Gas Analysis by GC

Not surprisingly, GC is the method of choice for the analysis of mixtures of gases (i.e., compounds which are vapors at room temperature). The procedures for sampling and introduction to the GC instrument sometimes differ from the standard syringe injection or SPMB techniques (Section 12.3) employed for neat liquids or dilute solution samples. Gases at trace levels in air or dissolved in water may be present at levels too low to measure with the volumes one might sample and inject with a syringe even one of several milliliter volume. Certainly one would not wish to introduce several milliliters of water into a GC column  

Trace levels of gases in air can be sampled by drawing large volumes of air (up to cubic meter volumes) with a pump through a trap (a tube or plate filled with a special sorbent packing) for periods up to an hour or more. One especially useful sorbent is a packed bed of Tenax particles. These are beads of polyphenyl ether polymer 

Encyclopedia of Chromatography, Marcel Dekker Inc. New York, 2001. 

Modern Practice of Gas Chromatography, 3rd ed. Wiley New York, 1995. 

Jennings, W. Mittlefehlt, E. Stremple, P. Analytical Gas Chromatography, 2nd ed. Harcourt Brace Orlando, FL, 1997. 

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Effects of Ion Trapping Agents. The effects of ion trapping agents were studied in experiments that used a Teflon vessel and gas analysis by GC-MS. Table III shows the results. The ion trapping agents that were used in this experiment were bismuth hydroxides. 

FIGURE 34. Natural gas analysis by GC-C-MS (a) m/z 44 trace (quantification), (b) relative to standard, (c) CPDB for each alkane (analyzed by Finnigan-Mat)... 

Figure 5.16. Scheme of an instrument for manometric-gravimetric-oscillometric measurements of binary coadsorption equilibria in swelling materials (polymers, resins etc.) without sorptive gas analysis by GC or MS. 

The purpose of the experiment is to illustrate the application of derivatisation in the analysis of sugar and related substances by gas-liquid chromatography. The silylation method described is an almost universal derivatisation procedure for carbohydrate analysis by GC.79... 

The combination of chromatography and mass spectrometry (MS) is a subject that has attracted much interest over the last forty years or so. The combination of gas chromatography (GC) with mass spectrometry (GC-MS) was first reported in 1958 and made available commercially in 1967. Since then, it has become increasingly utilized and is probably the most widely used hyphenated or tandem technique, as such combinations are often known. The acceptance of GC-MS as a routine technique has in no small part been due to the fact that interfaces have been available for both packed and capillary columns which allow the vast majority of compounds amenable to separation by gas chromatography to be transferred efficiently to the mass spectrometer. Compounds amenable to analysis by GC need to be both volatile, at the temperatures used to achieve separation, and thermally stable, i.e. the same requirements needed to produce mass spectra from an analyte using either electron (El) or chemical ionization (Cl) (see Chapter 3). In simple terms, therefore, virtually all compounds that pass through a GC column can be ionized and the full analytical capabilities of the mass spectrometer utilized. 

Purge-and-trap methods have also been used to analyze biological fluids for the presence of trichloroethylene. Breast milk and blood were analyzed for trichloroethylene by purging onto a Tenax gas chromatograph to concentrate the volatiles, followed by thermal desorption and analysis by GC/MS (Antoine et al. 1986 Pellizzari et al. 1982). However, the breast milk analysis was only qualitative, and recoveries appeared to be low for those chemicals analyzed (Pellizzari et al. 1982). Precision (Antoine et al. 1986) and sensitivity (Pellizzari et al. 1982) were comparable to headspace analysis. 

In supercritical fluid chromatography (SFC) the mobile phase is a supercritical fluid, such as carbon dioxide [15]. A supercritical fluid can be created either by heating a gas above its critical temperature or compressing a liquid above its critical pressure. Generally, an SFC system typically has chromatographic equipment similar to a HPLC, but uses GC columns. Both GC and LC detectors are used, thus allowing analysis of samples that cannot be vaporized for analysis by GC, yet cannot be detected with the usual LC detectors, to be both separated and detected using SFC. SFC is also in other... 

Hcxanc can be determined in biological fluids and tissues and breath using a variety of analytical methods. Representative methods are summarized in Table 6-1. Most methods utilize gas chromatographic (GC) techniques for determination of -hexane. The three methods used for preparation of biological fluids and tissues for analysis are solvent extraction, direct aqueous injection, and headspace extraction. Breath samples are usually collected on adsorbent traps or in sampling bags or canisters prior to analysis by GC. 

A representative gas chromatogram with ECD of the analysis of various polar chlorinated pesticides isolated from cod liver oil [179] is shown in Fig. 13. Determination of the polar chlorinated pesticides in cod liver oil required clean up of the lipid matrix with a dimethylformamide/water/hexane liquid-liquid partitioning procedure followed by isolation using a normal-phase LC procedures, and final analysis by GC-ECD [179]. 

