WP4- NA4: Trace gases networking: Volatile organic carbon and … standards... · 2013-07-26 ·...

Measurement Guidelines VOC Version: Draft: 2012/07/18

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

Deliverable D4.1: Draft for standardized operating procedures (SOPs) for VOC

measurements Summary:

This SOP provides a guideline for good measurement practice for the analysis of 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....................................................................................................................................................2 2. Sampling .......................................................................................................................................................................3

2.1. off-line sampling....................................................................................................................................................3 2.1.1. VOCs...............................................................................................................................................................3 2.1.1.1 adsorption tubes..........................................................................................................................................3 2.1.1.2 stainless steel canisters................................................................................................................................3 2.1.2 OVOCs (DNPH) ................................................................................................................................................6

2.2. on-line sampling inlet line (VOCs and OVOCs)......................................................................................................6 3. Preconcentration..........................................................................................................................................................6

3.1. removal of water/O3/CO2......................................................................................................................................6 3.1.1 Water removal ................................................................................................................................................6 3.1.2 Ozone removal................................................................................................................................................7 3.1.3 CO2 removal ....................................................................................................................................................7

3.2. Trapping ................................................................................................................................................................7 3.3. capillary columns for GC analysis of VOCs and OVOC ..........................................................................................8

4. Analysis.........................................................................................................................................................................9 4.1. GC-FID....................................................................................................................................................................9 4.2 GC-MS...................................................................................................................................................................10 4.3 PTR-MS (provided by R. Holzinger Uni Utrecht and T. Petäjä) ............................................................................10

5. Quality Assurance and Quality Control......................................................................................................................11 5.1 Standards and Scale .............................................................................................................................................12 5.2 Zero Gas................................................................................................................................................................13 5.3 Data Quality Objectives........................................................................................................................................13 5.4 Method for Measurement of Standards .............................................................................................................14 5.5. Method for Measurement of zero gas (blanks)..................................................................................................16 5.6. Method for Detecting Effects of Ozone on Reactive Compounds .....................................................................17 5.7 Logs at each station..............................................................................................................................................18

6. Post-analysis ...............................................................................................................................................................19 6.1 Data checks of final mixing ratio data..................................................................................................................19 6.2 Uncertainty evaluation.........................................................................................................................................24

6.2.1. Calculation of mixing ratios for linear detection systems ...........................................................................24 6.2.2 Determination of Precision...........................................................................................................................25 6.2.3 Determination of Uncertainty ......................................................................................................................25 6.2.4 Determination of detection limit..................................................................................................................29

6.3 Data submission ...................................................................................................................................................33 7.References...................................................................................................................................................................33 8. Appendices .................................................................................................................................................................36


1. General introduction

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

extensively presented for example in the GAW Reports 111 and 171, Helmig et al., EOS, 2009). In

brief summary, VOCs are, besides CO, the major group of reduced gaseous carbon-containing

compounds emitted into the atmosphere by natural and anthropogenic sources. Typically, more than

100 compounds can be measured with mixing ratios in the low ppt range up to a few ppb. They have

multiple roles in atmospheric chemistry, and of main interest is their contribution in photochemical

processes like generation of photo-oxidants, e.g. ozone, their impact on the oxidizing capacity of the

atmosphere and their contribution to secondary organic aerosol (SOA) production. 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 thus among the long-

term monitoring parameters in GAW (GAW Report 172) and regional programs like EMEP.

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

measurement guideline: the non-methane hydrocarbons (NMHCs), the oxygenated organic

compounds (OVOCs) and biogenic VOCs (mainly isoprene and the group of monoterpenes, often

summarized as BVOCs). Key substances of these groups have been identified in GAW Report 171 and

detailed guidelines for their measurements are provided in this paper, following the general QA

recommendations and the strategic plan by GAW (GAW Report 172). As analytical systems for the

measurement of the different groups of VOCs (see above) are generally capable of analyzing not only

the key species identified in GAW Report 171 but also a list of chemically similar compounds, this

guideline covers a broader range of compounds than specified in GAW Report 171. This is in line with

the EMEP objectives and addressed in the ACTRIS Description of Work.

Measurement of VOCs is mostly done by gas chromatographic methods, but there also exist well

established methods for carbonyl OVOCs by DNPH cartridge sampling and HPLC and the relatively

new on-line method PTR-MS. As gas chromatography is most widely used, this method will be in the

focus of this measurement guideline but the other techniques are also covered.

The measurement of VOCs by gas chromatography 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

removal/ozone removal system and then concentrated using 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 gaschromatographic column and finally analysed by FID or MS

(or any other suitable detector).

For quantification a standard should be used which either contains VOCs in ambient air, or contains

artificially mixed VOCs in N2. Concentrations should be in the region of the expected ambient

concentrations. If needed the standard has to be diluted with zero air or N2 into a concentration

which is in the range of concentrations measured at the specific station.


2. Sampling

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

using either adsorption tubes or stainless steel canisters. Samples are subsequently transported to

the lab where they are analyised. The specific requirements of the different methods are described

below.

2.1. off-line sampling

2.1.1. VOCs

2.1.1.1 adsorption tubes

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

commercially available products. For this draft it is advised to follow the specific prodecures of the

individual products.

2.1.1.2 stainless steel canisters

For analyzing NMHCs in ambient air using canisters the procedures described in a new Standard

Operation Procedure (SOP) valid in the future for the WMO GAW network may be applied.

Publication by WMO is expected to take place in the second half of 2012. For immediate further

information on the SOP contact the WCC-VOC (imk-ifu.kit.edu/wcc-voc). In the following only the

sampling procedure is described as listed in this SOP. It has to be noted, that a sound analyses of

NMHCs in air samples requires carefully preconditioning steps for the canisters. These procedures

are described in detail in the above mentioned SOP. 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 recommendations from US-EPA (1998, 1999) on

determination of volatile organic compounds (VOCs) in ambient air. Further, beside this SOP, many

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

National Oceanic and Atmospheric Administration Cooperative Global Air Sampling Network

(Pollmann et al., 2008)).

General considerations

Generally, the use of materials other than stainless steel, glass, silica coated stainless steel, PFA and

PTFE 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 effects. The recommendations given below

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 i.e. more flexibility in air sampling and is recommended. The inner

surface of canisters is passivated, e.g. electro-polished. Stainless steel valves (e.g. Swagelok) shall be

used to seal the canisters inlets and outlets, respectively. The sampling procedures described here

are valid for two valve canisters.

Air samples collected for measurements of NMHCs in small canisters shall be pressurized. Pumps

used for filling the canisters should reach a final pressure of > 3 bar ensuring a minimum gas flow

rate at ambient pressure of > 10 l/min, e.g. MB-158 (metal bellows) or N 022 STE (KNF; PTFE). All

parts of the pump with air contact must be of stainless steel or PTFE. The pressure should be checked

by using an oil free pressure gauge.


