

stations and GAWTEC can provide advice on instrumentation that has performed successfully.

Manufacturers’ instrument manuals should be available on site for all instruments used at the site.

3.4.1 Instrument replacement As long as an instrument performs within the specifications and the DQOs (Section 2), there is no

necessity for replacement. If the instrument performance requires a replacement, the new and old

system should run parallel for some time (6 month) if possible.

Since IT equipment is subject to fast evolution, back-up equipment should be available and

appropriate updates should be carried out depending on the availability of financial resources.

3.4.2 Instrument control and data acquisition software Instrument control and data acquisition usually depends on the available manufacturers’ software

for the VOCS instrument.

3.5 Air inlet and sample lines

The air inlet is an essential component of the ACTRIS monitoring system and any compromises made

with regard to the inlet will affect all subsequent data. There are two key components of the inlet

system, the location of the inlet and the materials of the inlet. In analytical chemistry terminology, the

location of the inlet is an aspect of sampling and the passage of the air through the inlet corresponds to

pre-treatment of the sample. � See Section 4.

3.6 Associated key measurements

Key measurements that will help in the interpretation of VOCs measurements include those used for

processing the VOCs data, data selection and those related to VOCs chemistry. To understand the

influence of nearby sources, to undertake data selection according to meteorological conditions and to

quality control, the following additional parameters are useful:

• Wind speed and direction;

• Air temperature;

• Humidity;

• Particle number concentrations;

• Carbon monoxide mole fraction;

• Nitrogen oxides mole fractions;

• Radon concentration.

To interpret the atmospheric chemistry processes affecting the observed ozone mole fractions, the

following parameters are useful:

• Nitrogen oxides mole fractions;

• O3/CO/CH4 /OH mole fractions;

• Water vapour concentration;

• Air temperature;


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• Spectral distribution of solar radiation (suitable for determining molecular photolysis

rates)/solar radiation.

Where VOCs measurements are undertaken at ACTRIS stations, consideration should be given to

measurement of these additional parameters. The measurement techniques for these parameters are

presented in GAW Report No.143 Global Atmosphere Watch Measurements Guide (WMO 2001b)) and

in individual measurement guidelines (WMO, 2007a; WMO, 2010b; WMO, 2010c; WMO, 2011a).

3.7 Environmental issues that affect ACTRIS stations and VOCs observations

The environmental conditions/hazards that affect VOCs observations include the following:

• Inlet blockage at polar and high-altitude sites, due to ice riming and blowing snow;

• Pollution events nearby roads, agriculture, biomass burning, industry etc.;

• Access limited by environmental conditions such as flooding, severe weather etc.;

• Lava flow for stations located on active volcanoes;

• Tourist activities.

Consideration should be given to minimising the effect of the factors listed above where possible when

setting up the station, while it is clear that the impact of natural hazards cannot be completely avoided.


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4. Sampling

The air from which VOCs are analysed can be sampled on-line at the measurement site or off-line,

using either adsorption tubes or stainless steel canisters. Off-line samples are subsequently

transported to the lab where they are analysed. The specific requirements of the different methods

are described below.

4.1 Location of the inlet

The height of the air sample inlet is critical to this sampling of representative air. The optimum inlet

height depends on the surrounding area (type vegetation, orography, soil, water, snow). New stations

should, if possible, for a trial period sample VOCs at 2-3 different heights to determine which inlet height

is suitable.

4.2 Off-line sampling

4.2.1 Sampling inlet lines (NMHCs and OVOCs)

For the sampling of NMHCs the inlet line should be either Silco-treated steel or made of stainless

steel. In the case of stainless steel, the line has to be heated up to 70 °C to prevent condensation of

VOCs on internal surfaces (Hopkins et al., 2011). Transfer lines for the analysis of OVOCs should be

either silco-steel, PFA (perfluoralkoxy), PEEK (polyether ether ketone), or electro-polished heated

stainless steel but not untreated stainless steel. Silco-treated steel should be humidified before first

usage (e.g. by passing humid ambient air).

The inlet line has to be as short as possible and the diameter should not be larger than 1/8 inch in

order to minimize the dead volume of the sampling line unless it is permanently flushed. The

residence time in the inlet should not exceed a few seconds. It is recommended to have a mesh with

a maximum of 5 µm mesh size in the line in order to hold back particles. This has to be exchanged

regularly. Experience from an urban site (Zurich) is that the grid has to be exchanged every 4 months.

4.2.2 Adsorption tubes

Off-line sampling of NMHCs by adsorption tubes is a well-established method. Different providers have

commercially available products. For this SOP it is advised to follow the specific procedures of the

individual products. It has to be proven, however, that artefacts due to adsorption tube blanks

(especially for aromatic compounds) are reproducibly low compared to the range of concentrations

encountered at the sampling site. Due to the mentioned blank issues, it is generally not recommended

to use the adsorption tube technique in very clean air. Moreover this method is not recommended

within the EMEP or GAW programmes, and not yet tested in intercomparisons. However, for some

compounds, like terpenes, adsorption tubes or canister measurements are useful to characterise these

compound class, which are often not routinely analysed with on-line GC-systems.

4.2.3 Stainless steel canisters

In the GAW Report No. 204 (WMO, 2012) a new Standard Operation Procedure (SOP) valid in the

WMO GAW network was described. For the analysis of NMHCs in canister samples, this procedure

can be applied. This SOP is based largely on the recommendations from the “Accurate

Measurements of Hydrocarbons in the Atmosphere” project AMOHA (Plass-Dülmer et al., 2006) and

from US-EPA (1998, 1999) on determination of VOCs in ambient air. Beside this SOP, many other


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methods exist for whole air sampling for VOCS analysis (e.g. glass flask air sampling in the US

National Oceanic and Atmospheric Administration Cooperative Global Air Sampling Network

(Pollmann et al., 2008)). Generally, the use of materials other than ultra-pure stainless steel, glass,

silica coated stainless steel, PFA, and PTFE (polytetrafluorethylene) should be avoided for the

measurement of NMHCs in air samples. Especially, plastics other than PFA and PTFE shall not be used

to prevent memory and outgassing effects. The recommendations in the SOP constrain on electro-

polished stainless steel (ss) canisters. A variety of ss canisters with one or two valves may be

purchased from several suppliers, e.g. Restek or TO-Can® Air Monitoring Canisters. The use of two

valve canisters allows more flexibility in air sampling and is recommended. The inner surface of

canisters should be passivated, e.g. electro-polished and stainless steel valves (e.g. Swagelok) shall be

used to seal the canisters inlets and outlets, respectively. The canister conditioning and sampling

procedures are described in detail in GAW Report 204 (WMO, 2012). In this report problems of this

method are discussed, too. Briefly, major problems usually arise from inappropriate conditioning,

canister leaks, adsorptive losses of C7 and higher boiling compounds, long storage times, and artefact

formation of low boiling alkenes. These problems are linked to the canisters and depend strongly on

the type of canisters used. Thus, this technique is recommended only for C2-C6 alkanes, isoprene and

benzene. However, when canisters of the new generation like Silonite® (Entech) or SilcoCan® (Restek)

are used improvements for higher boiling compounds were found. Nevertheless, this has to be

tested in detail for the used canister.

4.2.4 OVOCs (DNPH)

Off-line sampling of OVOCs by DNPH-coated samplers with subsequent liquid chromatography (LC) is a

well-established method within EMEP, therefore the chapter 3.8 of the EMEP manual for sampling and

chemical analysis (Determination of aldehydes and ketones in ambient air, Revision Nov 2001) should be

used.

The SEP-PAK DNPH-Silica Cartridges (WATERS®) are recommended to measure individual aldehydes

(from C1-C6) and ketones (C3-C5) over a concentration range (0,05-10 µg/m3) encountered in

background atmosphere. Since the blank can differ from a cartridge to another, it is strongly

recommended to determine the blank by analyzing a set of minimum 7 cartridges per batch. For each

measurement, the mean bank value representative of the batch is then systematically subtracted to

the sample. Air flow through the DNPH cartridge may change during the sampling due to particles.

Then the sampling device should include a flow controller and a pump efficient enough to maintain a

constant flow rate of 1500 sccm over a minimum sampling time of 3 hours. The post sampling

procedure (extraction) is fully described in the above mentioned EMEP Manual. The extract is

analyzed by a high performance liquid chromatography (HPLC) equipped with a quaternary solvent

supply system and a UV detector. The calibration procedure mainly consists in analysing a certified

standard solution (e.g. SUPELCO® Carbonyl-DNPH Mix). However, it is recommended to validated the

whole method and determine accuracy and precision using a synthetic mixture obtained by diluting a

certified gaseous standard (OVOCs standard) with humid air.

4.3 On-line sampling/Quasi continuous observations

On-line sampling avoids storage issues and minimizes leak issues, however, requires an analytical system

at the sampling site and thus restricts the sampling intervals to the capabilities of the analytical system.

The air sample is directly transferred via a sampling line into the VOCs instrument. Concerning sampling

inlet lines see Section 4.1.1.


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5. Measurement techniques

For on-line and off-line analyses of VOCs from ambient air GC systems are the method of choice. The

advantages are medium cost, high sensitivity, excellent reproducibility, and, depending on the applied

chromatographical details, large resolving power. Disadvantages are the need for well-trained and

experienced operators, the restricted time resolution, and problems in analyzing more polar, surface-

sticky compounds. An alternative discussed in section 5.2 are the PTR-MS systems capable of high time

resolution measurements which are surface-contact free and allow analyses of the aforementioned

problematic polar compounds, e.g. OVOCs. Disadvantages of PTR-MS systems are the high cost of the

instrument, the high skills required to operate the systems and the fact that PTR-MS can practically only

analyse OVOCs and certain unsaturated NMHCs. Furthermore, it cannot separate isobaric compounds

like different monoterpenes.

5.1 GC technique

As VOCs are only occurring in the atmosphere in the range of pmol/mol (ppt) up to some nmol/mol

(ppb) they have to be preconcentrated before the analysis, using GC-FID or GC-MS. Preconcentration of

VOCs is performed on a trap which contains enough of a suitable material or a combination of different

adsorbents for fully retaining VOCs at a given temperature (section 5.1.2). The preconcentrated

compounds are subsequently injected onto the analytical column where they are separated depending

on the characteristics of the chosen column (Section 5.1.3). In the final step the compounds reach the

detector (FID or MS, see Section 5.1.4).

5.1.1 Removal of water/ozone/carbon dioxide/particles

Prior to preconcentration, additional trapping devices may be required: Water (H2O) in ambient air

affects the adsorption capacity of the preconcentration trap (see Appendix 2), the chromatography

(peak shapes and retention times) and leads to ice formation in the preconcentration unit, when

temperatures <0°C are applied. Ozone may react with unsaturated NMHCs such as alkenes (e.g. ethene,

isoprene, monoterpenes) during the preconcentration step and form OVOCs. Furthermore, ozone could

react with the adsorbent material itself (see Appendix 1). CO2 can distort the chromatography or effect

detector sensitivity in case of sample preconcentration at adsorption temperature <-78°C. Furthermore,

particle filters should be used to avoid contamination of the system with particles.

5.1.1.1 Water removal/management

Water management can be achieved by different methods such as Nafion® dryer or a water trap

(Table 3). Regardless, which water management system is chosen, its efficiency, potential artefacts

(e.g. blind values) and the recovery of water soluble compounds (e.g. alcohols) needs to be tested

(see Standard-addition measurements in Section 7.1.3).

If hydrophobic adsorbents (see Appendix 2) at above ambient air temperature are used in the

preconcentration trap, prior water removal can be neglected. In combination with a dry purging step

- flushing of the preconcentration trap in the sample flow direction with dry gas (e.g. purified helium

(He) 5.0 or He 6.0) subsequent to sampling – residual water is further removed. This kind of sampling

is applicable for C4 and higher boiling compounds, and is regularly used for BVOCs sampling.

Table 3 Methods to remove Water from the sample. Method Comments Recommended for


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Nafion ® Dryer with a volumetric counter-flow of dry air or N2, which is around 3 times higher than the flow of humid ambient air

removes H2O effectively and substantial parts of the polar OVOCs and monoterpenes. Potential artefacts in C2-C4- alkenes may occur depending on the status of the Nafion® Dryer. ((Gong and Demerjian, 1995; Plass-Dülmer et al., 2002) and references therein).

NMHCs C2-C7 ( sometimes C8)

Water trap @ T < Tambient H2O is adsorbed or frozen-out but not the analytes. The dew point should be measured and it should be at least 10°C lower than the trapping temperature of the preconcentration unit. For cryogenic trapping of VOCs, the dew point should be below -30°C. In NMHCs, BVOCs, and selected OVOCs analysis, freeze-out water traps are widely used (e.g. Cape Verde (Hopkins et al., 2003)).

NMHCs

BVOCs

OVOCs (risk of potential losses of highly water soluble compounds like alcohols (e.g. methanol*))

*When measuring highly water soluble compounds (e.g. methanol), the recovery of these compounds needs to be

tested.

5.1.1.2 Ozone removal

To avoid artefact formation from the reaction of unsaturated VOCs with ozone (O3), several methods

are available to eliminate ozone from the sample. Table 4 lists the most common methods. A more

thorough compilation of available methods and their evaluation can be found in Appendix 1.