In common with all capillary-based techniques, modem GC offers high-efficiency separations, allowing analyte resolution even with relatively low selectivity differences. Flame ionisation is now the most common form of detection used for organic analytes in GC and is universal for hydrocarbon-containing species. Although achiral GC is used widely in the pharmaceutical industry for the analysis of residual solvents and volatile analytes, its apphcation to chiral analysis tends to be limited to chiral raw materials and smaller intermediates. As the mobile phase is a gas, only volatile analytes are applicable to analysis by GC, often precluding its use for the analysis of relatively large, complex API molecules. Moreover, analyte molecules also need to be thermally stable to the temperatures required to ensure volatilisation [114]. 

Ion pair extraction has also been used to extract polar analytes in bioanalytical procedures. Figure 15.2 exemplifies the determination of the amino acid taurine by gas chromatography-mass spectrometry (GC-MS) this figure also illustrates a useful property of amines (and phenols), which is that they will react more rapidly than water with an acylating reagent in an aqueous environment thus improving their organosolubility. After acylation and ion pair extraction with tetrabutyl ammonium sulphate the taurine is converted to an amide prior to analysis by GC-MS. 

Calcium carbide desulfurization slag has a distinctive odor. Since pure acetylene is odorless, the odor must be produced by other trace constituents in the off-gases. A calcium carbide desulfurization slag sample from one ductile foundry was treated with water at a 1 1 solid-to-liquid ratio, and the gas was collected in a Tedlar bag for analysis by GC-MS. Several trace gases were identified, including arsine, divinyl sulfide (CHj-CH S, ethanethiol (ethyl mercaptan), methane, phosphine, and carbon monoxide. 

A stream-splitter attached to the exit tube of a thermal conductivity detector can be used to identify the functional groups of gas chromatographic effluents. Table 4.1 lists functional group tests and limits of detection. A review of elemental analysis by GC is given by Rezl and Janak (9). 

Determination of urea pesticides has been performed by gas chromatography (37,56,160, 161), a technique that has highly sensitive and selective detectors. Nevertheless, some substituted ureas are thermolabile and decompose during the analysis by GC (102). Therefore, direct determination of unchanged compounds by GC is possible only for some urea pesticides under determined chromatographic conditions (161). In other cases, a degradation product is quantified instead of the parent compound (29,162-164), or these substituted ureas must be derivatized before GC determination (37,102). 

As already mentioned, it is the volatile constituents that serve to identify fruit type and variety. Broadly speaking, qualitative analysis will identify the principal substances present in the volatiles fraction as representative of a particular fruit type, but it is the relative proportions of these substances that will reflect the variety. Alcohols, volatile acids, esters, carbonyl compounds, and low-boiling hydrocarbons are the principal groups represented. Analysis by GC-MS (gas chromatography coupled with mass spectroscopy) can be used to provide quantification and identification of the various constituents. 

It is unlikely that the unchanged nerve agent would be detected in the blood or tissues of a casualty unless samples were collected very soon after the exposure. A number of methods have been reported for the analysis of nerve agents in blood, for application to animal studies. These involve simple liquid or SPE extraction, for example, using chloroform (sarin, soman) (47), C18 SPE (sarin, soman) l48 49 , ethyl acetate (VX) (50), usually after precipitation of proteins, and analysis by GC/MS or gas chromatography/nitrogen-phosphorus detection (GC/NPD). Sarin bound to cholinesterase and displaced with fluoride ion was extracted by C18 SPE (see Part B) (51). 

Due to their highly polar nature, barbiturates require derivatization prior to analysis by GC-MS. In such cases, the sample is dissolved in methanol, centrifnged, the supernatant recovered and placed in a derivatizing vial and is then blown down under nitrogen. The derivatization procednre, using 0.2 M trimethylanilin-ium hydroxide in methanol, is, in principle, the same as that used for other pre-column derivatizations. However, with this system the reaction does not occnr immediately because insufficient activation energy is available at ambient temperature for this to take place. Direct transfer of the reaction mixture onto the heated injection block of the gas chromatograph overcomes this problem and the derivatization reaction can then proceed. Snch a reaction, an example of flash alkylation is illustrated in Scheme 9.1. 

Compound analysis by GC. Individual compounds are determined by gas chromatography (GC), GC in combination with a Mass-Spectrometer (MS) is preferred, GC in combination with a Flame Ionisation Detector (FID) with internal standard calibration is an alternative. 

S.A. Wudy, J. Homoki, W.M. Teller, Clinical steroid analysis by GC-MS, in W.M.A. Niessen (Ed), Current practice in gas chromatography-mass spectrometry, 2001, Marcel Dekker Inc., New York, NY. 

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