A filter in the sampling system is recommended to protect the pressurizing pump and the canisters

from contamination by micro-organisms and particles. A PTFE filter (pore size 0.45 µm, diameter 25

to 50 mm, stainless steel filter holder; e.g. Pall or Millipore) shall be placed at the inlet of the

pressurizing pump. Prior to use, a new filter shall be purged with pure nitrogen (15 min with 1 L/min)

or alternatively with sample air after assembling it in the sampling line because it may release traces

of low volatility hydrocarbons (C7 to C12 and oxygenated VOC). The filter membrane shall be

exchanged depending on exposure (guideline: every 25 samples for 1 L canisters).

Canisters must be checked for leaks and blank values before air sampling. The entire sampling system

and the main inlet tubing shall be clean and tight. The material and the sampling pump used must

comply with specifications above.

An air monitoring station on different platforms (e.g. aircraft, shipboard, tower) typically use a main

inlet tubing line and a downstream pump with high flow rate (several cubic meters per hour) to pull

ambient air to a number of sampling ports. The gas samples are then usually drawn from a manifold

near to the sampling device with a smaller flow rate (0.1 l/min to 0.5 l/min).

When sampling air with canisters an ozone scrubber during sampling sometimes is used (e.g. Plass-

Duelmer et al., 2006). Hence only reactive alkene components such as 1,3-butadiene tend to oxidize

during sampling and storage. For less reactive compounds e.g. alkanes no loss in stainless steel

canisters has been observed. Surprisingly, this has also been reported for isoprene when ambient air

with approximately 100 μg/m³ ozone was sampled into 1 L electro-polished stainless steel canisters

(Leibrock, 1996; Palluau et al., 2007). This result may be explained by a rapid destruction of ozone on

the stainless steel surface of sampling line and the canister. For the GAW NMHC target compounds

the effect of ozone on sample integrity is assumed to be negligible and an ozone scrubber during air

sampling is not required.

Canister preparation prior to sampling

The set up of continues flow air sampling system for two valve canisters is schematically shown in

Figure 1. Cleaned canisters shall be prepared at the sampling location prior to air sampling. For this

purpose all canisters are first flushed with sample ambient air prior to sampling (see Section below).

Sampling procedure

Procedure for continues flow air sampling with 2 valve canisters (Figure 3):

1) Connect sampling system to a sampling port.

2) Open the canister valve (2).

3) Open canister valve (1), switch pump on and flush canister with sample air for a period

sufficient to exchange the volume of the canister ten times at the given sampling flow rate.

4) Close valve (2) and pressurize the canister with sample air up to 2.0 bar (29 psi).

Watch pressure gauge (3).

5) Open valve (2) to release the pressurized sample air.

6) Repeat Step 3 and 4 three additional times.

7) Finally, pressurize the canister (valve (2) closed) up to 2.0 bar (29 psi) and close valve (1) to

retain the air sample.

8) Pack the canister in an appropriate shipping case (see below) and ship the canister

with a copy of the sample data sheet (see Section below) to the analysing laboratory.


Note 1: Eight pressurization/release cycles are required when dry air at temperatures below 0 ºC is

sampled to stabilize higher boiling compounds.

Note 2: The technically specified maximum pump pressure shall be at least 20% above the sample

pressure of 2 bar (29 psi) for avoiding an overheating of the air during sampling. Sampling/pressuri-

zing times shall be checked every 10 samples for consistency. Increasing sampling times indicate the

need for pump service.

Note 3: When sampling under high humidity air conditions water may condense during the

pressurization step which is not a problem when analysing NMHC.

Fig. 1: Flushing and repeated pressurization/release for 2-valve-canisters (valves closed/open

according to the process step).

Sampling protocol

The protocol shall contain at least the following information:

(1) Complete identification of the canister (each canister must have an engraved number).

(2) The sampling location and sampling time period.

(3) Final sampling pressure.

(4) Any unusual features noted during the sampling (e.g.: canister was not pre-pressurised).

Canister leak test

Canisters must be leak tested. First, canisters are pressurized to approximately 3 bar (44 psi) with

clean nitrogen. The canister valves are kept closed and the initial pressure is measured. After 24 h

the final pressure is checked. It is recommended that the leak rate for 1 L canisters should not exceed

0.01 mbar/h. Depressurize and condition the canister as described in Sections 5.2 or 5.5. It is

recommended to perform a leak test after every 10th sample cycle.

Transport

To prevent contamination of the inlet and outlet tubing sealing with appropriate caps is obligatory. It

is also recommended to use a special transport case to avoid damage to cylinders and shut-off

valves.


Equipment and canister storage

The sampling equipment shall be stored after sampling with capped in- and outlets in normal

conditions (extreme temperatures and humidity should be avoided) away from organic solvents. The

canisters should be analyzed as soon as possible and storage time shall not exceed 30 days (US-EPA,

1999). Storage time shall be documented on the sampling protocol for each canister.

Check of storage stability of NMHCs

With increasing number of carbon atoms, the NMHCs tend to be lost on the canister surface during

transport and storage. To check for the stability of the NMHCs, the canisters are filled with an

ambient air sample. The aliquots of the sample are then analyzed immediately after sampling and

repeatedly during anticipated storage time from sampling to measurement.

Note: Ample evidence suggests that, in addition to the canister type, the NMHC stability in canisters

is strongly dependent on the humidity of the sample. A minimum relative humidity of about 12% at

room temperature is required to achieve stable NMHC concentrations (McClenny et al., 1991).

2.1.2 OVOCs (DNPH)

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

established method within EMEP, therefore the chapter 3.8 (Determination of aldehydes and ketones in

ambient air) of the EMEP manual for sampling and chemical analysis (Revision nov 2001) should be used.

2.2. on-line sampling inlet line (VOCs and OVOCs)

For VOCs the inlet line should be either silco-treated steel or stainless steel. In the case of steinless

steel, the line has to be heated up to 70 °C to prevent condensation of VOCs on internal surfaces

(HOPKINS, 2011). Transfer lines for the analysis of OVOC should be either silco-steel or PFA

(Perfluoralkoxy), but not stainless steel. Silco-treated steel should be humidified before first usage

(e.g. by passing ambient air).

The 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.

3. Preconcentration

3.1. removal of water/O 3/CO2

3.1.1 Water removal

Different systems for reducing the water content of the sample can be used. The dew point should

be measured and it should be at least 10°C lower than the trapping temperature in case of

adsorptive sampling, as otherwise adsorbed water on the trap will lead to a smaller breakthrough

volume or even a blockage of the trap. For cryogenic trapping the dew point should be below -30°C.

Systems in use are (1) a Nafion ® Dryer with a volumetric counterflow of dry air or N2, which is

around 3 times higher than the flow of humid ambient air and (2) a water trap at subambient

temperature, where water is adsorbed but not the analytes.

Examples: Rigi, Jungfraujoch (Nafion), water trap at -40°C in 1/8“ silcosteel line (Hohenpeissenberg)


3.1.2 Ozone removal

For the measurement of compounds reactive to ozone, such as alkenes including isoprene and the

terpenes, the O3 in the sampled ambient air should be destroyed before analysis. Several methods

are used.

