Gas Chromatography

Carrier Gas

Flow Control

Injector Port

Column

Column Oven

DetectorRecorder

Recorder

The GC system consists of gas supplies for the mobile phase (and the detector, if needed), flow controls for the gasses, an injector, an oven for heating the column, a detector, and a data recording device.

1. Direct Injection of Gasses2. Dynamic Headspace3. SPME (Solid Phase Microextraction)4. As a Liquid Via a Syringe

Sample Preparation

Direct Injection of Gasses

Gasses can be sampled with a simple gas-tight syringe and introduced directly into the GC injector.

Another method of direct injection of gasses, uses a sample loop similar to the ones used in HPLC. The sample loop is filled with the gas sample and the valve is turned to divert the sample to the GC column.

Direct Injection of Gasses

Unlike HPLC, temperature and pressure must be carefully controlled because the mass of the sample will depend on these factors as gasses expand and contract depending on the temperature.

Cryofocusing:Larger volumes of gasses can be introduced into a specialized injector where the analytes are focused on a cold surface and then volatilized by rapid heating.

While direct injection of gasses can be used, many volatile analytes are often in too low of a concentration to be quantified using most common detectors. For this reason, volatiles in the headspace are often concentrated using dynamic headspace or purge-and-trap methods.

Dynamic Headspace

Dynamic Headspace

Typically the sample is placed in a closed container and a flow of inert gas such as nitrogen is used to purge the headspace onto an adsorbent or a cryogenic trap. Cryogenic traps are more complex than adsorbent traps but they are the least selective and will contain virtually any aroma compound in the sample. However, cryogenic traps will also concentrate water (the most abundant volatile in food), and the aqueous sample must be further processed.

Dynamic Headspace

Common adsorbents, do not trap much water, and the traps can be thermally desorbed directly into the injector of a gas chromatograph. Tenax is one of the most common trapping materials because its high thermal stability makes it the best choice for thermal desorption.

Dynamic Headspace

Table 1. Physical Properties of Some Adsorbants Used for Preconcentration of OrganicVolatiles from the Vapor Phase.Compound Surface area

(m2/g) Mean pore Dia.(Å) TemperatureLimit(oC)

Tenax GC 19-30 720 450Chromosorb 102 300-400 90 250Chromosorb 105 600-700 500 200Porapak Q 630-840 75 250XAD-4 750 50 200XAD-7 458 80 150

Source. Sucan et al. (1).

Solid Phase Microextraction

Sampling devices (manual and autosampler) consist of a coated silica fiber inside a hollow needle. The coating on the fiber adsorbs and concentrates volatiles from the headspace inside the sample container. The needle protects the fiber which can beinserted through the septa of the GC injector for direct analysis.

Coating Material Uses TypePDMS Nonpolar semivolatiles (MW 60-275) Absorbent, NonpolarPolyacrylate Polar semivolatiles (MW 80-300) Absorbent, PolarPDMS/Carboxen Trace-level volatiles (MW 30-225) Adsorbent, BipolarPDMS/DVB Volatiles, nitrogen compounds (MW 50-300) Adsorbent, BipolarCW/DVB Alcohols and polar compounds (MW 40-275) Adsorbent, PolarDVB/Carb./PDMS Volatiles and semivolatiles (MW 40-275) Adsorbent, Bipolar

PDMS = Polydimethylsiloxane, DVB = Divinylbenzene, CW = Carbowax

Some Common Fiber Coatings Used with SPME

External calibration curves can only be used if the standards are made up in the same matrix as the test sample. For complex samples such as foods, internal calibrations such as isotopic dilution or standard addition should be used. Care must be taken to ensure that the response is linear in the concentration range of the sample and the spiked sample, and multiple standard addition is advised whenever practical.

Quantification of Analytes with SPME

Compound 15% NaCl 36% NaCl 42% NaClEthanol 38 94 150Z-3-Hexanol 220 1000 1040Hexyl Alcohol 200 700 4200Ethyl Butyrate 170 920 880Linalool 300 2170 2230Limonene 25 32 -25-Pinene -37 -26 -49Source. Steffen and Pawliszyn (6).

Percent Change in Peak Area Due to the Addition of Salt

Injection of Liquid Samples in Gas Chromatography

Much more difficult than you would think!

The most common method of introducing a sample into the GC is with a microliter syringe, hence this process has become known as injection.

Syringe Loading Methods

Pull the sample up into the barrel of the syringe and then pre-heat the needle for several seconds before injecting. This reduces the condensation of high boiling point compounds on the walls of the needle.

Hot Needle Injection

Can help reduce bias in the injection of complex samples

Split/Splitless Injectors

...are probably the most common types

Splitless Injection, (where the split vent is closed) attempts to transfer all of the sample to the column and isused for trace analysis.

Split Mode, only a small portion (maybe 1-10% of the sample moves into the column, and the rest is sent to waste. This is used when the analytes are in high concentration and would overload the column.

How a Split/Splitless Injector Works

Backflash

When a liquid solvent is transferred into the hot injector, it vaporizes and expands. If the injector volume is not large enough to contain the expansion, a problem called backflash can occur. The sample expands so much that it can reach to the septum purge area and part of the sample can be lost through the septum purge. The sample may also condense on the cooler area of the upper injector, and may re-volatilize slowly. Backflash can result in poor peak area reproducibility, solvent peak tailing, ghost peaks or non-linear

increases in peak area as injection volume increases.

Determining Liquid Volumes for Injection

Table 4. Liner VolumesTheoretical Effective

1.0 mm ID 59 L L2.0 mm ID 236 L L3.0 mm ID 530 L L4.0 mm ID 942L L

Table 5. Solvent Expansion Volumes( injector 250oC and 10 psig)Injection volume Expansion Volumes L (vaporized) (Liquid) H2O CS2 CH2Cl2 Hexane Isooctane0.5 L 710* 212 200 98 781.0 L 1420* 423 401 195 1552.0 L 2840* 846* 802* 390 3105.0 L 7100* 2120* 2000* 975* 775*

Solvent Expansion Volumes

Ways to Reduce Backflash

1. Optimize sleeve Pack with glass wool or use double gooseneck2. Inject less3. Decrease injector temperature4. Increase head pressure (higher flow rate into column)

Cool On-Column Injection

This method places the liquid sample directly on a cooled column. and then the column inlet is rapidly heated to vaporize the sample.

Advantages:

highly reproducibleminimal molecular weight discriminationbackflash is not a problemminimal thermal degradation

Disadvantages:

difficult to handle the small needles requiredsolvent overload (peak splitting)contamination with non-volatiles

Temperature Programmable Injector

Temperature programmable injectors are a combination of split/splitless injectors and cool on-column inlets.

Advantages:more flexibility than on-column injectionbetter than on-column injection for dirty samplescan concentrate sample using solvent purge mode

Disadvantages:costcomplexity