Sampling headspace

Sample injection in gas chromatography often seems deceptively simple a microlitre aliquot is rapidly injected into an inlet system, and elution and detection follow. Samples containing substantial amounts of non-volatile material, however, require one or more preparation steps in order to isolate volatile analytes from non-volatiles that would otherwise contaminate the inlet system and column, eventually leading to impaired chromatographic performance. Examples of such procedures include liquid-liquid extraction, solid-phase extraction and filtration. The use of a pre-column (viz. a retention gap or a guard column) is often required, even if prepared samples are used. 

sample vial is introduced into a thermostatted oven where the vapour-sample equilibrium is established  

after a pre-programmed time the vial is pressurised to column head pressure by the sampling needle and 

the valves are switched so that the carrier gas flow to the column is interrupted and sample vapours from the vial pass directly onto the column, sample volume is determined by the time the valves are in the sampling position. 

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Although aimed at the introductory class, this simple experiment provides a nice demonstration of the use of GG for a qualitative analysis. Students obtain chromatograms for several possible accelerants using headspace sampling and then analyze the headspace over a sealed sample of charred wood to determine the accelerant used in burning the wood. Separations are carried out using a wide-bore capillary column with a stationary phase of methyl 50% phenyl silicone and a flame ionization detector. 

The principle of headspace sampling is introduced in this experiment using a mixture of methanol, chloroform, 1,2-dichloroethane, 1,1,1-trichloroethane, benzene, toluene, and p-xylene. Directions are given for evaluating the distribution coefficient for the partitioning of a volatile species between the liquid and vapor phase and for its quantitative analysis in the liquid phase. Both packed (OV-101) and capillary (5% phenyl silicone) columns were used. The GG is equipped with a flame ionization detector. 

MONITORING RANCIDITY OF FARM MILK USING HEADSPACE SAMPLING... 

In the Netherlands, milk from every farmer is tested twice a year for the extent of lipolysis, using the BDI method. However, the BDI method only detects long chain FFA, which does not induce off-flavor. On the other hand, headspace sampling does detect the short chain FFA. The aim of this study is to compare the BDI method to headspace sampling. 

Raw milk was heated at 40°C and mixed in a blender for 1 min. This milk was added in different quantities (0-5 ml) to fresh raw milk to induce lipolysis. After 3 days, the milk was analyzed using the BDI method and headspace sampling. 

Solid-phase microextraction (SPME) was used for headspace sampling. The FFA were extracted from the headspace with PA, Car/PDMS, and CW/DVB fibers. It was examined whether addition of salt (NaCl) and decreasing the pH by addition of sulphuric acid (H SO ) increased the sensitivity. FFA were analyzed using gas chromatography coupled to mass spectrometry in selected ion monitoring. 

Whether the headspace sampling produces comparable results to the BDI method is subject of current research and the results of these test will be presented during the conference. 

Deveaux M, Huvenne J-P. 1987. Identification of solvents of abuse using gas chromatography/fourier transform infrared spectrometry after headspace sampling. Chromatographia 23 626-630. 

Prom the experimental point of view the static headspace sampling technique is very simple. The sample, either solid or liquid, is placed in a glass vial of appropriate size and closed with a Teflon-lined silicone septum. The 1 is carefully... 

Dynamic headspace sampling employs the continuous removal of... 

Gas phase stripping (purge-and-trap) techniques can iaq>rove the yield of organic volatiles from water or biological fluids by facilitating the transfer of volatiles from the liquid to the gas phase it is also more suitable than dynamic headspace sampling when the sample volume is restricted (320 23,347-351). Tbe technique is used routinely in many laboratorl B for the analysis... 

Small solid seuaples can be analyzed directly by dynamic headspace sampling using a platinum coil and quartz crucible pyrolyzer and cold trap coupled to an open tubular column (341,369,379). This method has been used primarily for the analysis of mineral samples and of additives, catalysts and byproducts in finished polymers which yield unreliable results using conventional headspace techniques owing to the slow release of the volatiles to the headspace. At the higher temperatures (450-1000 C) available with the pyrolyzer the volatiles are more readily and completely removed from the sample providing for quantitative analysis. 

Where is the initial analyte concentration in the liquid phase, C( the concentration of analyte in the gas phase, K the gas-liquid partition coefficient for the analyte at the analysis temperature, V, the volume of liquid phase, and V, the volume of gas phase (318-321,324,325). From equation (8.3) it can be seen that the concentration of the analyte in the headspace above a liquid in equilibrium with a vapor phase will depend on the volume ratio of the geis and liquid phases and the compound-specific partition coefficient which, in turn, is matrix dependent. The sensitivity 1 of the headspace sampling method can be increased in some instances adjusting the pH, salting out or raising the... 

Figure 8.26 A, valve diagram of the Hewlett-Packard 19395A Beadspace Sampler. B, Headspace sampling with constant headspace pressure, as carried out with the Perkin-Elmer HS-lOO Automatic Headspace Sampler.

Two basic methods are used for SPME direct immersion of the fibre into the sample and headspace sampling. Experimental parameters comprise the polarity of the sample matrix and coating material, solvent and salting-out. Other parameters for optimisation of SPME conditions include desorption time, injector port temperature and initial oven temperature. 

Sample pretreatment Occasional matrix clean-up Occasional matrix clean-up not for headspace sampling... 

Table 4.25 Main features of headspace sampling Advantages...

Table 4.25 lists the main characteristics of headspace sampling. In HS-GC sample preparation is very often limited to placing a sample in a vial. Sample extraction, clean-up and preconcentration are not necessary. Elimination of solvents in preparing samples for GC has several benefits ... 

Static headspace may also be carried out by substituting the heating step by a microwave treatment. In this procedure the material is immersed in a solvent that is transparent to microwaves relative to the sample in order to impart most, if not all, of the microwave energy to the sample [208]. Another configuration of MAP gas-phase extraction relates to dynamic headspace sampling. 

HS-GC methods have equally been used for chromatographic analysis of residual volatile substances in PS [219]. In particular, various methods have been described for the determination of styrene monomer in PS by solution headspace analysis [204,220]. Residual styrene monomer in PS granules can be determined in about 100 min in DMF solution using n-butylbenzene as an internal standard for this monomer solid headspace sampling is considerably less suitable as over 20 h are required to reach equilibrium [204]. Shanks [221] has determined residual styrene and butadiene in polymers with an analytical sensitivity of 0.05 to 5 ppm by SHS analysis of polymer solutions. The method development for determination of residual styrene monomer in PS samples and of residual solvent (toluene) in a printed laminated plastic film by HS-GC was illustrated [207], Less volatile monomers such as styrene (b.p. 145 °C) and 2-ethylhexyl acrylate (b.p. 214 °C) may not be determined using headspace techniques with the same sensitivities realised for more volatile monomers. Steichen [216] has reported a 600-fold increase in headspace sensitivity for the analysis of residual 2-ethylhexyl acrylate by adding water to the solution in dimethylacetamide. 

In static headspace sampling [301,302] the polymer is heated in a septum-capped vial for a time sufficient for the solid and vapour phases to reach equilibrium (typically 2 hours). The headspace is then sampled (either manually or automatically) for GC analysis, often followed by FID or NPD detection. Headspace sampling is a very effective method for maintaining a clean chromatographic system. Changing equilibrium temperature and time, and the volumes present in the headspace vial can influence the sensitivity of the static headspace system. SHS-GC-MS is capable of analysing volatile compounds in full scan with ppb level... 

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