- Heated stainlesssteel line or a metal grid in the ambient air flow (>70°C)

- Addition of a flow of NO (O3 + NO�NO2) into the ambient air flow

- Small cartiridges filled with or filters impregnated with special salts like sodium thiosulfate or

potassium-iodide

A compilation of available method and their evaluation can be found in Appendix 1 (Ozone removal

techniques for GC analysis of OVOC (oxygenated volatile organic compounds) in ambient air samples

(by J. Englert)).

3.1.3 CO2 removal

If the trapping temperature is lower than -78°C it is advisable to remove CO2 either from the ambient

air flow (by using an adsorbent in-line (e.g. Ascarite) or between trapping and analysis (e.g. by slowly

heating the trap to a temperature high enough for the CO2 to be released but not for the analytes). If

CO2 is trapped and released into the analytical system this could lead to distortion of

chromatography and/or diminished sensitivity of the detector.

3.2. Trapping

As VOCs are only occurring in the atmosphere in the range of ppt (parts per trillion, 1 x 10-12) up to

some ppb (parts per billion, 1 x 10-9) VOCs from ambient air have to be preconcentrated before the

analysis, using gas chromatograph-flame ionization detection (GC-FID) or gas chromatograph-mass

spectrometry (GC-MS). On-line sampling 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. Either a combination of week adsorbents with low sub-ambient temperature can be

chosen or stronger adsorbent with ambient temperature, often also multi-bed adsorbents with

increasing adsorbent strength in sampling flow direction are used. For each system break-through

volume has to be tested, using either increasing amounts of humidified synthetic standards or of

ambient air spiked with standards.

A compilation of different trapping adsorbents and their usage is done in Appendix 2 (Adsorbents for

sorbent-based enrichment of VOCs and OVOCs (oxygenated volatile organic compounds) in ambient

air samples (by Jenny Englert))

For the trapping procedure a pump should be used after the trap connected to a critical orifice or a

mass flow controller (or any other suitable instrument) to regulate the flow over the trap. It is

essential to determine the sampling volume with low uncertainty either by regularly calibrated

massflowcontrollers 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.

Examples: Perkin Elmer TD system (Empa, OHP), ADS (Empa, selfmade, Rigi),


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 counterflow. 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

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 too large traps or to

slow heating rates, 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.

3.3. capillary columns for GC analysis of VOCs and OVOC

Several possible analytical columns are discussed in the Appendix 3 (chromatographic separation (by

J. Englert).

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

inertness. Despite their lower capacity they are suitable for most applications in environmental

research. There are basically two types of capillary columns that are currently used for the analysis of

OVOC: 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 VOC 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 retain saturated aliphatic NMHCs as they

have little or limited interactions with the surface of the stationary phase. By this way OVOC are

isolated and generally no co-elutions with NMHC 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 humidity management. Otherwise there would be

a sharp water peak that co-elutes with OVOCs, e.g. propanal and acrolein on GS-OxyPLOT (Agilent)

and for most PLOT columns a shift in retention times depending on the moisture content of the

ambient air sample. Furthermore, some PLOT columns may occasionally lose particles of the

stationary phase (problem especially for MS), but this effect was decreased by better bonding 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

rate of diffusion into the stationary phase and solubility 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 OVOC with NMHC 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 NMHC

have lower retention so that there are less co-elutions with OVOC. But a drawback is the fact that

aldehydes have also low retention. Furthermore polyethylene glycol columns have short lifetimes,

susceptible to damage upon overheating or exposure to oxygen and they cannot be operated at sub-

ambient oven temperatures.

4. Analysis

4.1. GC-FID

Currently most GC systems are operated with FID detection since FID’s have a number of advantages:

- FID’s are very sensitive, robust, simple in design and easy to use

- they perform very stable with typically less than 2% sensitivity drift over one month

- they have a response 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

- they are not sensitive to traces of water, N2 and O2, and noble gases from the sample gas

FIDs need air, make-up (typically N2) and H2 gas supplies and flow rates should be well controlled to

achieve stable operation of the detector. Detector air can be generated by catalytically cleaning (Pd or Pt

catalyst at 350-450°C) ambient air, or synthetic air can be used. Essential is to have low VOC levels or at

least low fluctuation in VOC levels. Hydrogen can either be supplied by gas cylinder or by H2-generator.

The most commonly used make up gas is nitrogen from cylinders, but also other gas may be 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. GC-FID can be operated with most carrier gases, e.g. He, H2

or N2, however for reasons of good separation, hydrogen or helium should be used. Operating very

active PLOT columns with hydrogen carrier gas may cause problems in analysis of alkene or alkyne

compounds due to catalytic reactions at active surface sites. The sensitivity of an FID is generally

sufficient to do analysis in background atmosphere at ppt levels, e.g. detection limits of GC-FID systems

for analyzing 1 Liter of air are typically better than 3 pptv (e.g. Plass-Dülmer et al., 2002).

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 ppb of ethane have the same integrated response as 1 ppb

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 artifacts by making use of the known carbon

response (=peak area/(Volume sample*mixing ratio*carbon number)). When the carbon responses for

the various organic compounds are calculated, they should agree within a few %. 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 (few percent of

high concentrated standard in ambient air sample) are of high value as they characterize interferences

with other constituents of ambient air like water vapor and ozone.

Disadvantage of GC-FID systems, however, is that the FID is not substance-specific. Thus it requires a

very good chromatographic separation in order to minimize problems due to co-eluting peaks.

Accordingly, GC FID is a suitable system whenever unambiguous identification can be achieved simply

based on the retention times, otherwise combination with MS is recommended.

4.2 GC-MS

The analysis of VOCs with GCMS is possible, although not ideal for isomers of smaller compounds

such as pentenes or isohexanes and isoheptanes with similar boiling point/retention times on

analytical columns. As the sensitivity of the mass spectrometer decreases relatively rapidly,

calibrations using a working standard have to be performed nearly as often as ambient air

measurements. At Jungfraujoch for example the following system is in use: std-air-air-std-air-air-std-

air-air-std-

This ensures that the decline in sensitivity is accurately tracked. Furthermore, target tank

measurements should be performed bi-weekly and blank measurements should be performed

monthly. After a considerable change of the system (e.g. source replacement, changing of analytical

column or trap) blank measurements have to be performed as soon as the system runs in a steady

mode again.

The source has to be tuned after the loss of sensitivity has reached a certain degree, but at least

every second month. 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.

4.3 PTR-MS (provided by R. Holzinger Uni Utrecht an d T. Petäjä)

Inlet (recommendation):

~4mm ID PFA tubing, max 15m, flow 2 L/min, protected by PFA filter holder with 47mm PTFE filter 5

μm pore size

PTR-MS samples from this inlet line with a short low volume (1/8“or 1/16“) line. Recommended

materials: PFA, PEEK

Background determination:

The 2L main sample flow is passed through a Pt catalyst at a temperature of 350 C.