Table 4 Ozone removal methods and recommendations. Method Comments Recommended for

(e-polished) stainless steel @ T > 70°C

Losses of OVOCs can occur (Hopkins et al., 2011; Englert et al., 2014)

NMHCS, OVOCs*

Titration with NO NO (O3 + NO�NO2) into ambient air flow

Slow reaction, alcohol losses were observed, poisonous reactant (Helmig, 1997;Komenda et al. 2003; Legreid 2006)

NMHCs, BVOCs, OVOCs*

Cartridges filled or filters impregnated with sodium thiosulfate (Na2S2O3)

(Plass-Dülmer, 2002) NMHCs, BVOCs

potassium-iodide (KI) Has to be implemented after water removal**. Blank values for formaldehyde, acetaldehyde, alcohol losses (Helmig and Greenberg, 1994; Leibrock, 1996)

NMHCs,?

sodium sulfate (Na2SO3) Removes methyl vinyl ketone (MVK) and macrolein (??) (MAC), efficiency depends on H2O vapour content of air stream, humidity increases efficiency (Helmig, 1997).

NMHCs,?

Manganese-Oxide Work in progress *Recovery has to be tested.

** the ozone scrubber is efficient with a minimum moisture in the gas stream (Kliendienst et al, 1995). However special cautions have to be taken when the scrubber is used for extensive periods of time at high RH. The preparation procedure includes drying the tube properly.

5.1.1.3 Carbon dioxide removal


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Methods to remove carbon dioxide (CO2) from the ambient air flow are listed in Table 5.

Table 5 CO2 removal

Method Comments Recommended for

Cartridge with Ascarite substance is hygroscopic, trap should be installed behind a water trap to avoid liquefaction; artefact might be possible

To be determined

Preconcentration trap slow heating of preconcentration trap to a temperature high enough for the CO2 to be released but not for the analytes.

NMHCs BVOCs

5.1.1.4 Particle filters

In order to avoid contamination of the system with particles, filters (Table 6) can be used in the analysis

of VOCs.

Table 6 Particle filters used in GC systems. Method Comments Recommended for

PTFE membrane filter Pore size: 20-30 µm, Metron Technology, Aschheim, Germany (used at Hohenpeißenberg) No artefacts are detected for recommended compounds (see Section 6.6.). Not suitable for OVOCs.

NMHCs (C2-C14) BVOCs

5.1.2 Sample Preconcentration

A compilation of different trapping adsorbents and their usage is provided in Appendix 2. Either

cryogenic adsorption on glass beads, a combination of week adsorbents with low sub-ambient

temperature or stronger adsorbent with higher, up to ambient temperature can be chosen, often

also multi-bed adsorbents with increasing adsorbent strength in sampling flow direction are used.

For each system break-through volumes have to be tested, using either increasing amounts of

humidified synthetic standards or of ambient air spiked with standards (Section 7.6).

For the trapping procedure a pump should be used downstream of the trap connected to a critical

orifice or a mass flow controller (or any other suitable instrument) to regulate the flow through the

trap. It is essential to determine the sampling volume with low uncertainty either by regularly

calibrated mass flow controllers or by pressure rise measurement in a defined reference volume. If

the pump is used before the trap it has to be ensured that no additional contamination is produced

by the pump.

After trapping, the trap should be flushed in forward mode at the same temperature for an adequate

amount of time to allow the purge out of remaining water and potentially adsorbed gases (e.g. CO2,

noble gases) from the trap.

Release of the analytes from the trap is normally done by heating the trap (either by ohmic

resistance or by other means of heating) in counter-flow. The final temperature should be reached as

fast as possible and should be high enough to release all analytes. Analytes are transferred to the gas

chromatography system by carrier gas flow. After transfer of the analytes the trap should be


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reconditioned (e.g. by flushing it further with carrier gas and heating it to a higher temperature than

needed to release the analytes, the flow of cleaning gas is vented to the environment). In case that

analyte injection is not rapid enough to obtain sharp peaks which may be due to large trap volumes

or slow heating rate of the trap, a second focusing trap should be installed between

preconcentration and column. This again may be adsorptive or cryogenic but needs to have a

substantially smaller internal volume than the preconcentration trap.

Following systems (Table 7) have taken part successfully in an ACTRIS intercomparison experiment

(Hoerger et al. 2014):

Table 7 List of successfully employed GC systems. Adsorbents Temperature and flows Sample

Volume

Systems Recommended for

custom made thermo desorption systems

Glass beads in 1/8” Silcosteel tubing

Ads. -180°C and 50 ml/min (LN2 cooling) Des. 340°C and 5ml/min

750ml Hohenpeißenberg (Plass-Dülmer et al., 2002)*

NMHCs (C2-C8)

Fused Silica beads, Caboxene® 1003,Carboxene® 1016, Carboseive® S-III in stainless steel tube

Ads. -45°C Des. 235°C

600ml Rigi, EMPA NMHCs (C2-C8)

Carbopack®BHT Ads. -120°C Des. 200°C

400ml WCC-VOC, KIT Garmisch

NMHCs (C2-C6)

Tenax TA/ CarbopackX/Carboxene569 in fritted glass tube

Ads. 30°C, 80 ml/min Des. 200°C, 20 ml/min**

1500ml Hohenpeißenberg BVOCs (sabinene depletion on TenaxTA**), NMHCs (C4-C14)

Commercial thermo desorption systems

Markes UNITY TD Carbopack®B, Carboxen®1000

Ads. -20°C, Des. 350°C,

1000ml Cape Verde, (Hopkins et al., 2003) EMPA

NMHCs (C2-C8), OVOCs,

ENTECH TD Glass beads

Ads. -120°C, Des. 70°C,****

360ml EMD NMHCs (C2-C8)

* Reference systems during ACTRIS intercomparison ** Refocussing on Methyl Silicone Capillary, XX -180°C 20ml/min, des. 60°C, 2.5ml/min *** needs to be tested regularly, depletion process increases with age of td tube

**** Refocussing on glass beads, Tenax®, Ads. -50°C, Des. 220°C

5.1.2.1 Split injection

During the ACTRIS NMHCS intercomparison experiment systems with split injection seemed to have a

poorer performance than systems without (Hoerger et al., 2014). Reasons (e.g. variable split flows) are

not understood, yet.

Currently, it is recommended to inject directly onto the column, without split injection.

5.1.3 Capillary columns for GC analysis of VOCs and OVOCs

Capillary columns exhibit better separation efficiencies and higher inertness compared to packed

columns. Despite their lower capacity they are suitable for most applications in trace gas analysis.


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There are two types of capillary columns that are most widely used for the analysis: PLOT (Porous

Layer Open Tubular) and WCOT (Wall Coated Open Tubular) columns.

Several possible analytical columns are listed in the Appendix 3. Table 8 lists a number of columns

which have been successfully employed in VOCs analysis.

Table 8 List of VOCS columns.

VOCS Column Trange Typ.

Dim Comments Citation

NMHCs C2-C8 AL2O3/KCL PLOT -100°C – 200°C

50m x 0.53mm

Plass-Dülmer et al., 2002

OVOCS

CP-LOWOX* 0°C – 350°C 20m x 0.53mm

strong selectivity but H2O dependent retention time shifts

Hopkins et al., 2003

CP-Porabond-U -100°C – 300°C

25m x 0.32mm

Long lifetime, retention times stable but co-elutions with aliphatic NMHCs (e.g.)

Legreid, 2006 and Englert et al. (to be submitted)

BVOCS, NMHCS C5 and higher

DB-1** -60°C – 350°C

50m x 0.32mm Co-elution with

OVOCs

Riemer et al., 1998

DB-5** 50m x 0.22mm

-

* or similar columns as listed in table 1 in Appendix 3 ** or similar columns as listed in table 2, in Appendix 3

5.1.4 Detection

Two detection systems are mostly used in VOCs analysis: Flame Ionization Detection (FID) and/or Mass

Spectrometry (MS). Advantages and disadvantages of these detectors are listed in Table 9.

Table 9 Advantages and disadvantages of detection systems.

FID MS

Advantages + sensitive, robust, simple in design and easy to use + very stable performance with typically less than 2% sensitivity drift over one month + response is proportional to the mass or carbon number and allow easy quantification + with the effective carbon number (ECN) concept of the response, they allow for effective QA (see Section 8.1.1) + not sensitive to traces of water, N2 and O2, and noble gases from the sample gas + less expensive

+ compound identifying capabilities + substance-specific: overlaying peaks are detectable by compound specific mass tracks

Disadvantages - not substance-specific � Co-elution of peaks GC

- variable sensitivity requires more frequent calibration measurements - instruments needs regular tuning - expensive

A FID is the favourable detection system whenever identification can be achieved simply based on the

retention times. If the resolution of the chromatographic system does not allow unambiguous


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identification of different compounds based on retention time alone, a mass spectrometer is

recommended as detector for its compound identifying capabilities.

5.1.4.1 Operating conditions: Flame Ionization Detector (FID)

The operation principle is based on the ionization of organics in a hydrogen (H2) flame. A FID needs

thus air and H2 to produce the flame and a make-up gas for proper operation; the flow rates should

be well controlled to achieve stable operation of the detector. Essential is to have low VOCs levels or

at least low fluctuation in VOCs levels in the operating gases. Table 10 lists the suitable operating

condition for FIDs:

Table 10 Operating conditions for FID

GAS Supply Flow rate* Temperature

Air Synthetic air (quality 5.0) or ambient air catalytically cleaned (Pd or Pt catalyst at 350°C-450°C)

300-350 ml

TFID **>= Tcolumn,max to avoid or minimize deposition of column residues

H2 Cylinder (H2 quality 5.0) or H2 generator

30 ml

Make Up Gas (e.g. N2)

Cylinders, grade 5.0 or higher 30 ml

*The suitable flows might vary depending on the FID used; it is important to check the total flows of the individual gases, including the carrier gas, and stay within the specified margins by the FID manufacturer. ** but within the specification of the manufacturer

The sensitivity of an FID is generally sufficient to do analysis in background atmosphere at pmol/mol

levels (ppt), e.g. detection limits of GC-FID systems for analyzing 1 litre of air are typically better than 3

pmol/mol (e.g. Plass-Dülmer et al., 2002).

For GC-FID systems, it is recommended to perform a calibration every 2 weeks in order to secure

high data coverage, however, at least every 2 months.

5.1.4.2 Operating conditions Mass Spectrometer (MS)

In a MS the analytes are ionized in the ion source either by chemical (CI) or electron ionization (EI). In

VOCs analysis, usually EI is used. The resulting gas-phase ions are measured depending on their

specific mass-to-charge ratio. Thus, even overlying peaks can thus be separated by analyzing

different, compound specific mass tracks.

Operating temperatures of a MS system are listed in table 11:

Table 11 Operating temperatures of a MS system.

MS System Ttransfer line Tion source Tquadrupol

AGILENT inert XL 150°C - 200°C 230°C - 250°C High temperatures minimize the residence time and adsorption effects of compounds in the source

150°C - 180°C

Other MS systems To be determined To be determined To be determined

The sensitivity of a MS is not stable and the signal depends on a set of tunable parameters (e.g.

repeller voltage, lenses, multiplier voltage), which influence ionization and ion transmission process

as well as the detection of the charged ions at an electron multiplier. Usually a decrease of MS


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sensitivity is observed over time which results in a decrease of peak area. Three measures are thus

required:

i) Tracking the sensitivity with frequent working standard measurements. Frequency of

the working standard measurements should ensure that the decline in sensitivity is

accurately tracked over time (e.g. if continuous measurements are performed it is

recommended to perform a working standard measurement every 2-4 sample; at least

daily close in time to the ambient air sample).

ii) Regular auto-tuning of the MS: weekly to monthly, depending on the drift strength

observed in the individual systems but at least every second month.

iii) If boundary conditions of the source (repeller voltage, lenses) do not allow a proper

tuning of the source anymore it has to be cleaned using the procedure specified by the

manufacturer.

5.2 PTR-MS (provided by R. Holzinger, T. Petäjä, S. Dusanter)

PTR-MS techniques minimize potential losses of VOCs since ambient air is directly analysed without any

preconcentration as in gas chromatography. Ambient air passes a drift tube where VOCs are ionized by

proton transfer from hydronium ions (H3O+), providing that the VOCS proton affinity is higher than that

of water. Product ions are then detected and quantified by mass spectrometry at the targeted VOCS

masses plus 1 amu. For more detailed description of PTR-MS techniques see e.g. Blake et al. (2009), De

Gouw and Warneke (2007) or Wisthaler et al. (2006).

The following recommendations are preliminary, further work is in needed and in progress. A focus

on PTR-MS measurements is planned for ACTRIS-2.

Inlet (recommendation):

The length of the inlet line depends on the measurement place and height. However, it should be as

short as possible to minimize the residence time in the inlet line. Recommended materials are PFA, PEEK

and PTFE. Typical inlet diameters (ID) are 4-8 mm. Inlet flow varies from few l/min to some tens of l/min,

depending on the inlet diameter and length. The inlet flow should be turbulent.

The sampling line should also be heated at 40-60°C to minimize wall-gas interactions. I think it is critical

for sticky compounds such as carboxylic acids. In addition, it is important to make sure that there is no

cold region between the sampling line and the PTRMS.

PTR-MS sampling from this inlet line should be short and have a low volume (1/8“or 1/16“) line. The

PTR-MS sample flow should be ca. 100 ml/min or more.

Background determination:

The instrumental background is determined by measuring VOCS free air (zero air), which is produced

by pumping ambient air through a catalytic converter (Pt catalyst with heating). The background should

be measured every hour, but at least once a day. The background signals are subtracted from the


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measured signals. Background signals should be measured at the same RH than ambient air because

signals measured at some masses are RH-dependent.

The quality of the zero gas is essential as residual mole fraction of a few pmol/mol to a few tens of

pmol/mol can lead to negative offsets of the same magnitude for ambient measurements.