Critical instrument settings (recommendation):

Drift tube pressure: 2.2 mbar

Drift tube voltage: 600V

Drift tube temperature: 50C

Voltage between last drift ring and exit lense: 30V (instrument dependent, maybe define a procedure

to optimize this)


Ratio O2+/H3O+ below 0.03

PTR-TOF-MS:

�SV valve setting (describe optimazation procedure)

� Peak shape standards

� MCP voltage

Q-PTR-MS

�Tuning of mass scale and resolution

� SEM voltage

Working standard (recommendation)

Methanol, m33; Acetonitrile, m42; Acetaldehyde, m45; Acetone, m59; MBO, m87, m69, m41; MVK,

m71; MEK, m73; Benzene, m79; Toluene, m93; Xylene, m107, TMB, m121; a-pinene, m137, m81,

Trifluorobenzene, m133; Trichlorobenzene, m181, m183, m185

Intercalibration standard (recommendation): same compounds

Prepared and distributed by EMPA twice per year. (I think it is not necessary to ship pressurized

cylinders ~1-2 liters of gas standard should be sufficient)

In field operation:

When and how often:

� background measurements

� working standard

� Full mass scan

Calculation of VMR

� recommended rate constants

� tranmission

� recommended procedure

5. Quality Assurance and Quality Control

Quality assurance (QA) follows the principles of the GAW QA system (see Figure 2 and

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

1. 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).

2. Full traceability to the primary standard of all measurements made by Global, Regional and

Contributing GAW stations.

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

4. 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).

5. Establishment of MGs or SOPs for these measurements.

6. Use of detailed log books for each parameter containing comprehensive meta information

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


7. 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).

8. 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.

Fig. 2: Principles of the GAW QA system.

5.1 Standards and Scale

The nomenclature of standards should be used as follows: The “primary standard”, “secondary

standard” etc. should only be used at the Central Calibration Laboratory (CCL), they comprise a system of

standards defining the “scale” for VOC in GAW.

Generally, “tertiary standards” are provided by the CCL to the stations and laboratories and they are

used there as “laboratory standards” or “working standards”. The highest level standard at a given site

is called the “laboratory standard” (the laboratory standard is not necessarily a tertiary standard from

the CCL, though it is recommended). The laboratory standard is supplemented by a system of working

standards at the stations that may consist of synthetic or whole air mixtures, certified or custom-made.

“Traveling standards” are generally working standards with reference mole fractions determined by the

CCL, WCC or a certified laboratory used in comparisons and round robins.

The term “target gas” is used for a working standard which is treated as a sample of unknown

compositions, e.g. the data evaluation procedures of a station are used to determine the VOC mixing

ratios and the deviation from the assigned values is a test for the quality of measurements. Accordingly,

a target gas should be close to ambient gas composition which means a whole air standard with similar

concentration levels.

The scale is kept by the CCL by means of a system of standards (see above). The scale is transferred to

the stations and labs by the tertiary standards. In case a station does not hold a tertiary standard by the


CCL, it has to demonstrate that the laboratory standard is linked to the 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 mixing ratios)

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 is a whole-air working standard calibrated versus laboratory standard, other

standards or by other means

Though not recommended, there are minimum requirements for a station that need to be fulfilled:

1. A laboratory standard to define the 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 are needed.

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

5.2 Zero Gas

“Zero gas” is a hydrocarbon free gas from either catalytically cleaned ambient air (Pt or Pd catalyst at

400°C) or alternatively, not as good but easier to handle, synthetic gas of at least 5.0 quality.

Catalytically cleaned air is preferred as this is identical to the sample gas matrix. Often, trace

amounts of hydrocarbons in the pmol/mol range are present as impurities in the zero gas. Stations

should go for optimum zero gases by comparing the blank values obtained in measurements of

different hydrocarbon free gases aiming at the lowest levels.

The routine measurement of zero gas is part of the QA program to be followed at all stations. It

yields information about artifacts due to release of adsorbed hydrocarbons or leaks in the sample

path. Blank values should be as low as possible. However, there is an inherent problem when using

zero gas with trace level impurities of hydrocarbons: these can not easily be separated from blank

artifacts as mentioned before. Accordingly, care has to taken to identify the origin of blanks found in

zero gas measurements.

Zero gas should be applied at the inlet of the sampling system (on-line syst.) or to flasks and canisters

via any used sampling line under field use conditions. Often a high flow inlet manifold is used which

cannot be easily flushed by zero gas, then zero gas can be introduced after the sampling split to the

instrument, but checks with independent sampling not over the routinely used inlet should be

performed.

5.3 Data Quality Objectives


The following data quality objectives have been approved by the GAW-VOC expert group:

The enhanced DQOs are the objective for good performing stations. Within ACTRIS, generally the

GAW DQO 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 standards comprising

multiple measurements.

5.4 Method for Measurement of Standards

Calibration using a working standard is required every 2 months using a GC-FID. If possible and for

securing highest possible data coverage it is suggested to perform the calibration every 2 weeks. As

mass spectrometers drift considerably, it is recommended to perform at least one working standard

measurements per day. For achieving optimal performance a more frequent application of the

working standard is recommended. Furthermore, a target tank has to be run once a week. This tank

can either be a certified standard mixture in air or N2 or a compressed ambient air within a cylinder,

with known concentrations.

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. Furthermore, it is strongly recommended to

use no liquid leak tester solutions as they might contaminate the system.

If a working standard/target gas cyclinder is newly connected, at least 24 hours before the

measurement the pressure regulator and the transfer line with capped fitting on the GC connection

side should be mounted. The transfer line should be made of Silcosteel 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

GAWUncert.

%

GAWRepeat.

%

Enhanced*Uncert.

%

Enhanced*Repeat.

%

alkanes 10 5 5 2

Alkenes incl. IsopreneMonoterpenes 20 15

510

25

Alkynes 15 5 5 2

Aromatics 15 10 5 2

OVOC 20 15 10 5

Acetonitrile and DMS 20 15 10 5

Mole fraction < 0.1ppb +/-20 ppt +/- 15 ppt +/- 10 ppt +/- 5 ppt

GAWUncert.

%

GAWRepeat.

%

Enhanced*Uncert.

%

Enhanced*Repeat.

%

alkanes 10 5 5 2

Alkenes incl. IsopreneMonoterpenes 20 15

510

25

Alkynes 15 5 5 2

Aromatics 15 10 5 2

OVOC 20 15 10 5

Acetonitrile and DMS 20 15 10 5

Mole fraction < 0.1ppb +/-20 ppt +/- 15 ppt +/- 10 ppt +/- 5 ppt


(follow the mounting instructions of the manufacturer). After installation, the regulator needs to be

flushed. For this, the transfer line is uncapped, the regulator’s low pressure port is gently opened and

the cylinder valve is opened very short such that the primary pressure jumps up and goes down again

to ambient. This procedure should be repeated at least 2 times. After flushing, plug the transfer line

and pressurize the pressure regulator another time by shortly opening the cylinder valve. Check the

pressure for a few minutes; if not constant, tighten the 30 mm nut and repeat until pressure is

constant. Then, apply a low pressure (0.2-0.5 bar) to the transfer line, as well. As an initial leak check,

close the regulator. After it is closed, watch the low pressure port for some 10 min. If not constant,

check plug and connection to the transfer line, tighten gently, and repeat the check. Finally, the

pressure regulator and the transfer line (plugged at the end) should be 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 loosing 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. 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.

In many set-ups the standard cylinder is permanently connected to the GC system. 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). To

stay within the DQO, 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,

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.

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 VOC 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.