Critical instrument settings (recommendation):

Drift tube pressure: 2.0-2.3 mbar

Drift tube voltage: 400-650V

Drift tube temperature: 50-60°C

Voltage between last drift ring and exit lense: 30 V (instrument dependent, optimization procedures

have to be defined)

The ratio of O2+ to H3O

+ should be below 0.03

Table 12 Further settings to be discussed System PTR TOF-MS Q-PTR-MS

IONICON �SV valve setting (describe

optimization procedure, this is

applicable for the instruments with

the switchable reagent ion (SRI))

� Peak shape standards

� MCP voltage

�Tuning of mass scale and resolution

� SEM voltage

Other To be determined To be determined

Working standard (recommendation)

A recommended calibration standard includes: methanol (m33); acetonitrile (m42); acetaldehyde

(m45); acetone (m59); isoprene (m69); methyl vinyl ketone (MVK; m71); methyl ethyl ketone (MEK;

m73); benzene (m79); toluene (m93); xylene (m107); trimethylbenzene (TMB; m121); α-pinene

(m137, m81), trifluorobenzene (m133); trichlorobenzene (m181, m183, m185)

A dilution of ca. 1/50 should be used.

Calibration in field operation:

When and how often:

� background measurements, at least once a day

� calibration (working standard)

� Full mass scan

Calculation of VMR

� recommended rate constants

Typically experimentally obtained values provided by Zhao and Zhang (2004) are used. For unknown

compounds the usually recommended value is 2·10-15m3s-1.

� transmission

� recommended procedure


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6. Reference materials

The calibration scale is kept by the Central Calibration Laboratories (CCLs) by means of a system of

standards (see below). The calibration scale is transferred to the stations and labs by tertiary standards.

In case a station does not use a tertiary standard from the CCL, it has to demonstrate that the laboratory

standard is linked to the calibration scale by regular and direct comparison.

It is recommended that each station or laboratory holds the following calibration gases:

1. A laboratory standard which should be a multi-component laboratory standard (synthetic

mixture) that covers the main components and should presumably be produced by the CCL (NPL) or

another (NMI) linked to the CCL.

2. A certified multi-component working standard (synthetic mixture with certified mole fractions)

with similar components as the laboratory standard.

3. Multi-component working standards that cover all components measured and which are

calibrated versus laboratory standard, travelling standards, or other methods (carbon response FID,

permeation/diffusion source, mixtures). One of these working standards should be of high mole

fractions (upper nmol/mol range) for standard addition measurements.

4. A target gas which preferably is a whole-air working standard calibrated versus laboratory

standard, other standards or by other means, but may also be a synthetic mixture.

Minimum requirements for a station that need to be fulfilled:

1. A laboratory standard to define the calibration scale for each component measured at the

station

2. One working standard (which may be custom-made) to check for drifts in the scale. In case

of a GC-FID where calibration can in many cases be reasonably transferred from the

laboratory standard to other compounds not present in the laboratory standard by means

of the carbon response concept, a well-documented procedure to assign calibration factors

and uncertainties to these compounds is needed.

3. A target gas which is presumably whole air but could also be a synthetic mixture

The CCLs are

• for NMHCs, the National Physical Laboratory (NPL, http://www.npl.co.uk/)

• for terpenes, the National Institute for Standards and Technology (NIST,

http://www.nist.gov/) and

• for OVOCs, a CCL is not determined, yet.


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7. Quality assurance

Quality assurance (QA) follows the principles of the GAW QA system

(http://www.wmo.int/pages/prog/arep/gaw/qassurance.html):

i) Network-wide use of only one reference standard or scale (primary standard). In

consequence, there is only one institution that is responsible for this standard (CCL).

ii) Full traceability to the primary standard of all measurements made by Global, Regional

and Contributing GAW stations.

iii) The definition of data quality objectives (DQOs).

iv) Establishment of guidelines on how to meet these quality targets, i.e., harmonized

measurement techniques based on Measurement Guidelines (MGs) and Standard

Operating Procedures (SOPs).

v) Establishment of MGs or SOPs for these measurements.

vi) Use of detailed log books for each parameter containing comprehensive meta

information related to the measurements, maintenance, and 'internal' calibrations.

vii) Regular independent assessments (system and performance audits, Performance audit:

check measurements versus DQOs and traceability System audit: overall conformity of a

station with the principles of GAW).

viii) Timely submission of data and associated metadata to the responsible World Data Centre

as a means of permitting independent review of data by a wider community.

7.1 Calibration measurements

Frequent calibration measurements are essential for performing good measurements. Furthermore

and as first QC measure, target tank measurements should be made. If results of target gas

measurements are not in the ACTRIS DQO, the instrument and quality assurance system have to be

optimized in order to achieve better results with potential consequences on more frequent

calibration, blank and target gas measurements. In the following table 13 recommended calibration

frequencies are listed.

Table 13 Recommended calibration frequencies

System Laboratory

Standard

Working Standard blank Target

GC-FID 2/year (1/year)

2/month (1/month) 1/week (1/month) 1/month

GC-MS 2/year (1/year)

Every 2-4th sample (1/day)

1/week (1/month) 1/month

To stay within the DQOs, the sensitivity of a GC system should not drift by more than 3% between

calibrations. Similarly, blank values (see below) and reproducibility should not change such that they

introduce more than 3% effects on the measured data. As both, calibration and target gas

measurements enable to detect drifts in the system it is up to the operators to decide the share of

these measurements. Another issue is the reproducibility of such standard measurements. Often,


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first measurements are off in a set of measurements due to insufficient equilibration of internal

surfaces of sampling lines. For such conditions, series of standard measurements are to be

performed containing at least one appropriate measurement.

If a drift in the laboratory standard is observed as determined by a drift/inconsistency with a working

standard or a discrepancy with a new laboratory standard beyond the combined uncertainties, the

discrepancy has to be resolved as soon as possible. Options in such a situation are:

• send the laboratory standard for recalibration to the CCL or WCC

• ask other stations for a high level standard for an independent check

• check available results from past intercomparisons.

Anyway, station operators should try to identify where the drift occurred and apply a correction for

those periods in which the drift can be well described. If this is not possible, the uncertainty during

this period needs to cover the range of unexplained drift.

In case stations use working standards/target gases not comprising all components measured, it is

justified to determine the sensitivity drift of the instrument by this reduced compound mix if it

comprises major constituents of the various groups of VOCs and it covers the range of volatility and

polarity encountered in the samples. Calibration factors of compounds not present in the working

standard may then be scaled by calibration factors of physically similar behaving compounds present

in the standard.

OVOCs calibration and target gases might indicate lower repeatability and reproducibility as surface

equilibria need more time to be established and slight changes in pressure may affect these

equilibria. Accordingly, it might be necessary to apply temperature control to the cylinder valve,

pressure regulator and transfer line. Also, frequently used dynamic dilution systems might require

substantial warm-up times and it is recommended to keep them running all time.

7.1.1 Method for measurements of Laboratory/Working standards and Target gases

Generally it is recommended to leave pressure regulators and transfer lines attached to the working

standard/target gas cylinders in order to minimize the risk of contamination and reduce equilibration

times. Laboratory gloves (i.e. powder-free latex) should be worn whenever working with parts in

contact with test gases in order to avoid contamination.

In order to set the stage for good calibration measurements several issues should be considered:

Transfer line and ferrule material

• Silco steel or Sulfinert or other stainless steel tubing with a passivated internal surface.

• The use of Vespel/Graphite (VG) ferrules is recommended as these provide a tight sealing

while not damaging the tubing. They can be used several times and should only be replaced

in case that sealing or contamination problems are present (follow the mounting instructions

of the manufacturer).

Installation of a new standard gas cylinder


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• Pressure regulator and the transfer line with capped fitting on the GC connection side should

be mounted at least 24 hours before the measurement.

• After installation, the regulator and transfer line need to be flushed at least 3 times with the

calibration gas.

• Initial leak check: After flushing, pressurize the pressure regulator (cylinder pressure) and the

plugged transfer line (at level pressure needed for the measurement set up). With the

cylinder valve closed, check the pressure for a few minutes; if not constant, check all

connections, tighten gently, and repeat the check.

It is strongly recommended to use no liquid leak tester solutions as they might contaminate

the system.

Equilibration

• For equilibration keep the pressure regulator and the transfer line (plugged at the end)

pressurized with the standard gas for at least 24 hours. During this equilibration time, the

cylinder valve is closed to avoid back diffusion of potential contaminants into the cylinder

and to avoid losing sample through possible leakages. This setup also serves as a static leak

test as the upstream regulator pressure should not change during the 24 h equilibration

period.

Connection to the instrument

• Connect the test gas cylinder to an appropriate instrument inlet port. Then flush the whole

inlet line for at least another 3 times and leave the gas cylinder connected to your

instrument. It is recommended to open the standard cylinder valve only during the sampling

periods unless you use an automated measurements sequence in unattended operation.

• It is recommenced to leave the standard cylinder permanently connected to the GC system.

If this is not possible:

1. Leave the pressure regulator mounted on the cylinder, keep it pressurized and

repeat the “connection to the instrument” method every time you connect the

cylinder to the standard port.

2. If you have to dismount the pressure regulator, it is recommended to follow the

complete “installation of a new gas cylinder” – method every time.

Measurement procedure

The standard gas measurement should follow your typical measurement procedure. However, the

measurement of the standard gas should be performed after an initial flushing period through the GC

valve system which is sufficiently long to achieve equilibration in the lines (typically 10 min with 30

ml/min are sufficient for NMHCs).

7.1.2 Zero Gas

In this context, “Zero gas” is a hydrocarbon free gas. The routine measurement of zero gas is part of

the QA program to be followed at all stations. It yields information about artefacts due to release of

adsorbed hydrocarbons or leaks in the sample path. Blank values should be as low as possible. As

“zero gas” you can use


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• catalytically cleaned ambient air (Pt or Pd catalyst at 400°C), which is preferred as this is

identical to the sample gas matrix.

or

• synthetic gas (e.g. He or N2) of at least 5.0 or higher quality.

This method is not as good but easier to handle. In N2 5.0 quality, often methanol can be

observed. To reduce impurities in synthetic gas a post-cleaning is recommended (e.g. cooled

charcoal and molecular sieve cartridges).

• For offline sampling humidified zero gas is needed. High quality water has to be used.

Often, trace amounts of hydrocarbons in the pmol/mol range are present as impurities in the zero

gas. This creates an inherent problem: blank values caused by impurities cannot easily be separated

from blank artefacts as mentioned before. Accordingly, care has to be taken to identify the origin of

blanks found in zero gas measurements. Stations have to test zero-gases by comparing the blank

values obtained in measurements of different hydrocarbon free gases aiming at the lowest levels.

As blank values might vary over time, it is recommended to conduct weekly zero gas measurements.

Figure 1 represents the behaviour of blank mole fractions over time; in the here shown example He

was used as zero gas in weekly measurements. Blank mole fractions were determined by applying

the same calibration factors as for ambient air samples. Shown blank measurements were performed

with the set up as depicted in Figure 2. Except for some single events and benzene, most blank values

are observed at a rather constant level below 5 pmol/mol. The observed benzene variability is

captured by the frequent measurements. Single events (e.g. as observed in June when propane and

propene increased drastically) are recorded as well and yield valuable information about the system

status.

Some occasionally observed blank substances are listed in table 14 below.

Table 14 Occasionally observed blank substances

Compound Cause

various column peaks, column bleeding leakages contamination

benzene are occasionally observed and are generally associated to some kind of overheating of traps but the nature of this contamination is not really understood

C2-C4 alkenes often observed in systems using Nafion® Dryers ((Gong and Demerjian, 1995; Plass-Dülmer et al., 2002, Hoerger et al., 2014) and references therein)

acetaldehyde KI –ozone filter (Helmig and Greenberg, 1994; Leibrock, 1996) formaldhyde KI –ozone filter (Helmig and Greenberg, 1994; Leibrock, 1996)


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Figure 1 Blank mole fractions from zero gas measurements performed with He and the configuration described below in Figure 2 (Hohenpeissenberg Observatory)

7.1.2.1 Method for Measurement of zero gas (blanks)

For blank measurements, a zero gas is sampled via the usual air sample path as depicted in Figure 3.

Thus, the zero gas passes the ozone and particle filter (if present), the water trap, and sampling unit

just like ambient air samples. The sample volume for zero gas should be the same as for ambient air

samples. Often a high flow inlet manifold is used which cannot be easily flushed by zero gas. In this

case, zero gas has to be introduced after the sampling split to the instrument. However, checks for

this set-up should be performed with independent sampling. Figure 3 describes a possible set-up to

perform a zero gas measurement. The zero gas is applied at an open “T” into the sampling line at a

flow rate sufficiently higher than the sample flow; e.g. with a sampling flow of about 80 ml/min

towards the GC, a zero gas flow of 100ml/min yields a 20 ml/min overflow towards the ambient air

sample manifold. It cannot be excluded though, that small amounts of ambient air diffuse into the He

against the He flow (at the open “T”-position).

0

5

10

15

20

25

30

01 02 03 04 05 06 07 08 09 10 11 12

year 2009

He-

blan

k, p

pt

Ethane

Ethene

Propane

Propene

Acetylene

1-Butene

Benzene

i-Butene


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Figure 2 Example for a zero gas measurement set-up (Hohenpeissenberg Observatory).

7.1.3 Method for Detecting Effects of Ozone on Reactive Compounds

In order to check for interferences with ozone and other reactive constituent of the ambient air

sample gas, it is suggested to perform “standard addition measurements”. Briefly, a high

concentrated standard gas mixture (e.g. VOCS at 100 nmol/mol level) is added into the ambient air

stream such way that the ambient air peak areas are negligible, while the gas matrix itself is

dominated by the O3 rich ambient air (>90%). The standard mixture should contain ozone reactive

compounds (e.g. alkenes). If the O3 rich ambient air matrix does not have an effect on the sample,

the peak area ratio of the standard addition and a pure standard measurement is defined only by the

dilution factor and the C-response factor is constant (see Sections 8.1 and 8.3).

The set up shown in Figure 3 can be used for the standard addition measurements.

Figure 3 Set-up for standard addition measurements as used by Hohenpeissenberg. In this specific set-up a quartz capillary (red arrow) is used to add the low standard gas flow into the ambient air stream.

It is recommended to perform standard additions on a monthly basis but at least 4 times per year.

Strongest effects are expected in high ozone periods (e.g. summer).


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7.2 Audit procedures

Audits are performed by the WCC-VOC (KIT Garmisch).

7.3 Measurement protocol

It is required that each station has the following log sheets/book either in electronic or paper-based

form:

1. Instrument logbook with all operation parameters, significant changes, characterizations,

tests results, etc.

2. Measurement logbook with all measurements including the type of measurement, the time

of measurement in UTC (start sampling, end sampling, start GC run), sampled volume (dry

volume), and comments (anything unusual).

3. A Log of the used calibration factors and blank value determinations from zero gas

measurements.

4. A Log of all working standard and target gas measurements.

5. An Error Log with ascribed uncertainty contributions to compound measurements due to

peak-overlap, scatter of blank values, unusual low reproducibility, instable sensitivity and so

on as well as all other unexplained deviations from normal instrument performance.

6. Meteorological data Log (temp, humidity, wind velocity and direction)

7.4 Measurement uncertainties

This section describes the routine assessment of measurement precision and uncertainty. While the

precision reflects random errors in the measurement process, the uncertainty includes also possible

systematic errors in the measurement. In the following it is illustrated which factors influence precision

and uncertainty and how they are derived following the concept of the “Guide to the Expression of

Uncertainty in Measurement” (GUM, 2008).

Derived uncertainty values are 1σ errors, for the expanded uncertainty the values have to be multiplied

by the coverage factor k=2 (representing the 2σ error). The error calculation is based on the calculation

of mole fractions for linear detection systems as presented in section 6.2.1.

For data submission the precision (1σ) will be reported as well as the total expanded uncertainty (2σ).

7.4.1 Calculation of mole fractions for linear detection systems

For substances quantifiable via a standard gas mixture, the mole fraction χsample,i of a compound “i” in a

sample is calculated via:

icalsample

iblankisampleisample f

V

AA,

,,, *

−=χ (F1)

With the calibration factor


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26

irespinumiblankical

icalcalical CCAA

Vf

,,,

,, *

1*=

−=

χ (F2)

Asample,i= peak area of sample measurement of compound “i”

Acal,i = peak area of calibration gas measurement of compound “i”

Ablank,i=possible blank value of compound “i” determined in zero gas measurements

χcal,i = certified mole fraction of calibration gas standard

Vcal = sample volume of calibration gas

Vsample = sample volume of sample

Cinum = Number of C atoms in the molecule “i” (e.g. for i = n-Pentane, Cnum = 5)

Cresp,i = mean C-response factor of compound “i”

In case of substances not quantifiable by the standard gas mixture, the mole fraction is calculated via the

mean C-Response factor respC , which is determined from selected compounds in the standard gas

measurements averaging the respC factors for those substances.

The mole fraction of a substance is then

respinumisample

isampleisample

CCV

A

**,

,, =χ (F3)

7.4.2 Determination of Precision

The precision can either be derived from the target gas or working standard (whole air) measurements

or series of air samples taken in similar, stable air mass conditions (meteorological and chemical) in

series of 5 or more measurements.

It covers the random errors of peak integration, volume determination and blank variation. In case of

canister or adsorption tube sampling it includes the reproducibility of the sampling system as well.

The precision precδχ is determined as the standard deviation of a series of measurements of a sample

sampleσχ :

sampleprec σχχ =∆

The above given value represents the instruments precision only at the concentration level and

complexity of the sample gas. When working with highly variable concentrations, hence variable peak

areas, a more general description of the precision has to be applied for routine measurements:


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samplerel

prec DL χσχχ *31 +=∆

Where

DL = detection limit of the system (as described below)

C = mole fraction of considered peak

samplerel χσ = relative standard deviation of the sample (e.g. working standard)

Thus, the DL is the dominant factor for rather small peaks, while the reproducibility of the instrument

becomes the important term for larger peaks.

7.4.3 Determination of Uncertainty

The total uncertainty uncχ∆ of a measurement does not only include the random errors described by

the precision but also the systematic errors systematicχ∆ of the measurement.

222systematicprecunc χχχ ∆+∆=∆

Possible systematic errors are:

- uncertainty in the standard gas mole fraction calδχ

- systematic integration errors (due to peak overlay or bad peak separation) intAδ

- systematic errors in sample volume determination Vδ

- further instrumental problems (e.g. sampling line artefacts, possible non-linearity of the

detector (MS), changes of split flow rates) instrumentδχ

- offline sampling errors

Following Gaussian error propagation, the overall systematic error is then described as 222

int22

instrumentvolcalsystematic χχδχχ ∆+∆+∆+∆=∆ I

Referring to equations F1 to F3, the single error contributions are determined for each analysed

compound as

calcalsample

calsamplecal AV

VAδχχ *

*

*=∆ ,

where calδχ includes the certified relative uncertainty of the standard gas (or the working standard) and

possible drifts of the standard.


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2

int,2

2

int,2int *

*

***

+

=∆ cal

calsample

calcalsamplesample

sample

cal AAV

VAA

V

f δχ

δχ ,

where calAint,δ represents the relative error in peak area due to integration of the calibration

measurement and sampleAint,δ the integration error of the sample measurement, respectively.

volχ∆ , the systematic error of the sample volume, can be neglected, since calsample VV δδ = , and thus the

error cancels in equation F1. The statistic volume error is covered by the measurement precision.

instrumentχ∆ , the error in mole fraction due to specific instrumental problems has to be evaluated for

each site individually. These errors can be derived by tests or intercomparison measurements.

7.4.4 Determination of detection limit

Due to impurities or analytical problems or limits the baseline of your gas chromatographic system is

usually to a certain degree noisy. Thus, the lowest quantifiable quantity of a substance - the

detection limit of the measurement system – is different from zero.

A simple way to calculate the detection limit is, to integrate a baseline signal over a time interval

similar to the average peak width. This integration is performed for a statistically significant number

of times (min 10 times). The derived standard deviation of the integrated area multiplied by a factor

of 3 represents the detection limit.


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8. Data Management

8.1 Data evaluation

8.1.1 FID: effective carbon number

The effective carbon number concept (ECN) (Sternberg et al., 1962, Dietz et al., 1967) states that the

response (peak area) of the FID is proportional to the number of molecules times the effective number

of carbon atoms per analyte molecule, e.g. 2 nmol/mol of ethane have the same integrated response as

1 nmol/mol of butane (when comparing identical sample volumes). If other than hydrogen or carbon

bonds occur, the response of the respective carbon atom is adjusted to yield an effective carbon

number. For example, an alcohol-group (-O-H) at a terminal C, like in ethanol, results in an effective

response of 0.5 for this carbon or a total of 1.5. ECN are listed in original literature (see above), e.g. 1.0

for carbon in aliphatic and aromatic bonds, 0.95 per C in olefinic bonds, 1.3 in acetylenic, 0 for carbonyl,

and 0.5 for primary, 0.25 for secondary and 0.75 for tertiary alcohol (Sternberg et al., 1962). Using the

ECN-concept, reliable calibration factors for compounds not present in the calibration gas mixture can

be estimated.

A GC-FID system can be easily characterized for losses or artefacts by making use of the known carbon

response Cresp

With the peak area A, sample volume V, substance mole fraction and the number of carbon atoms Cnum.

When the carbon responses for the various organic compounds are calculated, they should agree within

a few percent. Deviations are often due to bad peak separation, adsorptive losses in the system, or

artificial changes at active sites. Efforts should be taken to optimize the system. Especially standard

addition measurements are of high value as they characterize interferences with other constituents of

ambient air like water vapour and ozone.

8.1.2 Time series of calibration gas measurements

Time series of the calibration factor, peak area or especially for GC-FID systems the C-response factor are

valuable tools to monitor the system status over time. As mentioned above, the C-response factor

should agree for different compounds within a few percent. And as long as the FID conditions do not

change, the C-response factor is expected to be constant. Since a MS is more variable these time series

are expected to show drifts and steps due to sensitivity changes. But as for the C-response factor, a

similar behaviour is expected for similar compounds.

In Figure 4, a time series of C-response factors for a number of NMHCs is shown. Several features can be

observed in this example: With the exception ethyne, all shown substances agree within 3% and

resemble the same behavior over time. A reason for the drift (~ -4%) might be e.g. a slow change of FID

characteristics which are, however, captured by the frequent standard measurements. Ethyne reveals a

different C-response behavior; it is not only more variable but shoes a sharp increase. The latter was

connected to a change of working standard, which affected only the ethyne response. As a consequence

�� = �� × × ��


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30

for ethyne, the average C-response factor is used, as it cannot be determined properly in the calibration

measurements.

Figure 4 Time series of C-response factors of bi-weekly working standard measurements for several NMHCs with the C2-C8 GC-FID system of Hohenpeissenberg (Plass-Duelmer et al., 2003) during the year 2013.

8.1.3 Target gas measurements

In Figure 5, a series of target gas measurements at Hohenpeissenberg (DWD, Germany) is shown.

Here, the determined mole fraction for all analysed compounds is plotted over time in a log scale.

Relative changes are detectable as linear deviations from constant values. The first plot shows

compounds with mole fraction of more than 50 pmol/mol, the second one C3-C6 hydrocarbons with

mole fractions below 50 pmol/mol, and the third one C6-C9 hydrocarbons with mole fractions below

50 pmol/mol. Except for 2-methylpropene (due to blank values), it should be pointed out that

repeatability gets poorer for compounds with higher molecular weight and towards lower mole

fractions. However, the repeatability is still mostly within 2 pmol/mol or a few percent. This plot

shows monthly repeatability of a series of 5 replicates, and monthly reproducibility throughout the

year for ambient air mole fraction levels and ambient air matrix.

6

6.2

6.4

6.6

6.8

7

7.2

7.4

7.6

7.8

8working standard C-response GC Hohenpeissenberg, 2013

ethane, AS_NPL

ethene, AS_NPL

propane, AS_NPL

propene, AS_NPL

2-methylpropane,AS_NPL

ethyne, AS_NPL

n-butane, AS_NPL

t-2-butene, AS_NPL

1-butene, AS_NPL

c-2-butene, AS_NPL


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31

Figure 5 Measurements of compressed whole air from cylinder (“Referenzluft-04” or “Ref-04”) through 2009; generally 5 replicates are measured once a month.

8.1.4 Results of standard addition measurements

The standard addition measurement is compared to a pure standard measurement of the same

standard gas mixture. If the O3 rich ambient air matrix does not have an effect on the sample, the

calibration factor (for FID systems the C-response factor) should be the same for both measurements

and thus

1 =��

� ��

With �� =�� !"!�# being the average peak area ratio for non O3-reactive compound with

low mole fraction in the ambient air (e.g. alkanes like). This concept is applicable for all linear GC-

systems, without knowledge of the exact flows or mole fractions in the standard gas mixture.

In Figure 6, results of a standard addition measurement performed in 2009 at Hohenpeissenberg are

shown. Plotted is the normalized peak ratio as described above. Some of them have fairly high

contributions from ambient air (ratios up to about 3), but the important alkenes are clearly

dominated by the added standard. If ozone interferences (losses) exist, these reactive alkenes should

show lower ratios than 1. None of the alkenes shows any significant deviations from 1 and thus no

indication of reactive losses with O3. In case alkene measurements exhibit a normalized peak area

ratio rnorm < 1, the GC system is further checked and if necessary the O3 filter is replaced.

10

100

1000

10000

mol

e fr

actio

n [p

mol

/mol

]

Target Gas

ethane

ethene

propane

propene

2-methylpropane

Ethyne

n-butane

n-pentane

3-M-Pentan

2-methylpropene

2-methylbutane

2-methylpentane

methylcyclopentane+2-2-dimethylbutane.toluene


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32

Figure 6 Results of NPL standard addition measurements from 2009 performed once per month. Typically, 2-10% of sample volume is added to an ambient air.

8.1.5 Data checks of final mole fraction data in time series

VOCs should be grouped in a convenient number (typically 3 or 4) of functionally similar compounds,

e.g. alkanes or alkenes, in a log plot over a time interval of half a year or a year. The procedure is

illustrated in Figures 7 and 8, where quality checks were performed for Hohenpeissenberg data in

year 2009.

Generally, it is expected that the variability of the data should increase with higher reactivity

(variability-lifetime-relation) and changes should be more pronounced for shorter lived compounds

(lower background). Spikes in positive direction may be attributed to plumes with local/regional

pollution and should be checked for consistency with other compounds from similar sources, if not

consistent, the raw data should be rechecked. Spikes in negative direction stand for clean air and

should again be checked for consistency with other compounds from similar sources. If they are

found to be not consistent with other compounds, the peak integration, breakthrough in trap or

other potential loss problems should be checked once again. (e.g. the negative spike in ethane at the

beginning of august (Fig. 8) was due to breakthrough; several positive spikes in November are found

in all time-series and are thus most likely an atmospheric feature).