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

to send the laboratory standard for recalibration to the CCL or WCC, ask other stations for a high

level standard for an independent check, or check available results from past intercomparisons.

Anyways, 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 Figure 3 an example of a series of working standards measured at Hohenpeissenberg (DWD,

Germany) is shown. Here, the determined mixing ratios for all analysed compounds are plotted over

time in a log scale. Relative changes should become detectable by non-constant values. The first plot

shows compounds with mixing ratios of more than 50 ppt, the next C3-C6 hydrocarbons with mixing

ratios below 50 ppt, and finally the plot of C6-C9 hydrocarbons with mixing ratios below 50 ppt.

Except for some severe i-butene problems (associated with blank values), it should be pointed out

that repeatability gets worse for compounds with higher molecular weight and towards lower mixing

ratios, but still repeatability is mostly within some 2 ppt or a few percent. This plot demonstrates

repeatability within each series of 5 replicates (monthly), and reproducibility throughout the year for

ambient mixing ratio levels and ambient air matrix.

10

100

1000

10000

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

Ethan

Ethen

Propan

Propen

i-Butan

Acetylen

n-Butan

n-Pentan

3-M -Pentan

i-Buten

i-Pentan

2-M -Pentan

M-C-pentan+2,2-DiM -B.

Toluol

Benzol

m-Xylol

Referenzluft-04, ppt

1

10

100

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

t-2-Buten

c-2-Buten

Cyclopentan

Propin

1,3-Butadien

1-Penten

Cyclohexan+?

2,3-DiM -Butan

1-Buten

Isopren

Referenzluft-04, ppt

1

10

100

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

n-Hexan

Isopren

1-Hexen

M -C-Hexan+2,2,3-TriM -B.2,3-DiM -Pentan

2+3-M -Hexan

n-Heptan

3-M -Heptan

n-Oktan

n-Nonan

Ethylbenzol

o-Xylo l

Fig. 3: Measurements of compressed whole air from cylinder (“Referenzluft-04” or “Ref-04”) through

2009, generally 5 replicates are measured once per month.

5.5. Method for Measurement of zero gas (blanks)


For blank measurements, a zero gas is sampled via the usual air sample path. Thus, it passes the

ozone and particle filter, the water trap and sampling unit just like ambient air samples. At

Hohenpeissenberg, Helium is applied at an open “T” into the 1/8” sampling line at a flow rate of

about 100 ml/min yielding a sample flow of about 80 ml/min towards the GC and 20 ml/min towards

the ambient air sample manifold. A similar amount of zero gas is analysed as in the ambient air

samples.

For convenience, we use purified He 5.0 (charcoal and mole sieve at -40°C) as zero gas as we use the

same gas as carrier gas and purge gas. Occasionally, we also test the same line using catalyst-purified

ambient air (Pd at 400°C) and essentially observe the same results as with He.

The Figure 4 gives an example of the behaviour of blanks over time. Peaks in 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. Additionally, it can not be excluded that small

amounts of ambient air diffuse into the He against the He flow (at the open “T”) and some of the

ethane, ethylene, propane and benzene is attributed to ambient air mixing into the He.

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

Fig. 4: Results of blank peak areas from He blanks transferred to mixing ratios by applying the same

calibration factors as for other samples.

5.6. Method for Detecting Effects of Ozone on React ive 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 4 times a year. In these, 1-5

ml/min of NPL standard are added via a fused silica capillary into the ambient air sample flow of 50

ml/min. Thus, roughly 2-10% of the whole gas sample is NPL standard, however, in the ambient air

matrix including ozone. It is not so important to know the flows exactly, as the mixing ratios of


hydrocarbons with low concentrations in ambient air and no reactivity towards ozone (e.g. propyne,

cyclohexane and n-heptane) can be used from the NPL standard to scale the mixing. Then the ratios

of the mixing ratios in standard addition measurement compared to a pure NPL standard are plotted

after normalizing them to the corresponding ratios of the above mentioned compounds (Fig. 4).

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 in the log (upper)

and linear plot (Fig. 5) any significant deviations from 1 and thus no indication of reactive losses.

NPL-Standard-Addition

0.1

1.0

10.0

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

Ethan Ethen Propan Propen i-Butan Acetylen

n-Butan t-2-Buten 1-Buten i-Buten c-2-Buten Propin

1,3-Butadien t-2-Penten c-2-Penten Cyclohexan+? n-Hexan Isopren

n-Heptan Benzol Toluol Ethylbenzol p,m-Xylol o-Xylol

NPL-Standard-Addition

0.5

1.0

1.5

2.0

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

Ethan Ethen Propan Propen i-Butan Acetylen

n-Butan t-2-Buten 1-Buten i-Buten c-2-Buten Propin

1,3-Butadien t-2-Penten c-2-Penten Cyclohexan+? n-Hexan Isopren

n-Heptan Benzol Toluol Ethylbenzol p,m-Xylol o-Xylol

Fig. 5: Results of NPL standard addition measurements from 2009 performed once per month.

Typically, 2-10% of sample volume is added to an ambient air sample by continuous addition of a

small flow of NPL standard via a quartz capillary to the ambient air.

5.7 Logs at each station

It is required that each station has the following logs either in electronic or paper-based form:


1. Instrument Log with all operation parameters, significant changes, characterizations, tests

results, etc.

2. Measurement Log 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 a.s.o. as

well as all other unexplained deviations from normal instrument performance.

6. Meteorological data Log (temp, hum., wind velocity and dir.)

6. Post-analysis

6.1 Data checks of final mixing ratio data

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 the Figures 6-, where the quality checks were performed for the Hohenpeissenberg data

in year 2009 and Rigi 2011 data. When looking at the plots please consider that at Hohenpeissenberg

usually measure at 1:00 and 13:00 CET which explains part of the variability in the data as diurnal

variations. Furthermore, Hohenpeissenberg is on top of a hill, thus we are frequently at night

decoupled from the boundary layer and do not observe accumulations of trace gases as observed at

flat terrain sites.

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, we check the peak integration, break through in

trap or other potential loss problems (e.g. the negative spike in ethane at the beginning of august

(Fig. 6) was due to break through).


10

100

1000

10000

01.01. 31.01. 03.03. 02.04. 03.05. 02.06. 03.07. 02.08. 02.09. 02.10. 02.11. 02.12.

Ethan, ppt Propan, ppt i-Butan, ppt n-Butan, ppt

Fig. 6: Time series (annual cycle) of C2-C4 alkanes measured at Hohenpeissenberg, generally,

measurements from 1:00 and 13:00 CET are shown.

For compounds with similar relative annual cycles but on different mixing ratio levels the ratio plots

as shown below can be used (Fig. 7).

In such plots we either compare structural similar compounds (as shown above for pairs of alkanes,

alkynes, and aromatics) or compounds originating from similar sources or compounds having similar

lifetimes. We check all the spikes with the log book to identify irregular operation conditions, recheck

peak integration, check with other compounds deviating in these individual measurements and try to

identify the reason for the spike. For example, in mid April the propyne/acetylene spike can be

attributed to small wood fires in the vicinity of the station,), the n-pentane/i-pentane spikes are due

to occasional n-pentane plumes (we do not know where they originate from but we observe them in

two independent GC-systems), the broad minimum in propyne/acetylene ratios in spring is due to

the longer lifetime of acetylene 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.