0.91.01.11.21.31.41.5

1.61.71.81.92.0

NPL-Standard-Addition

ethane ethene propanepropene 2-methylpropane acetylenen-butane t-2-butene 1-butene2-methylpropene c-2-Buten2 propyne1,3-butadiene t-2-pentene n-hexane


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Figure 7 Time series (annual cycle) of C2-C4 alkanes measured at Hohenpeissenberg, generally, measurements from 1:00 and 13:00 CET are shown. Due to the orographic situation on the top of a hill, Hohenpeissenberg is frequently decoupled from the boundary layer at night time and accumulations of trace gases as found at flat terrain sites are usually not observed.

For compounds with similar relative annual cycles but with different mole fraction levels the ratio

plots as shown below can be used (Fig. 9). In such plots, either structural similar compounds (as

shown above for pairs of alkanes, alkynes, and aromatics) are compared or compounds originating

from similar sources or compounds having similar lifetimes. For each distinct spike/outlier or

deviations from the ratio, following checks are performed:

i) logbook entries to identify irregular operation conditions

ii) peak integration

iii) other compounds deviating in these individual measurements and try to identify the reason

for the spike

For example, the n-pentane/2-methylbutane spikes are due to occasional n-pentane plumes

(observed in two independent systems but the origin of the plume is unknown), the broad minimum

in propyne/ethyne ratios in spring is due to the longer lifetime of ethyne and the correspondingly

relaxed reaction to the OH annual cycle, and of the aromatics only benzene has a well-developed

summer minimum, all others are flattened in summer but consistently as seen in the xylene/toluene

ratios.

10

100

1000

10000

ethane, ppt propane, ppt2-methylpropane, ppt n-butane, ppt

2011


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Figure 8 Time series of ratios between pairs of hydrocarbons with similar structure; data from Hohenpeißenberg, 2011.

8.1.6 QC in xy-plots (used at Rigi, Switzerland by Empa)

A similar approach as used at Hohenpeissenberg is applied at the Rigi site by Empa. Beside time

series, many xy-plots are generated to check for consistency with former years and within the year.

In the correlation plots, compounds which are structural similar, or compounds originating from

similar sources, or compounds having similar lifetimes are plotted against each other. In Figures 9

and 10 examples are shown of correlation plots using toluene vs. benzene and benzene vs. ethyne.

Table 15 xy-plots used at the Rigi (Switzerland) site by Empa

propane/ethane butenes/ethene isoheptane/isohexane

n-butane/ethane pentenes/ethene 1,3-butadiene/isoprene

propane/n-butane ethenes/butenes toluene/benzene

2-methylpropane/n-butane methylpropane/2-methylbutane m/p-xylene/benzene

2-methylbutane/n-pentane 1,3-butadiene/ethene ethylbenzene/mp-xylene

propene/ethene n-hexane/n-pentane o-xylene/mp-xylene

ethyne/ethene isohexane/n-hexane o-xylene/ethylbenzene

ethyne/benzene

0.01

0.1

1

10

n-pentane/2-methylbutane propyne/ethyne

toluene/benzene p-m-xylene/toluene2011


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Figure 9 Toluene vs benzene at Rigi (Switzerland). Blue 2011 data, brown 2008-2010 data

Figure 10 Ethyne vs benzene at Rigi (Switzerland). Blue 2011 data, brown 2008-2010 data

8.1.7 QC in repeatability and reproducibility:

One more check for the plausibility of the measurements is related to the repeatability (sets of 3-5

measurements each) and reproducibility (monthly or bimonthly) of the measurements of calibration

gases or target gases.

In Figure 11, the relative (%) and absolute (pmol/mol) standard deviations are plotted for 3 different

target gases on a log scale versus the mixing ratio for all identified compounds. Such a presentation

helps to identify problems with individual compounds. In the lower figure there are two curves

y = 0.9762x + 0.0305R² = 0.5713

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 0.2 0.4 0.6 0.8 1 1.2 1.4

tolu

en

e

benzene

Toluol * Toluol (11) Linear (Toluol (11))

Chrom. okC5/C2-Benzene also higher

y = 0.3465x + 0.0084R² = 0.9864

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5

be

nze

ne

ethyne

Benzol * Benzol (11) Linear (Benzol (11))


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added that represent a variability by 1 pmol/mol + 1% (red line) and 3 pmol/mol + 3 % (broken red

line). For a number of compounds the first line represents a good fit. However, some of the

compounds exhibit higher scatter but are generally within the 3 pmol/mol + 3% line which still is a

quite good level for the reproducibility of measurements of cylinder gases and within the ACTRIS

DQOs as shown in Section 2. Compounds with higher deviations should be checked for the reason of

the deviation, often peak-overlap or –integration problems are associated with worse reproducibility.

Also, heavier compounds tend to be less reproducible due to adsorption/desorption problems.

Figure 11 Standard deviations obtained from all measurements of target gases performed at Hohenpeissenberg Observatory in 2007 versus compounds’ mole fractions in pmol/mol in the respective standard; the upper panel shows the relative standard deviations, and the lower panel the pmol/mol standard deviations versus mole fractions (pmol/mol); two lines in the lower part show parameterizations of the standard deviations for “good” results (1 pmol/mol + 1%) and as an upper limit for most of the “not so good” compounds 3 pmol/mol + 3%.

reference gases in 2007rel. standard dev. versus mixing ratio

0%

1%

10%

100%

1000%

0.1 1 10 100 1000 10000 100000

Ref-04

SUPELCO

74-NMHC

abs. standard dev. versus mixing ratio

0

1

10

100

1000

10000

0.1 1 10 100 1000 10000 100000

Ref-04

1ppt+1%

SUPELCO

74-NMHC

3ppt+3%

Target 1 Target 2 Target 3

Target 1 Target 2 Target 3


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37

8.1.8 Recommended QC and minimum QC, thresholds for flagging the data

As a minimum requirement of the QC we suggest to visually control time series of calibration gas (7.2),

target gas (7.3) and ambient air measurements (7.5) and to generate xy-plots for the above mentioned

compounds (7.6) and to compare the data to previous years� Template is available from EMPA.

Table 16 ACTRIS data flags

Flag Data Valid (V) or invalid (I) Description

000 V Valid measurement

147 V Below theoretical detection limit or formal Q/A limit, but a value has been measured and reported and is considered valid

420 V Preliminary data

457 V Extremely low value, outside four times standard deviation in a lognormal distribution

458 V Extremely high value, outside four times standard deviation in a lognormal distribution

651 V Extremely high value, outside four times standard deviation in a lognormal distribution

652 V Construction/activity nearby

999 I Missing measurement, unspecified reason

8.2 Metadata

Link to EBAS Meta data ( http://ebas-submit.nilu.no/)

8.3 Ancillary data

� EBAS

8.4 Data archiving at the station or laboratory

It is recommended to perform daily backups of the raw data.

8.5 Data submission

All ACTRIS trace gases measurements are reported to, and stored in the EBAS atmospheric database

http://ebas.nilu.no. The EBAS database, originally designed for the European Monitoring and Evaluation

Programme (EMEP), today archives data on atmospheric composition from ground stations around the

globe, as well as aircraft and ship platforms. All datasets in EBAS are associated to one or more

projects/frameworks, having individual rules for data disclosure. Most data stored in EBAS are

originating from programs encouraging an unlimited and open data policy for non-commercial use. Offer

of co-authorship is made through personal contact with the data providers or owners whenever

considerate use is made of their data. In all cases, an acknowledgment must be made to the data


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38

providers or owners and to the project name when these data are used within a publication. The data

policy for ACTRIS near-surface data is harmonized with the data policy for GAW, and available from

EBAS, and from the ACTRIS data portal and also web: http://www.actris.eu/language/en-

GB/ProjectResults/Dataconcept.aspx.

The ACTRIS data portal links EBAS data, together with data from the two other ACTRIS databases,

EARLINET DB and CloudNet DB, into one common data portal. The portal facilitates the combined

analysis of all ACTRIS data, offering advanced tools for plotting and combining ACTRIS data from the

three topic databases, and mapping tools for user defined visualization of distribution atmospheric sites

and variables across networks and projects.

The following section provides a summary of the data submission procedures for trace gas data to EBAS.

The text below only address the main points as defined by August 2014, for a complete and, at any time,

updated document please reference http://ebas-submit.nilu.no/.

Trace gas near-surface data are qualified as ACTRIS data only if the measurement data are reported to

EBAS by using the templates and formats recommended by the ACTRIS trace gas community, and

following the procedures described in the current document. The data providers are responsible for the

quality of the data submitted and the templates ensure proper and sufficient documentation of the

data. ACTRIS partners shall label their contribution two EBAS with project/framework "ACTRIS". The data

can also be associated to other programs and frameworks like GAW-WDCGG-node, EMEP, InGOS etc.

Data submitted to EBAS need to be formatted in the EBAS NASA-Ames format by the data provider. The

EBAS NASA-Ames format is based on the ASCII text NASA-Ames 1001 format, but contains additional

metadata specifications ensuring proper documentation of the setup and procedures for each

measurement principle. The term VOCs in ACTRIS consists of three subgroups: NMHCs (C2-C9

hydrocarbons), OVOCs (oxygenated volatile organic compounds), and terpenes (biogenic hydrocarbons

with a terpene-structure), which can be measured by several different measurement principles; on-line

and off-line traps and off-line canisters.

An overview of all ACTRIS variables and the associated recommended methodologies are available at the

ACTRIS web in the document “ACTRIS Data, concept, and variables”:

http://www.actris.eu/Portals/97/Publications/data%20concept/ACTRIS_data_concept.pdf.

Specific templates for each of these are available from http://ebas-submit.nilu.no/ under the tab Submit

Data -> Regular Annual Data Reporting -> VOCS.

An EBAS NASA Ames file consists of two parts; a metadata header and a column formatted data part.

The header section contains a number of important metadata items describing the measurement site,

data variable, instrument, measurement principle and operating procedure. If nothing changes in the

measurement set up, the header will remain the same from year to year, and the measurement data will

be visible as one continuous dataset in the database. The data section of an EBAS NASA Ames file

consists of a fixed column number format ASCII table, including time stamp, data value and flag for each

single measurement point or data average point. The data formatting templates give the user a detailed

line-by-line explanation of what metadata that should be included on which line of the header, in terms

of correct procedure and wording:

http://ebas-submit.nilu.no/SubmitData/RegularAnnualDataReporting/VOCS/NMHCSonline.aspx


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Further information is available by clicking on the respective line number from the template. Flagging of

data should be done according to the ACTRIS trace gas guidelines. For time being only flags from the

tables at the format template pages are recommended, but a complete list of flags available in EBAS is

located at http://www.nilu.no/projects/ccc/flags/flags.html

The data centre recommends first to create the data table and then add the header. Name the file

overusing the filename stated in the header.

The data submission deadline for the ACTRIS project is following the EMEP submission deadline, this is

July 31 for data from the year before. Example: 31. July 2014 is reporting deadline for all 2013 data. The

files containing the data submissions must be uploaded to the EBAS anonymous FTP site, accessible at:

ftp://ebas-submissions.nilu.no/incoming using the submitters email as password. This site is for security

reasons a blind drop page, so the submitter will not be able to see the data after submission. An auto-

mail from the system will be sent to the data submitter if the submission was successful.

The submitted data will be collected, checked and inserted to EBAS. The data submitter will be notified

in case of needs for correction in the submitted data.

Status report to ACTRIS activity leaders will be made by the end of the year.


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9. References

Apel, E.C. et al. (2003): A fast-GC/MS system to measure C2 to C4 carbonyls and methanol aboard aircraft. Journal of Geophysical Research 108 (D20): 15-1 – 15-12.

Barkley, C.S. et al. (2005): Development of a Cryogen-Free Concentration System for Measurements of Volatile Organic Compounds. Analytical Chemistry 77 (21): 6989-6998.

Blake, R. S., Monks, P. S., Ellis, A. M. (2009): Proton-Transfer Reaction Mass Spectrometr. Chem. Rev. 109: 861-896. De Gouw, J., Warneke, C. (2007): Measurements of volatile organic compounds in the earth’s atmosphere using proton-transfer-reaction mass spectrometry. Mass Spectrom. Rev. 26 (2): 223–257.

Graus, M., Müller, M., Hansel, A. (2010): High Resolution PTR-TOF: Quantification and Formula Confirmation of VOCS in Real Time. J. Am. Soc. Mass Spectrom. 21 (6): 1037–1044.

Dietz W.A. (1967): Response factors for gas chromatographic analyses, J. of Gas Chromatography 5, 68.

Folkers, A. (2002): Oxygenated volatile organic compounds in the troposphere: Development and employment of a gas chromatographic detection method. Report of the Research Centre Jülich 3998. Dissertation University of Köln, Jülich.

Greenberg, J.P., Zimmermann, P.R., Pollock, W.F., Lueb, R.A., Heidt, L.E. (1992): Diurnal variability of atmospheric methane, nonmethane hydrocarbons, and carbon monoxide at Mauna Loa. Journal of Geophysical Research 97: 10,395-10,413.

Helmig, D. (1997): Ozone removal techniques in the sampling of atmospheric volatile organic trace gases. Atmospheric Environment 31 (21): 3635-3651.

Helmig, D., Greenberg, J.P. (1994): Automated in situ gas chromatographic-mass spectrometric analysis of ppt level volatile organic trace gases using multistage solid-adsorbent trapping. Journal of Chromatography A 677: 123-132.

Hoerger, C.C., S. Reimann, A. Werner, C. Plass-Duelmer, E. Weiss, R. Steinbrecher, S. Sauvage, J.R. Hopkins, J. Aalto, J. Arduini, N. Bonnaire, A. Borowiak, J.N. Cape, A. Colomb, R. Connolly, J. Diskova, P. Dumitrean, C. Ehlers, V. Gros, H. Hakola, M. Hill, M.K. Kajos, J. Jäger, R. Junek, M. Leuchner, A.C. Lewis, M. Maione, D.Martin, E. Nemitz, S. O'Doherty, P. Pérez Ballesta, T. Petäjä, J-P. Putaud, N. Schmidbauer, G. Spain, E. Straube, M. Vana, M.K. Vollmer, R. Wegener, A. Wenger, Volatile organic compounds (VOCs) intercomparison experiment in Europe (within ACTRIS), (2014): submitted to Atmos. Meas. Tech.

Hopkins J.R., Lewis A.C., Read K.A. (2003): A two-column method for long-term monitoring of non-methane hydrocarbons (NMHCs) and oxygenated volatile organic compounds (o-VOCs). Journal of Environmental Monitoring, volume 5, issue 1.

Hopkins, J.R., Jones, C.E., Lewis, A.C. (2011): A dual channel gas chromatograph for atmospheric analysis of volatile organic compounds including oxygenated and monoterpene compounds. Journal of Environmental Monitoring DOI: 10.1039/c1em10050e.

Koppmann, R., Johnen, F.J., Khedim, A., Rudolph, J., Wedel, A., Wiards, B. (1995): The influence of ozone on light nonmethane hydrocarbons during cryogenic preconcentration. Journal of Geophysical Research 100: 11,383-11,391.

Kuster, W.C, Goldan, P.D., Albritton, D.L. (1986): Ozone interferences with ambient dimethyl sulfide measurements: The problem and a solution. Eos 67: 887.

Lee, J.H., Batterman, S.A., Jia, C., Chernyak, S. (2006): Ozone artifacts and carbonyl measurements using Tenax GR, Tenax TA, Carbopack B, and Carbopack X adsorbents. Journal of the Air & Waste Management Association 56: 1503-1517.


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Leibrock E. (1996): Entwicklung eines gaschromatographischen Verfahrens zur Spurenanalytik von oxidierten Kohlenwasserstoffen in Luft, Wissenschafts-Verlag Maraun, Frankfurt a.M., McClenny, W.A., Pleil, J.D., Evans, G.F., Oliver, K.D., Holdren, M.W., Winberrry, W.T. (1991): Canister-based method for monitoring toxic V0Cs in ambient air, J. Air Waste Manage. Assoc. 41, 1308-1318

McClenny W.A., Pleil J.D., Evans G.F., Oliver K.D., Holdren M.W., Winberry W.T. (1991): Canister-based method for monitoring toxic VOCs in ambient air J. Air Waste Man. Ass., 41, 1308–1318.

Palluau F., Ph Mirabel , M Millet (2007): Influence of ozone on the sampling and storage of volatile organic compounds in canisters, Environ Chem. Lett. 5, 51–55.

Plass-Dülmer, C.; Michl, K.; Ruf, R.; Berresheim, H. (2002), C2 - C8 hydrocarbon measurement and quality control procedures at the Global Atmosphere Watch Observatory Hohenpeissenberg. J. Chromatogr. 953, 175-197.

Plass-Dülmer C., N. Schmidbauer J. Slemr, F. Slemr, H. D'Souza (2006): European hydrocarbon intercomparison experiment AMOHA part 4: Canister sampling of ambient air, J. Geophys. Res., 111, D04306, doi:10.1029/2005JD006351.

Pollmann J., D. Helmig, J. Hueber, Ch. Plass-Dülmer, P. Tans (2008): Sampling, storage, and analysis of C2–C7 non-methane hydrocarbons from the US National Oceanic and Atmospheric Administration Cooperative Air Sampling Network glass flasks, J. Chromatogr., A 1188, 75-87.

Sternberg J.C., W.S. Gallaway and D.T.L. Jones, in N. Brenner (1962), The mechanism of response of Flame Ionization Detectors, in J. Callen and M.D. Weiss (Editors) Gas-Chromatography, Academic Press, New York, p. 231-267.

Taipale, R., Ruuskanen, T. M., Rinne, J., Kajos, M. K., Hakola, H., Pohja, T., Kulmala, M. (2008): Technical Note: Quantitative long-term measurements of VOCS concentrations by PTR-MS – measurement, calibration, and volume mixing ratio calculation methods. Atmospheric Chemistry and Physics 8, 9435-9475.

US-EPA (1998): Technical assistance document for analysis of ozone precursors, US-Environmental Protection Agency, EPA/600-R-98/161.

US-EPA TO-14A (1999): Compendium of methods for the determination of toxic organic compounds in ambient air: determination of volatile organic compounds (VOCs) in ambient air using specially prepared canisters with subsequent analysis by gas chromatography, US-Environmental Protection Agency, Method TO-14A, 2nd ed., EPA/625/R-96/010b.

Wisthaler, A. et al. (2006): Recent developments in proton-transfer-reaction mass spectrometry. Photonic, Electronic and Atomic Collisions, 24th International Conference on Photonic, Electronic and Atomic Collisions, Rosario, Argentina: 462-469. Doi: 10.1142/9789812772442_0060.

WMO (1995), WMO-BMBF Workshop on VOCs - Establishment of a „World Calibration/Instrument Intercomparison Facility for VOCS“ to serve the WMO Global Atmosphere Watch (GAW) Programme, WMO Report, 111.

WMO (2001): Strategy for the Implementation of the Global Atmosphere Watch Programme (2001 – 2007), GAW Report No. 142 (WMO TD No. 1077), 62 pp., World Meteorological Organization, Geneva, Switzerland.

WMO (2007a): A WMO/GAW Expert Workshop on Global Long-Term Measurements of Volatile Organic Compounds, Geneva, Switzerland.

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WMO (2012): (GAW Report 204: Standard Operating Procedures (SOPs) for Air Sampling in Stainless Steel Canisters for Non-Methane Hydrocarbons Analysis (prepared by R. Steinbrecher and E. Weiss), 28 pp., Geneva, Switzerland.

Zhao, J. and R. Zhang, 2004: Proton transfer reaction rate constants between hydronium ion (H3O+) and volatile organic compounds. Atmospheric Environment, 38, 2177–2185. Blake, R. S., Monks, P. S., Ellis, A. M. (2009): Proton-Transfer Reaction Mass Spectrometr. Chem. Rev. 109: 861-896.


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10. Appendices

Appendix 1: Ozone removal techniques for GC analysis of OVOCS

Appendix 2: Adsorbents for sorbent-based enrichment of VOCs and OVOCs (oxygenated volatile

organic compounds) in ambient air samples

Appendix 3: Chromatographic separation


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APPENDIX 1: OZONE REMOVAL TECHNIQUES FOR GC ANALYSIS OF OVOCS (OXYGENATED VOLATILE

ORGANIC COMPOUNDS) IN AMBIENT AIR SAMPLES (COMPILED FROM LITERATURE J. ENGLERT)

Reactions of concentrated VOCs with ozone during sampling process may alter the quantities of the

target analytes and also contribute to the formation of artefacts which may mistakenly be

interpreted as atmospheric constituents.

Ozone reactions during cryogenic enrichment of VOCS:

Ozone melting and boiling points (at atmospheric pressure) are at -192.1°C and 111.9°C. During

cryogenic freeze-out of VOCs from ambient air samples ozone is concentrated together with the

target analytes, whereas the main constituents of air nitrogen and oxygen do not condense under

these conditions (boiling point of liquid nitrogen -196°C). Reactions of VOCs with ozone occur when

heating the cryogenic trap to transfer the analytes to the GC system. Alkenes, such as isoprene and

monoterpenes can be depleted in this reactions leading to artefacts like methacrolein and

methylvinylketone. By collecting ambient air into stainless steel canisters prior to the analysis with

cryogenic freeze-out techniques this effect is reduced because of the short lifetime of ozone in these

canisters (Helmig, 1997; Greenberg et al., 1992). But this method is not suitable for oxygenated VOCs

because of the high reactivity of these compounds on unheated stainless steel surfaces.

Ozone reactions during solid adsorbent sampling of VOCs:

Ozone artefacts are formed on and with some sorbents (e.g. graphitised carbon sorbents and Tenax®

TA) leading to both VOCs losses and formation (Lee et al., 2006; McClenny et al., 2001). Adsorbed

unsaturated hydrocarbons might for example undergo reaction with ozone during ambient sampling

leading to diminished alkene concentrations and the formation of oxygenated reaction products e.g.

acetaldehyde and formaldehyde. Products from ozone - Tenax® reactions include benzaldehyde,

phenol, acetophenone and n-aldehydes (Helmig, 1997).

Reactions with ozone can be reduced by selectively removing the oxidant in the sample flow prior to

the concentrating of the analytes of interest. The ozone removing system should be easy to use,

inexpensive, efficient in the ozone removal rate and have a high scrubbing capacity, long lifetime and

eliminate the effects of ozone without interfering with the analytes of the target compounds and

without introducing contaminants. Furthermore it should be universally applicable to allow the

analysis of a wide range of compounds. Commonly reported techniques for ozone scrubbers include

impregnated filters, impregnated glass wool, coated tubes and coated annular denuders.

Catalytic destruction of ozone on metal surfaces:

Aluminum, copper, lead and tin have low ozone depletion efficiency whereas silver, iron, zinc, gold,

nickel, mercury and platinum have high ozone destruction capacities. The ozone removal

acquirement of some metals is used e.g. by nickel tubing, which reduces ozone levels to less than 20


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% of ambient air level (Helmig, 1997). Koppmann et al. (1995) found up to 50% destruction of

ambient ozone by pulling the sample air through stainless steel inlet lines kept at 67°C.

Hopkins et al. (2011): All gas transfer lines within the system are made from stainless steel and

heated to 70°C to reduce ozone mixing ratios.

Disadvantage: Loss of OVOCs on the surface of stainless steel even at high temperatures (150°C).

Ozone deletion by nitric oxide (NO) titration:

Titration of the ambient air sample with a few ppm of NO prior to the concentration step is a very

efficient method to remove ozone. Ozone (O3) deletion performance depends on sufficient reaction

time and NO concentration in the mixing chamber. An example is the titration of the ambient air

sample for 20 seconds in a 1 litre glass reaction vessel with a small flow of 200 ppm NO in nitrogen

resulting in a NO concentration of 2 ppm. NO reacts with O3 to nitrogen dioxide (NO2) and oxygen

(O2) (Helmig, 1997). The reaction is: O3 + NO → O2 + NO2.

Disadvantage: slow reaction, alcohol losses (but constant)

Ozone deletion by potassium iodide (KI):

In many cases KI is used for O3 removal. This technique is very effective at ambient humidity levels

while capacity is reduced in dry air respected in following equation (Helmig, 1997): O3 + 2KI + H2O →

O2 + I2 + 2KOH. KI reacts with O3 to potassium oxide (K2O) and elemental iodine.

Example: PTFE-lined stainless steel or Silco steel capillary, OD 1/4″, 5 cm filled with KI-coated glass

wool.

Disadvantage: formaldehyde and acetaldehyde blank values, alcohol losses (Helmig and Greenberg,

1994; Leibrock, 1996)

Sodium sulphite (Na2SO3):

Most efficient in the presence of atmospheric water vapour and hence has to be positioned

upstream of a water trap – was found to remove 99% of the O3 in a humid ambient air stream but

inconsistent removal efficiencies from different suppliers and from different batches – testing of

individual O3 traps is required (Helmig, 1997)

Example: ¼“ glass tube filled with 1 g of Na2SO3 anhydrous crystals held in place by glass wool plugs

and maintained at 100°C to prevent clumping of the Na2SO3

Disadvantage: removal of methyl vinyl ketone (MVK) and methacrolein.

Sodium thiosulphate (Na2S2O3):


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The reaction between thiosulfate and O3 produces tetrathionate oxygen and water depends on the

pH level: 2S2O32- + O3 + 2H+ → S4O6

2- + O2 + H2O

Example: O3 filters were prepared by flowing a 10% solution of aqueous Na2S2O3 through commercial

glass fiber filters followed by dry purge with nitrogen and had capacities in excess of 1 m3 air at

ambient O3 levels

(Helmig, 1997)

Advantage: this glass fiber filters also reduce sampling artefacts from reactions with halogens

Other O3 removal agents are copper oxide (CuO), magnesium sulphate (MgSO4), manganese dioxide

(MnO2), potassium carbonate (K2CO3) and TPDDC (see Table 1).

In-line O3 scrubbers like granular KCl and crystalline Na2SO4 are prone to artefacts and require

regular maintenance so that they are not suited to long-term instrument deployments (Hopkins et

al., 2011).

Table 1: Ozone removal techniques for VOCS monitoring and their characteristics.