0.01

0.1

1

10

01.01. 31.01. 03.03. 02.04. 03.05. 02.06. 03.07. 02.08. 02.09. 02.10. 02.11. 02.12.

n-Pentan/i-Pentan Propyn/Acetylen Toluol/Benzol p,m-Xylol/Toluol

Fig. 7: Time series of ratios between pairs of hydrocarbons with similar structure

xy-plots for the data evalualtion (used at Rigi, Switzerland by Empa)

A similar approach as used at Hohenpeissenberg is applied at the Rigi site by Empa. In addition many

xy-plots are produced to check for consistency with former years and within the year. In Figures 8

and 9 examples are shown of correlation plots using toluene vs. benzene and benzene vs. ethyne.

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

propane/ethane butenes/ethene soheptane/isohexane

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

propane/n-butane entenes/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

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

acetylene/benzene


Fig: 8: toluene vs benzene at Rigi (Switzerland). Blue 2011 data, brown 2008-2011 data

Fig: 9: acetylene vs benzene at Rigi (Switzerland). Blue 2011 data, brown 2008-2011 data

repeatability and reproducibility:

One more check which is aplied at Hohenpeissenberg for the plausibility of the measurements is

related to the repeatability and reproducibility of the measurements. We use the reference gas

measurements of a complete year and determine the standard deviations for all compounds of the

whole air reference (“Ref-04”), a synthetic mixture prepared from liquid PIANO standard material

mixtures from Supelco mixed with N2 in a high pressure cylinder (“SUPELCO”), and the Apel-Riemer

standard (“74-NMHC”).which we use to check our laboratory standard from NPL. The relative (%) and

absolute (ppt) standard deviations are plotted on a log scale versus the mixing ratio for all identified


compounds. In the lower figure there are added two curves that represent a variability by 1ppt + 1%

(red line) and 3ppt + 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 3ppt +

3% line which still is a quite good level for the reproducibility of our measurements of cylinder gases.

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.

Such a presentation helps to identify problems with individual compounds. At Hohenpeissenberg, we

calculate the precision of the measurements by error propagation based on blank value scatter, peak

integration problems and other characterised problems. This precision estimate should then be

comparable or larger (conservative estimate) than the determined reproducibility (Fig. 10) to be on

the safe side with the precision estimate.

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%

Fig. 10: Standard deviations obtained from all measurements of standard (not NPL as this is the

laboratory standard used for quantification of the other standards and reference gases) and

reference gases in 2007 versus compounds’ mixing ratios in ppt in respective standard; the upper

panel shows the relative standard deviations, and the lower panel the ppt standard deviations versus

mixing ratios (ppt); two lines in the lower part show parameterizations of the standard deviations for

the “good” results (1 ppt + 1%) and as an upper limit for most of the “not so good” compounds 3 ppt

+ 3%.


6.2 Uncertainty evaluation

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 1 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 mixing ratios 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)

6.2.1. Calculation of mixing ratios for linear detection systems

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

sample is calculated via:

icalsample

iblankisampleisample f

V

AA,

,,, *

−=χ (F1)

With the calibration factor

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

cal,i = certified mixing ratio 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-reponse factor of compound “i”

In case of substances not quantifiable by the standard gas mixture, the mixing ratio 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 mixing ratio of a substance is then

respinumisample

isampleisample

CCV

A

**,

,, =χ (F3)


6.2.2 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:

samplerel

prec DL χσχχ *31 +=∆

Where

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

C = mixing ratio 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.

6.2.3 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:

• the uncertainty in the standard gas mixing ratio calδχ

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


• systematic errors in sample volume determination Vδ

• systematic errors in blank determination blankδχ

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

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

Following Gaussian error propagation, the overall systematic error is then described as 2222

int22

instrumentblankvolcalsystematic χχχχχχ ∆+∆+∆+∆+∆=∆

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

compound as described below (assuming blanksamplesample AAA −≅ ):

Systematic error due to the uncertainty of the calibration gas:

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.

Systemativ integration error:

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.

If the sample gas and the calibration gas chromatogram resemble each other (similar concentration

level, same peak shapes, etc.), the systematic integration error cancels.

How to determine the systematic integration errors:

i) Conservative estimate

A statistic integration error (random changes in baseline position) is covered by the precision of the

measurement.

However, for peaks strongly deviating from the theoretically expected Gaussian peak form (e.g. with a

strong tailing) or overlapping peaks a systematic integration error has to be determined.

For peaks with a pronounced tailing, some systematic error has to be expected as the slowly declining

slope generates some potential to erroneous peak integration:


(a) the tailing – thus the complete amount of the eluting substance – might not be completely captured

by the routine integration parameters or (b) some of the observed tailing is some baseline artefact

independent from the observed substance. (c) else.

Of course for overlapping (non-Gaussian) peaks the not visible peak end or start generates some

potential problem.

Very conservative estimates of systematic integration errors can be derived by integrating peaks such

that an extreme minimum and maximum peak area is determined as depicted in Fig. 1 and 2. After

determination of a maximum and a minimum peak area, the standard deviation intσ , of the derived

peak areas (min,max, norm) is calculated. 3intσ

conservatively describes the possible systematic error of

peak integration.

Tailing peaks:

For tailing peaks an estimate the minimum and maximum peak area can be derived by applying the

baselines as shown in Fig 1. While the blue baseline cuts off the tailing, the red baseline captures the

visible tailing.

MIN

MAX

Figure 11 Scheme for tailing peaks. The black baseline represents the best fit. Red and blue lines illustrate the baselines to derive the maximum and minimum peak area, repectively.

Overlaying Peaks:


Usually, for overlapping peaks the drop baseline integration (black straight and dotted baselines) is the

method of choice. For the theoretical Gaussian peak this procedure will exactly describe the expected

peak area. For non-Gaussian peaks it is a reasonable best estimate. A minimum for peak A can (very,

very) conservatively be derived by applying the green dashed baseline; for peak B the red dashed

baseline, respectively. Maximum areas are derived by integration of the total area of peaks A and B (via

black straight baseline) and subtraction of the tangent peaks; e.g. for peak A the maximum area is the

total area minus the minimum peak area of peak B and vice versa for peak B.

A B

Figure 12 Scheme for not separated peaks. The baseline in black (straight line) yields the area of peak A and B. Applying a baseline drop in the peak valley (black dotted line) will separate the peak areas and yield the best estimate for the overlayed peaks. Dotted baselines in in green and red can be used to derive the maximum and minimum expected peak areas.

ii) “educated estimate”; will be specified later.

For groups already applying a more educated and specific procedure to determine the systematic

integration error, it is advisable to continue to use your own procedure (and provide a description).

Systematic volume errors:

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.


Systematic error of blank value:

If a blank correction has to be applied, the error of this correction can be described as the deviation from

the mean blank value of the component:

1

1*

−=∆

nblankblank σχ

With the standard deviation σblank calculated from n zero gas measurements.