Technique Agent Characteristics

Coated annular denuder Potassium iodide (KI) Very efficient

Cellulose filter KI Improved formaldehyde and acetaldehyde recovery

Packed Teflon tubing Crystalline KI

Quantitative transmission of formaldehyde and acetaldehyde, partial loss of methacrolein and methyl vinyl ketone (MVK)

Impinger KI 2% aqueous, buffered KI solution

Impregnated glass wool KI Quantitative O3 removal, iodated artefacts

Coated tubing KI in copper tubing Commercial scrubber KI in polyethylene cartridge Low capacity at 5% RH

Impregnated glass fiber filter Sodium thiosulphate (Na2S2O3) High capacity, also reduces sampling artifacts from reactions with halogens

Coated copper screen Manganese dioxide (MnO2) High capacity, possible losses of terpenes (e.g. camphor, linalool), loss of formaldehyde

Packed copper tubes Anhydrous 20-60 mesh potassium carbonate (K2CO3), crystalline

100% transmittance of light hydrocarbons

Packed Teflon tubing K2CO3 ozone and water removal, 100% transmission of light hydrocarbons

Packed glass tube Crystalline sodium sulphite (Na2SO3)

Loss of unsaturated compounds prevented, most efficient in the presence of atmospheric water vapour

Cartridge Copper oxide (CuO), crystalline No losses of carbonyl compounds

Trap Crystalline magnesium sulphate (MgSO4)

Removal of at least 100 ppb, loss of O3 removal efficiency with sampling length

Gas-phase ozone titration Nitric oxide (NO) Very efficient, quantitative


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recovery of formaldehyde, formation of artifacts on Tenax exposed to elevated NOx levels, possible chromatographic interferences of NO and NO2 with NMHCS (Kuster et al., 1986), losses of alcohols, slow reaction

Metal tubing Nickel (Ni) O3 reduced to less than 20% of ambient level

Spiked cartridge TPDDC (Tetramethyl-1,4-phenylenediamine dihydrochloride)

Sampling of carbonyl compounds on microcartridges containing porous glass particles impregnated with dansylhydrazine (DNSH), agent added to the reagent solution at the time of cartridge preparation to serve as an O3 scavenger

Spiked cartridge 5% Na2S2O3 aqueous solution on Tenax

Direct pretreatment of the adsorbent, improved monoterpene recovery

Spiked cartridge Na2S2O3 Interferences eliminated


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APPENDIX 2: ADSORBENTS FOR SORBENT-BASED ENRICHMENT OF VOCS AND OVOCS (OXYGENATED

VOLATILE ORGANIC COMPOUNDS) IN AMBIENT AIR SAMPLES (COMPILED FROM LITERATURE BY J.

ENGLERT)

Sampling of ambient air with sorbent tubes or traps and subsequent thermal desorption to transfer

the sampled compounds to a GC system is widely-used for trace gas analysis of VOCs because of the

high sensitivity of this method.

There are two different sorbent-based sampling strategies: (1) on-line sampling of ambient air

directly into (cooled) sorbent focusing traps or transfer of air samples from containers (stainless steel

canisters or PTFE bags) into these (cooled) traps; and (2) off-line pumped (active) or diffusive

(passive) sampling onto adsorbent tubes or cartridges held at ambient temperature. In the case of

off-line sampling VOCs are transferred in a second step into a cooled focusing device (e.g. sorbent

trap). For oxygenated VOCs a method with short transfer from sampling device to the analysis system

is important because of the high losses of these analytes on surfaces, especially on unheated and not

inert ones like untreated surfaces of stainless steel.

When selecting a suitable sorbent or sorbent combination for VOCs and OVOCs several factors have

to be considered including sorbent strength, artefacts, hydrophobicity, inertness, thermal stability

and friability. It has to be verified that there is no breakthrough (most critical compounds methanol

and acetaldehyde), getting stuck or back-diffusion of target compounds. Some special, low volatile

analytes may also be lost through aerosol formation.

The sorbents must be strong enough to retain target analytes from a specific sample volume but

must also be weak enough to release them during thermal desorption. Sorbent strength is measured

in terms of breakthrough volumes that are defined as the litres gas per gram adsorbent required to

elute a VOCs off 1.0 gram adsorbent at an indicated temperature. This capacity of solid sorbents

depends on temperature and is typically specified at 20°C. It approximately halves for every 10°C

rise. Therefore, cooling the traps during sampling increases adsorbent performance. The lowest

possible temperature is limited by the dew point of the sampled air (Brown and Shirey, 2001; Helmig

and Greenberg, 1994; Woolfenden, 2010b).

When using hydrophilic sorbents (molecular sieves) or temperatures below the dew point for

ambient air samples some kind of water trap has to be installed in the sampling line. Otherwise there

would be a reduction of sorbent performance that might reach a factor of 10 at high humidity

conditions (90% RH) and after desorption of the trapped water moisture might interfere with the

following chromatographic analysis. Weak and medium strength sorbents (porous polymers and

graphitised carbon blacks) are hydrophobic and so they prevent trapping of excess water.

Some sorbents especially carbon blacks contain chemically active materials (trace metals) and are

unsuitable for labile (reactive) species. Most porous polymers except from Tenax® TA have high

inherent artefacts with blank peaks at 5-10 ng levels (Woolfenden, 2010b).

Ozone (O3) artefacts are formed on and with some sorbents (e.g. graphitised carbon sorbents and

Tenax® TA) leading to both OVOCs losses and buildings (Lee et al., 2006; McClenny et al., 2001). So

the aspect of O3 removal has to be considered in sorbent-based ambient air sampling.


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Quartz wool or silica beads are not able to retain most of the compounds. They are usually used in

multi-bed traps to prevent very high boilers to come in contact with a stronger adsorbent.

Porous polymers are weak or medium strength sorbents. None of them could retain the very volatile

analytes. In multi-bed traps they are often the first sorbent in sampling direction for the mid and

higher boiling point analytes beginning from benzene. Porous polymers are hydrophobic and so are

adequate for humid ambient air samples.

CarbopackTM, CarbotrapTM and CarbographTM are graphitised carbon blacks. The three different

types differ in mesh sizes. They are suitable for most of the VOCs depending on their different

sorbent strength. The strongest CarbopackTM X should have a weaker adsorbent in front of it when

sampling very high boiling point analytes. All graphitised carbon blacks are hydrophobic like porous

polymers and so are adequate for humid ambient air samples (Brown and Shirey, 2001).

CarboxenTM and CarbosieveTM adsorbents are very strong and not appropriate for analytes with

boiling points higher than benzene because they have very small pores. They should always be used

with a weaker adsorbent (porous polymer or graphitised carbon black) placed in front. Pore shape of

the CarbosievesTM is different from the CarboxensTM. Pores of CarbosievesTM may be blocked by

analytes with high boiling points. Both CarboxensTM and CarbosievesTM are not hydrophobic and so

do need water removal for sampling humid ambient air samples.

Charcoals are not suitable for thermal desorption because they are too strong to release most of the

analytes with only heat. However, they are sometimes used in multi-adsorbent traps for very volatile

analytes e.g. Halocarbon 12 and Chloromethane. Charcoals are hydrophilic (Brown and Shirey, 2001).

Multi-adsorbent traps with up to four different sorbents allow a wide range of volatile compounds to

be enriched simultaneously. Sorbents are arranged in order of increasing sorbent strength from the

sampling end. Thermal desorption is in reverse direction to sampling flow so that low-volatile

compounds do not come in contact with the stronger adsorbent for highly volatile analytes. Care

should be taken when choosing sorbents for multi-adsorbent traps or tubes. The temperature

required for conditioning the most thermally-stable sorbent must not exceed the maximum

temperature of any other. Migration of loosely bound analytes from weak to strong adsorbent (e.g.

from Tenax® TA to a carbon molecular sieve) has to be inhibited by extending the bed length of the

weaker sorbent or inserting a medium strength sorbent between (Woolfenden, 2010b). Multi-

adsorbent traps applied for oxygenated VOCs are for example CarbopackTM B : CarboxenTM 1000,

90 mg in total (Hopkins et al., 2003), 75 mg CarbopackTM B : 5 mg CarbopackTM X (Roukos et al.,

2009) or 70 mg Tenax® TA : 110 mg CarbotrapTM : 250 mg CarbosieveTM SIII (Folkers, 2002).

There are different adsorbent bed sizes and densities depending on application and analytes. To

allow high sampling flow rates coarse sorbent grain sizes (20/40 mesh) have to be used (Helmig and

Greenberg, 1994).

Important characteristics of the most common sorbents and their adequacy for OVOCs analysis are

summarized in table 1.


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Table 17

Sorbent Class Strength

Max. Temp. [°C]

Relative analyte size to n-alkanes

Adequacy for OVOCS (e.g. methanol, ethanol, ketones, aldehydes)

Characteristics

Quartz wool/silica beads

Fused silica Very weak

>450 C30-C40

No, too weak – but suitable for cryogen enrichment

Very inert, non-water retentive, hydrophobic, minimal inherent artefacts, friable, 40/60 mesh recommended to minimise back pressure

CarbographTM 2TD CarbopackTM C CarbotrapTM C

Graphitised carbon black

Weak >450 C8-C20 No, too weak

Very inert, hydrophobic, minimal inherent artefacts, friable, 40/60 mesh recommended to minimise back pressure, O3 artefacts

Tenax® TA Porous polymer

Weak 350 C6-C30

No, too weak (Leibrock, 1996; Woolfenden, 2010b)

Too weak for acetone and n-pentane, high benzene blank value, inert, hydrophobic, low inherent artefacts (e.g. aldehydes - Helmig and Greenberg, 1994), NMHCS, aldehyde and ketone artefacts in combination with O3 (Lee et al., 2006), prone to chemical degradation and aging effects (Helmig and Greenberg, 1994)

CarbographTM 1TD CarbographTM B CarbopackTM B CarbotrapTM


Weak/medium

>450 C5/6-C14

Yes (Hopkinset al., 2003; Roukos et al., 2009)

Hydrophobic, minimal inherent artefacts, friable, formation of fines, 40/60 mesh recommended to minimise back pressure, aldehyde and ketone artefacts in combination with O3 (Lee et al., 2006)

Chromosorb® 102 Porous polymer

Medium

225 C5-C12 Yes Inert, hydrophobic, high inherent artefact levels

PoraPakTM Q Porous polymer

Medium


Chromosorb® 106 Porous polymer

Medium


PoraPakTM N Porous polymer

Medium


HayeSepTM D Porous polymer

Medium

290 Yes (Legreid, 2006)

Inert, hydrophobic, high inherent artefact levels

CarbographTM 5TD


Medium/strong

>450 C3/4-C8 Hydrophobic, minimal inherent artefacts, friable, formation of fines, 40/60


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mesh recommended to minimise back pressure, retention of very volatile compounds e.g. 1,3-butadiene

CarbopackTM X Graphitised carbon black

Medium/strong

>450 C3-C9 Yes (Roukos et al., 2009)

Hydrophobic, minimal inherent artefacts, friable, formation of fines, 40/60 mesh recommended to minimise back pressure, retention of very volatile compounds e.g. 1,3-butadiene, no O3 artefacts (Lee et al., 2006)

CarboxenTM 569 Carbonised molecular sieve

Strong >450 C2-C5 Yes

Inert, less hydrophilic than most carbonised molecular sieves, minimal inherent artefacts

UnicarbTM Carbonised molecular sieve

Strong >450 C3-C8 Yes

Inert, hydrophilic, performance weakened in humid conditions, individual inherent artefacts, must be conditioned slowly, requires extensive purge to remove permanent gases

CarboxenTM 1003 Carbonised molecular sieve

Very strong

>450 C2-C5 Yes

Inert, hydrophilic, performance weakened in humid conditions, individual inherent artefacts, must be conditioned slowly, requires extensive purge to remove permanent gases

CarbosieveTM SIII Carbonised molecular sieve

Very strong

>450 C2-C5 Yes

Inert, minimal inherent artefacts, significantly water and CO2 retentive, performance weakened in humid conditions, cold trap not lower than 0°C, easily and irreversibly contaminated by higher boiling components – protect with front bed of weaker sorbent

Molecular sieve 5Å

Molecular sieve

Very strong

>400 C2-C5 No (not inert)

High inherent artefacts, significantly hydrophilic, not suitable in humid conditions, easily and irreversibly contaminated by higher boiling components

Molecular sieve 13x

Molecular sieve

Very strong

>400 C2-C5 No (not intert)

High inherent artefacts, significantly hydrophilic, not suitable in humid


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conditions, easily and irreversibly contaminated by higher boiling components

Charcoal Activated carbon

Very strong

>400 C2-C4 No (Woolfenden, 2010a)

Limited to solvent extraction (too strong and reactive for thermal desorption – metal content), hydrophilic, poor sensitivity – only for ppm level concentrations, analytical interference when using MS detection

Trademarks: Tenax® TA - Buchem bv, Netherlands; Chromosorb® - Celite Corporation, USA; PoraPakTM

– Waters Corporation, USA; CarbographTM – LARA s.r.l., Italy; UniCarbTM – Markes International Ltd.,

UK, USA; HayeSepTM – Hayes Separations Inc., USA; CarbotrapTM, CarbopackTM, CarboxenTM and

CarbosieveTM – Sigma-Aldrich, USA


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APPENDIX 3: CHROMATOGRAPHIC SEPARATION (COMPILED FROM LITERATURE BY J. ENGLERT)

There are two types of capillary columns that are most widely used for the analysis of volatile organic

compounds (VOCs): PLOT (Porous Layer Open Tubular) and WCOT (Wall Coated Open Tubular)

columns.

PLOT columns feature a solid stationary phase consisting of a thin layer of small and porous particles

(adsorbent) adhered to the surface of the tubing. Chromatographic results are achieved by

adsorption of the analytes on the surface of the stationary phase by either surface charge

interactions or shape selectivity and size exclusion interactions. PLOT columns in contrast to weaker

retaining dimethylpolysiloxane columns are able to separate VOCs at ambient and above ambient

oven temperatures which reduces liquid nitrogen consumption that is necessary in case of WCOT

columns. Special highly polar OVOCs PLOT columns do not essentially retain most NMHCs as they

have little or limited interactions with the surface of the stationary phase. By this way OVOCs are

isolated and generally no co-elutions with NMHCs will appear. Thus, in principle a non-specific

detector (flame ionisation detector FID) can be used as single detector.

The disadvantage of PLOT columns is the need for water removal from the sample gas, otherwise

sharp water peaks will co-elute with OVOCs, e.g. with propanal and acrolein on GS-OxyPLOT

(Agilent). Furthermore, most PLOT columns are sensitive to water with respect to shifts in retention

times depending on the moisture content of the ambient air sample. Another issue of PLOT columns

may be occasionally occurring mobilisation of particles from the stationary phase (problem especially

for MS), but this effect has decreased due to better bonding of the porous polymer layer.

WCOT columns have a liquid stationary phase. They separate the solutes with different polarities and

solubility depending on the physical properties of the stationary phase, e.g. in non-polar films the

analytes dissolve according to the boiling points. The polar/non-polar interactions are much weaker

than the adsorptive interactions in PLOT columns. There a two types of films: non polar

dimethylpolysiloxane or polar polyethylene glycol. Dimethylpolysiloxane columns are versatile, very

stable and can be operated at very low temperatures. But there are co-elution problems of OVOCS

with NMHCs and so there is the need for a specific detector (MS). Another disadvantage is the low

retention of alcohols on dimethylpolysiloxane columns.

On the contrary on polyethylene glycol columns alcohols have high retention. Concurrently NMHCs

have lower retention so that there are less co-elutions with OVOCs. But a drawback is the fact that

aldehydes have also low retention. Furthermore, polyethylene glycol columns have shorter lifetimes,

are susceptible to damage upon overheating or exposure to oxygen and they cannot be operated at

sub-ambient oven temperatures.


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1. PLOT columns

Table 1: PLOT columns

PLOT column equivalents GS-OxyPLOT (Agilent),

CP-LowOx (Varian)

CP-PoraBOND U

(Agilent resp. Varian)

AlO3 PLOT

(Agilent resp. Varian)

Polarity High polar Midpolar High polar

Composition Proprietary, salt

deactivated

Styrene-glycol

methacrylate

copolymer

Proprietary, salt

deactivated

Operable temperature

range 0°C to 350°C -100°C to 300°C -100°C to 200°C

Analysis of alcohols + + -

Analysis of aldehydes + + -

Analysis of ketones + + -

Analysis of ethers + + -

Analysis of esters + + -

Analysis of aromatics + + +

Analysis of alkanes - + +

Analysis of terpenes +/- +

Analysis of nitriles + +

Expected co-elution problems

Ethyl acetate+MVK+MEK

(2-butanone), water

peak+propanal and

acrolein

Methanol+n-butane,

butanal+benzene+

ethylacetate+MVK, 2-

butanol+MEK,

butylacetate+

ethylbenzene+m+p-

xylene+n-hexanal,

pentanal+toluene

n-butane and ethyne

isohexanes

isoheptanes

m/p-xylene

Advantage

Strong selectivity to

OVOCs, high retention of

OVOCs even at above

ambient oven

temperatures, no

retention of saturated

aliphatic NMHCS and so

no co-elutions with

Water resistance,

retention times not

influenced by water,

long lifetime

Strong selectivity on

light hydrocarbons


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OVOCs, long lifetime

Disadvantage

Need for humidity

management, retention

of water, tailing of

unsaturated OVOCs due

to reactions with the

polar column,

unsaturated NMHCs and

aromatics both with

carbon atom numbers

higher than eleven stick

in the column

Co-elutions of OVOCs

with aliphatic NMHCs,

retention of water

Not useful for OVOCs


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Examples of ambient air chromatograms

1A) Al2O3 (KCl) (from Rigi, Switzerland, Empa)


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Fig.1: Al2O3 (KCl): a typical chromatogram at Rigi (Switzerland).


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1B) LowOx

Fig.2: CP-LowOx (Varian), 10 m x 0.53 mm x 10.0 µm (Hopkins et al., 2003).


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Fig.3: CP-LowOx (Varian), 30 m x 0.53 mm x 10.0 µm (Roukos et al., 2009).


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Fig. 4: CP-LowOx (Varian), 30 m x 0.53 mm x 10.0 µm (measurements École des Mines de Douai,

Environmental & Chemistry Department, site: Paris suburban, 2010).

1C) PoraBOND U

Fig. 5: CP-PoraBOND U (Varian), 25 m x 0.32 mm x 7.0 µm (measurements Empa 2012; system

description: Ledreid, 2006):

4.0 min Methylether, 5.0 min Methanol, 5.1 min n-Butane, 5.5 min. 1,3-Butadiene, 5.9 min

Acetaldehyde, 7.9 min Ethanol, 9.2 min Isoprene, 9.9 min Acrolein, 10.0 min Propanal, 10.6 min

Methylacetate, 10.8 min Isopropanol, 11.1 min Acetone, 13.0 min MTBE, 13.3 Methacrolein, 12.6 n-

Propanol, 14.8 Ethylacetate, 14.9 Butanal + Benzene, 15.1 MVK, 15.5 2-Butanol, 15.6 MEK, 17.1 2-

Methyl-3-butene-2-ol, 17.8 n-Butanol, 19.8 Pentanal + Toluene, 24.1 Butylacetate + Ethylbenzene +

m+p-Xylene + n-Hexanal, 24.8 o-Xylene, 29.0 Benzaldehyde.


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2. Dimethylpolysiloxane columns

Table 2: Dimethylpolysiloxane columns

WCOT column equivalents

DB-1 (Agilent), CP-Sil 5 CB (Varian), Rtx-1 (Restek), BP-1 (SGE), SPB-1 (Supelco)

HP-5ms resp. DB-5 (Agilent), CP-Sil 8 CB (Varian), Rtx-5ms (Restek), BPX-5 (SGE), SPB-5 (Supelco)

DB-624 (Agilent resp. Varian), Rtx-624 (Restek)

Polarity Non-polar Non-polar Midpolar

Composition 100% Dimethylpolysiloxane

5%-Phenyl-95%-methylpolysiloxane

6% Cyanopropylphenyl-94%-dimethylpolysiloxane

Operable temperature range -60°C to 350°C -60°C to 350°C -20°C to 260°C

Analysis of alcohols Tailing Tailing +

Analysis of aldehydes + + +

Analysis of ketones + + +

Analysis of ethers - - -

Analysis of esters + + +

Analysis of aromatics + + +

Analysis of alkanes + + +

Analysis of terpenes + + +

Analysis of nitriles - + +

Expected co-elution problems

Propanal+acetone, ethanol+acetone, n-pentane+acetone, n-butane+ acetaldehyde, OVOCs+ NMHCs

n-butane+acet-aldehyde+ methanol, isobutene+ methanol, ethanol+isopentane, acetone+propanal+ isopropanol, butanal+MEK, OVOCs+NMHCs

Propanal+acetone, OVOCs+NMHCs

Advantage High thermal stability More selective than DB-1, high thermal stability

Good retention of alcohols, good selectivity, good thermal stability

Disadvantage

Low selectivity, tailing of alcohols and ketones, co-elutions of OVOCs with NMHCs

Tailing of alcohols and ketones, co-elutions of OVOCswith NMHCs

Co-elutions of OVOCs with NMHCs


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2A) DB-1

Fig. 6: DB-1 (Agilent J&W), 100 m x 0.25 mm x 0.5 µm (Riemer et al., 1998).


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2B) Rtx-1

Fig. 7: Rtx-1:

2C) BPX-5

Fig. 8: BPX-5 (SGE), 50 m x 0.22 mm x 1.0 µm (measurements at Hohenpeissenberg Meteorological

Observatory, 2011):

19.49 min isobutene + methanol, 19.55 min acetaldehyde, 19.60 min n-butane, 21.29 min ethanol,

21.49 min isopentane, 22.01 min CCl3F, 22.55 min n-pentane, 22.67 min acrolein, 22.82 min acetone.

toluène

tétradécane

octanal

α-pinène


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2D) DB-624

Fig. 9: DB-624 (Agilent J&W), 10 m x 0.18 mm x 1.4 µm (Apel et al., 2003).


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3. Polyethylene glycol column

Table 3: Polyethylene glycol column

WCOT column equivalents DB-WAX (Agilent), CP-WAX 52 CB (Varian), Rtx-WAX (Restek), BP-20 (SGE), SUPELCOWAX 10 (Supelco)

Polarity High polar

Composition Polyethylene glycol

Operable temperature range 20°C to 260°C

Analysis of alcohols +

Analysis of aldehydes +/-

Analysis of ketones +

Analysis of ethers +

Analysis of esters +

Analysis of aromatics +

Analysis of alkanes +/-

Analysis of terpenes +

Analysis of nitriles -

Expected co-elution problems Butanal+acetone, methanol+MEK+3-methylfuran, ethanol+benzene+MVK, methylbutenol+toluol, 2-pentanone+pentanal

Advantage High retention of alcohols, low retention of alkanes (less co-elution problems)

Disadvantage Low retention of aldehydes, short lifetime of the column, cannot be operated at sub-ambient temperatures


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3A) CP-WAX 52 CB

Fig. 10: CP-WAX 52 CB (Varian), 60 m x 0.25 mm x 0.5 µm (Folkers, 2002).

3B) Rtx-WAX

Fig. 11: Rtx-WAX (Restek), 60 m x 0.53 mm x 0.5 µm (Goldstein and Schade, 2000).


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3C) DB-WAX

Fig. 12: DB-WAX (Agilent J&W), 60 m x 0.32 mm x 0.5 µm (Lamanna and Goldstein, 1999).


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References:

Apel, E.C. et al. (2008): Intercomparison of oxygenated volatile organic compound measurements at

the SAPHIR atmosphere simulation chamber. Journal of Geophysical Research 113 (D20307).

Apel, E.C. et al. (2003): A fast-GC/MS system to measure C2 to C4 carbonyls and methanol aboard

aircraft. Journal of Geophysical Research 108 (D20): 15-1 – 15-12.

Apel, E.C. et al. (1998): Measurements comparison of oxygenated volatile organic compounds at a

rural site during the 1995 SOS Nashville Intensive. Journal of Geophysical Research 103 (D17): 22,295

– 22,316.

Folkers, A. (2002): Oxygenated volatile organic compounds in the troposphere: Development and

employment of a gas chromatographic detection method. Report of the Research Centre Jülich 3998.

Dissertation University of Köln, Jülich.

Goldan, P.D., Kuster, W.C. (2004): Nonmethane hydrocarbon and oxy hydrocarbon measurements

during the 2002 New England Air Quality Study. Journal of Geophysical Research 109 (D21309).

Goldstein, A.H., Schade, G.W. (2000): Quantifying biogenic and anthropogenic contributions to

acetone mixing ratios in a rural environment. Atmospheric Environment 34: 4997-5006.

Helmig, D. (1999): Air analysis by gas chromatography. Journal of Chromatography A 843: 129-146.

Hopkins, J.R., Lewis, A.C., Read, K.A. (2003): A two-column method for long-term monitoring of non-

methane hydrocarbons (NMHCs) and oxygenated volatile organic compounds (o-VOCs). Journal of

Environmental Monitoring 5: 8-13.

Komenda, M., Schaub, A., Koppmann, R., 2003. Description and characterization of an on-line system for long-term measurements of isoprene, methyl vinyl ketone, and methacrolein in ambient air. Journal of Chromatography A 995, 185–201.

Lamanna, M.S., Goldstein, A.H. (1999): In situ measurements of C2-C10 volatile organic compounds

above a Sierra Nevada ponderosa pine plantation. Journal of Geophysical Research 104 (D17):

21,247-21,262.

Legreid, G. (2006): Oxygenated volatile organic compounds (OVOCs) in Switzerland: From the

boundary layer to the unpolluted troposphere. Dissertation ETH No. 16982, Zürich, Dübendorf.

Leibrock, E., Slemr, J. (1997): Method for measurement of volatile oxygenated hydrocarbons in

ambient air. Atmospheric Environment 31 (20): 3329-3339.

Leibrock, E. (1996): Development of a gas chromatography system for trace analysis of oxygenated

volatile organic compounds in air. Scientific journal 40, Fraunhofer-Institute for Atmospheric

Environmental Research, Garmisch-Partenkirchen. Dissertation Hamburg University of Technology,

Hamburg-Harburg.


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Lewis, A.C. et al. (1995): Programmed temperature vaporization injection (PTV) for in situ field

measurements of isoprene, and selected oxidation products in a eucalyptus forest. Atmospheric

Environment 29 (15): 1871-1875.

Riemer, D. et al. (1998): Observations of nonmethane hydrocarbons and oxygenated volatile organic

compounds at a rural site in the southeastern United States. Journal of Geophysical Research 103

(21): 28,111 – 28,128.

Roukos, J. et al. (2009): Development and validation of an automated monitoring system for

oxygenated volatile organic compounds and nitrile compounds in ambient air. Journal of

Chromatography A 1216: 8642-8651.

Vickers, A. (2007a): A "solid" alternative for analyzing oxygenated hydrocarbons – Agilent´s new

capillary GC PLOT column. Agilent Technologies publication 20 (2).

Access: http://www.chem.agilent.com.

Vickers, A. (2007b): GS-OxyPLOT: A PLOT column for the GC analysis of oxygenated hydrocarbons.

Agilent technical overview. Access: http://www.chem.agilent.com.Zou, Y., Cai, M. (2008):

Investigation of the unique selectivity and stability of Agilent GS-OxyPLOT columns. Agilent

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