Further systematic errors:

instrumentχ∆ , the error in mixing ratio due to specific instrumental problems has to be evaluated for each

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

way to describe instrumental artefacts is illustrated in the Example below.

6.2.4 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.

Figure 13 Integration of noise.


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.

EXAMPLE FOR ROUND ROBIN MEASUREMENTS:

Benzene measured in Sample: VOC_in_N2 and NPL (Volsample = VolNPL = 401ml, certified mixing ratio 2ppb

± 2%; blank corrected Areas: Acal = 8263.6667; Asample = 4413.6) with the FID.

Figure 14 Benzene peak in the NPL standard (certified mixing ratio 2 ppb) in red and the VOC_in_N2 sample gas in black; both at a sample volume of 401ml.

Precision

Systematic

Error

Uncertainty

MR [ppb] 1 [ppb] [%] 1σ[ppb] [%] 2σ[ppb] [%]

Benzene 1.067 0.005 0.45 0.0123 1.2 0.026 2.5

The precision was calculated from the series of sample measurements:

Area/Vol [units] 1σ 1σ

sample 1 sample2 sample3 sample4 sample5 Average Sdev

rel.

Sdev

10.932836 11.025449 11.062344 10.982789 11.026474 11.005978 0.05 0.45%


Thus, the precision is ppbsampleprec 005.0067.1*0045.0 ===∆ σχχ

(The precision is automatically calculated in the ACTRIS data template for the round robin

measurements).

The uncertainty was calculated considering (i) the error of the NPL standard, (ii) the systematic

integrations errors for the NPL and the sample chromatogram, (iii) a blank value and (iv) the error due to

potential artefacts of the measurement system:

(i) Error of the NPL as given by the certificate is: 2% (this is the 2σ error). Hence, the 1σ uncertainty for

Benzene is 1%; ppbcal 02.0=δχ .

calcalsample

calsamplecal AV

VAδχχ *

*

*=∆

,

ppbppbmlunits

cal 01067.0)02.0(*ts8263.67uni* 401ml

401*6.4413 ==∆χ

(ii)

The min and max area for benzene in the NPL calibration gas were determined as described above (Fig.

11) for one chromatogram in the measurement series:

Area [units] (blank not substracted)

norm min max Avg

sdev/3

[units]

rel. sdev/3

[%]

Benzene 8472 8325 8557 8451 39 0.5%

The relative systematic integration error for the NPL is thus 0.5%.

Since the VOC_in_N2 gas has an identical matrix, includes the same components only at a lower mixing

ratio and chromatograms resemble each other (see Fig.4), the same relative integration error of 0.5% is

assumed for the sample, thus the error cancels for this specific measurement 1.

1 Assuming the gas mixtures, hence the chromatograms, did not resemble each other (e.g. as for the

analysis of an air relative to a VOC in N2 calibration gas mixture), and the errors did not cancel out, the

integration error would be derived following (assuming further an integration error of 1% for the

sample).


(iii)

The blank value was determined using 2 zero gas measurements (n=2); the standard deviation of the

blank value is σblank=0.012ppb.

012.012

1*012.0

1

1* =

−=

−=∆ ppb

nblankblank σχ

(iv)

The deviation from the mean C-response is used to describe the error due to instrumental artefacts:

E.g. a significantly lower than expected C-response for a substance can be observed due to several

instrumental artefacts, such as too weak or too strong adsorbents in the pre-concentration unit, losses

due to decomposition within the instrument or the calibration gas itself, active surfaces, etc.

The error due to instrumental artefacts influencing the C-response factor can be described as

calsample

calsamplecalinstrument AV

VACresp

*

*** χδχ =∆

We have calculated the mean C-response from C5 – C8 Alkanes and Benzene. For Benzene the deviation

from the mean is %91.0=Crespδ and thus the error in mixing ratio due to this deviation is:

ppbml

mlppb

AV

VACresp

calsample

calsamplecalinstrument 0097.0

401*6667.8263

401*6.4413*2*0091.0

*

*** ===∆ χδχ

The total expanded uncertainty (precision + systematic errors, factor 2) for benzene in the VOC_in_N2

sample is then:

ppbsystematicprecunc 033.00157.0005.0*2*2 2222 =+=∆+∆=∆ χχχ

2

int,2

2

int,2int *

*

***

*

*

+

=∆ cal

calsample

calcalsamplesample

calsample

calcal AAV

VAA

AV

V δχ

δχχ

2

2

22int )6667.8263*005.0(*

6667.8263

2*6.4413)6.4413*01.0(*

6667.8263

2

+

=∆χ 1


6.3 Data submission

Data submission has to be performed using the template provided by EBAS using the website:

http://ebas-submit.nilu.no/. The deadline for submission for data from one year, is 31 July of the

following year.

Only the names of the peak list below should be used for the submission of the data under ACTRIS-

EBAS. If single compounds are not resolved for butenes, pentenes, isohexanes and isoheptanes, the

sum of these compounds can be reported, if species are measured as a group.

Peak list:

ethyne cyclo-pentane styrene

ethene 2-methylbutane ethylbenzene

ethane n-pentane m-p-xylene

propyne 2-2-dimethylpropane o-xylene

propene benzene 2-2-4-trimethylpentane

propane cyclo-hexane n-octane

1-3-butadiene methyl-cyclopentane 1-2-3-trimethylbenzene

1-butene 2-methylpentane 1-2-4-trimethylbenzene

butenes 3-methylpentane 1-3-5-trimethylbenzene

cis-2-butene n-hexane p-cymene

2-methylpropene 1-hexene tricyclene

trans-2-butene 2-2-dimethylbutane alpha-pinene

n-butane 2-3-dimethylbutane camphene

2-methylpropane isohexanes sabinene

1-butyne toluene myrcene

isoprene methyl-cyclohexane alpha-pinene

1-pentene 2-methylhexane alpha-phellandrene

2-methyl-1-butene 2-2-dimethylpentane 3-carene

2-methyl-2-butene 2-4-dimethylpentane alpha-terpinene

3-methyl-1-butene 2-2-3-trimethylbutane ocimene

cyclopentene 2-3-dimethylpentane limonene

cis-2-pentene 3-3-dimethylpentane beta-phellandrene

pentenes isoheptanes gamma-terpinene

trans-2-pentene n-heptane terpinolene

7.References

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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.

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

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Helmig, D. (1997): Ozone removal techniques in the sampling of atmospheric volatile organic trace

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of ppt level volatile organic trace gases using multistage solid-adsorbent trapping. Journal of

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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

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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.

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.

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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

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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.

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Protection Agency, EPA/600-R-98/161.

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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.


8. Appendices

Appendix1: Ozone removal techniques for GC analysis of OVOC

Appendix 2: Adsorbents for sorbent-based enrichment of VOCs and OVOCs (oxygenated volatile

organic compounds) in ambient air samples

Appendix 3: Chromatographic separation (by J. Englert)


APPENDIX 1: OZONE REMOVAL TECHNIQUES FOR GC ANALYSIS OF OVOC (OXYGENATED VOLATILE

ORGANIC COMPOUNDS) IN AMBIENT AIR SAMPLES (BY J. ENGLERT)

Reactions of concentrated VOC 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 VOC:

Ozone melting and boiling points (at atmospheric pressure) are at -192.1°C and 111.9°C. During

cryogenic freezeout of VOC 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 VOC 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 freezeout 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

VOC because of the high reactivity of these compounds on unheated stainless steel surfaces.

Ozone reactions during solid adsorbent sampling of VOC:

Ozone artefacts are formed on and with some sorbents (e.g. graphitised carbon sorbents and Tenax®

TA) leading to both VOC 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:

Aluminium, 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

% 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 OVOC 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 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 ozone to nitrogen dioxide (NO2) and oxygen (O2)

(HELMIG, 1997). The reaction is: O3 + NO → O2 + NO2 .

Disdavantage: slow reaction, alcohol losses (but constant)

Ozone deletion by potassium iodide (KI):

In many cases KI is used for ozone 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 ozone to potassium oxide (K2O) and elemental iodine.

Example: PTFE-lined stainless steel or Silcosteel 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 ozone in a humid ambient air stream but

inconsistent removal efficiencies from different suppliers and from different batches – testing of

individual ozone 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 methylvinylketone and methacrolein.

Sodium thiosulphate (Na2S2O3):

The reaction between thiosulfate and ozone produces tetrathionate oxygen and water: 2S2O32- + O3

+ 2H+ → S4O62- + O2 + H2O


pH dependency of this reaction

Example: ozone 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 ozone levels

(HELMIG, 1997)

Advantage: this glass fiber filters also reduce sampling artifacts from reactions with halogens

Other ozone removal agents are copper oxide (CuO), magnesium sulphate (MgSO4), manganese

dioxide (MnO2), potassium carbonate (K2CO3) and TPDDC (see table 1).

In-line ozone 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 VOC 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 methylvinylketone

Impinger KI 2% Aqueous, buffered KI solution

Impregnated glass wool KI Quantitative ozone 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 ozone removal efficiency with sampling length

Gas-phase ozone titration Nitric oxide (NO)

Very efficient, quantitative recovery of formaldehyde, formation of artefacts on Tenax exposured to elevated NOx levels, possible chromatographic interferences of NO and NO2 with NMHC (KUSTER et al., 1986), losses of alcohols, slow reaction

Metal tubing Nickel (Ni) Ozone 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 ozone scavenger

Spiked cartridge 5% Na2S2O3 aqueous solution on Tenax

Direct pretreatment of the adsorbent, improved monoterpene recovery

Spiked cartridge Na2S2O3 Interferences eliminated


APPENDIX 2 (ADSORBENTS FOR SORBENT-BASED ENRICHMENT OF VOCS AND OVOCS

(OXYGENATED VOLATILE ORGANIC COMPOUNDS) IN AMBIENT AIR SAMPLES (BY JENNY 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 VOC 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 VOC are transferred in a second step into a cooled focusing device (e.g. sorbent

trap). For oxygenated VOC 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 VOC 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 dewpoint 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 artefacts are formed on and with some sorbents (e.g. graphitised carbon sorbents and Tenax®

TA) leading to both OVOC losses and buildings (LEE et al., 2006; McCLENNY et al., 2001). So the

aspect of ozone removal has to be considered in sorbent-based ambient air sampling.


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 VOC 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 VOC 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 OVOC analysis are

summarized in table 1.

Sorbent Class Strength Max. Temp. [°C]

Relative analyte size to n-alkanes

Adequacy for OVOC (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, ozone artefacts

Tenax® TA Porous polymer

Weak 350 C6-C30

No, too weak (LEIBROCK, 1996; WOOLF-ENDEN, 2010b)

Too weak for acetone and n-pentane, high benzene blank value, inert, hydrophobic, low inherent artefacts (e.g. aldehydes - HELMIG and GREENBERG, 1994), NMHC, aldehyde and ketone artefacts in combination with ozone (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 (HOPKINS et 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 ozone (LEE et al., 2006)

Chromosorb® 102

Porous polymer

Medium 225 C5-C12 Yes Inert, hydrophobic, high inherent artefact


levels

PoraPakTM Q Porous polymer

Medium 250 C5-C12 Yes Inert, hydrophobic, high inherent artefact levels

Chromosorb® 106

Porous polymer


PoraPakTM N Porous polymer


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 mesh recommended to minimise back pressure, retention of very volatile compounds e.g. 1,3-butadiene

CarbopackTM X


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 ozone 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


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


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


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 conditions, easily and irreversibly contaminated by higher boiling components

Charcoal Activated carbon

Very strong

>400 C2-C4

No (WOOLF-ENDEN, 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


APPENDIX 3: CHROMATOGRAPHIC SEPARATION (BY J. ENGLERT)

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-xlene


Advantage

Strong selectivity to

OVOC, high retention

of OVOC even at

above ambient oven

temperatures, no

retention of saturated

aliphatic NMHC and so

no co-elutions with

OVOC, long lifetime

Water resistance,

retention times not

influenced by water,

long lifetime

Strong selectivity on

light hydrocarbons

Disadvantage

Need for humidity

management,

retention of water,

tailing of unsaturated

OVOC due to reactions

with the polar column,

unsaturated NMHC

and aromatics both

with carbon atom

numbers higher than

eleven stick in the

column

Co-elutions of OVOC

with aliphatic NMHC,

retention of water

Not usefull for

OVOCs


Examples of ambient air chromatograms

1A) Al2O3 (KCl) (from Rigi, Switzerland, Empa)


Fig.1: Al2O3 (KCl): a typical chromatogramme at Rigi (Switzerland).


1B) LowOx

Fig.2: CP-LowOx (Varian), 10 m x 0.53 mm x 10.0 µm (HOPKINS et al., 2003).

Fig.3: CP-LowOx (Varian), 30 m x 0.53 mm x 10.0 µm (ROUKOS et al., 2009).


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: LEGREID, 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.


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, OVOC+ NMHC

n-butane+acet-aldehyde+ methanol, isobutene+ methanol, ethanol+isopentane, acetone+propanal+ isopropanol, butanal+MEK, OVOC+NMHC

Propanal+acetone, OVOC+NMHC

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 OVOC with NMHC

Tailing of alcohols and ketones, co-elutions of OVOC with NMHC

Co-elutions of OVOC with NMHC


2A) DB-1

Fig. 6: DB-1 (Agilent J&W), 100 m x 0.25 mm x 0.5 µm (RIEMER et al., 1998).


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 (own 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


2D) DB-624

Fig. 9: DB-624 (Agilent J&W), 10 m x 0.18 mm x 1.4 µm (APEL et al., 2003).


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


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).


3C) DB-WAX

Fig. 12: DB-WAX (Agilent J&W), 60 m x 0.32 mm x 0.5 µm (LAMANNA and GOLDSTEIN, 1999).


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

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methane hydrocarbons (NMHCs) and oxygenated volatile organic compounds (o-VOCs). Journal of

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Legreid, G. (2006): Oxygenated volatile organic compounds (OVOCs) in Switzerland: From the

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Leibrock, E., Slemr, J. (1997): Method for measurement of volatile oxygenated hydrocarbons in

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