Headspace Gas Chromatographic Methods

Solid-phase microextraction (SPME) is a sampling and concentration technique used to increase the sensitivity of HS methods. This technique is utilized for arson analysis and environmental monitoring purposes and also for clinical and forensic procedures. Short, narrow diameter, fused-silica optical fibers coated with stationary phase polymers are either immersed in the sample or the HS and compounds are adsorbed or absorbed (depending on 

One of the most commonly used fibers for VOC analysis is coated with a lOO-pm polydimethylsiloxane phase (PDMS) and is available as a unit with a stainless-steel guide rod housed in a hollow septum-piercing needle (Supelco, Bellefonte, PA, USA). The fiber can be withdrawn into the needle for protection during handling and the depth of fiber exposure can be controlled using the adjustable holder. In HS techniques, the needle can be used to pierce the septum of the sample vial and to introduce the fiber into the injection port of the gas chromatograph. In the sample vial, the extraction onto the fiber reaches equilibrium fairly quickly for volatile compounds. Mills and Walker s review includes tabular summaries of HS-SPME-GC methods for the detection of alcohols, drugs, solvents and chemicals in blood and urine. 

Another way to increase sensitivity is to increase the volume injected into the column. Lowering the temperature of the entire column in cryogenic oven trapping (COT) methods or for the injection port at the inlet of the column in cryogenic focusing (CF) methods allows the injection of as much as 10 times the typical sample volume. Liquid nitrogen or liquid carbon dioxide is used to lower the temperatures to well below 0 °C (—180 °C for nitrogen and 90 °C for carbon dioxide). These techniques result in better peak shapes in addition to increased sensitivity. 

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Christensen JM, Rasmussen K, Koppen B. 1988. Automatic headspace gas chromatographic method for the simultaneous determination of trichloroethylene and metabolites in blood and urine. J Chromatogr 442 317-323. 

Ulberth F (1998) A rapid headspace gas chromatographic method for the determination of the butyric acid content in edible fats. Z Lebensm Unters Forsch 2o6A 305-3oy. 

Neill et al. [22] have described a headspace gas chromatographic method for the determination of carbon dioxide (fugacity) in seawater. This method requires a small water sample (60 ml), and provides for rapid analysis (2 min). 

DeVries JW, Broge JM, Schroeder JP, et al. 1985. Headspace gas chromatographic method for determination of methyl bromide in food ingredients. J Assoc Off Anal Chem 68 1112-1116. 

Chai, X.S. and Zhu, J.Y. Indirect headspace gas chromatographic method for vapor-liqtrid phase equilibrium study, J. Chromatogr. A, 799(2) 207-214, 1998. 

Morales, M.T., Aparicio, R. and Rios, J.J. (1994) Dynamic headspace gas chromatographic method for determining volatiles in virgin olive oil. J. Chromatogr. A, 668, 455-462. 

Isopropanol and its metaboHte, acetone, may be determined by headspace gas chromatography or by NMR spec-troscopy. A headspace gas chromatographic method is included in the Chapter 34 Appendix that is found on the book s accompanying Evolve site (http //evolve.elsevier.com/ Tietz/textbook/). 

Ang, C. Y. W. and Young, L. L. (1989) Rapid headspace gas chromatographic method for assessment of oxidative stability of cooked chicken meat. Journal of the Association of Official Analytical Chemists, 72, 277-81. 

McCown, S. M. and Radenheimer, P. (1989) An equilibrium headspace gas chromatographic method for the determination of volatile residues in vegetables. LC-GC International, 2,28-31. 

Ulberth, F. and Roubicek, D. (1993) Evaluation of a static headspace gas chromatographic method for the determination of lipid peroxides. Food Chemistry, 46, 137-41. 

A modified headspace gas chromatographic method has been described for the determination of free methylmethacrylate monomer in contact lenses regardless of their... 

Chian et al. [69] point out that the Bellar and Iichtenberg [65] procedure of gas stripping followed by adsorption onto a suitable medium and subsequent thermal desorption onto a gas chromatograph-mass spectrometer is not very successful for trace determinations of volatile polar organic compounds such as the low molecular weight alcohols, ketones, and aldehydes. To achieve their required sensitivity of parts per billion, Chian et al. [69] carried out a simple distillation of several hundred ml of sample to produce a few ml of distillate. This achieved a concentration factor of between 10 and 100. The headspace gas injection-gas chromatographic method was then applied to the concentrate obtained by distillation. 

Low detection limits (low ng/mL) have been achieved using a headspace/gas chromatographic (GC) technique (Seto et al. 1993). The sample is acidified and incubated, and the headspace analyzed by GC with a nitrogen-specific detector (NPD) (Carseal et al. 1993 Levin et al. 1990 Seto et al. 1993). Reported recovery is good (>90%) (Carseal et al. 1993), and precision is good as well (<15% RSD) (Carseal et al. 1993 Levin et al. 1990 Seto et al. 1993). Blood samples may be treated with chloramine T priorto incubation to produce a derivative which can be determined by GC with electron capture detection (ECD). Cyanate and thiocyanate do not interfere in this method (Odoul et al. 1994). The detection limit is 5 pg/L (ppb) precision is good (<15% RSD) (Odoul et al. 1994). 

The analysis involves gas chromatographic methods such as purge and trap, vacuum distillation, and headspace (Askari et al., 1996). On the other hand, samples for the determination of semi- and nonvolatile hydrocarbons need not be collected in such a rigorous manner. On arrival at the laboratory, they require extraction by techniques such as solvent or supercritical fiuid. Some cleanup of... 

Seto Y, Tsunoda N, Ohta H, et al. 1993. Determination of chloroform levels in blood using a headspace capillary gas chromatographic method. J Anal Toxicol 17(7) 415-420. 

Cummins, T.M., Robbins, G.A., Henebry, B.J., Goad, C.R., Gilbert, E.J., Miller, M.E., and Stuart, J.D. A water extraction, static headspace sampling, gas chromatographic method to determine MTBE in heating oil and diesel fuel. Environ. Sci. Technol, 35(6) 1202-1208, 2001. 

Vitenberg et al. [623] have described a gas chromatographic method for the determination of traces (down to l-10pg kg ) of sulphur compounds, such as hydrogen sulphide, mercaptans, sulphides and disulphides, in industrial waste waters (kraft paper mill effluents) by a combination of headspace analysis and microcoulometiy. This method increases the analytical sensitivity 102-103 times without any preliminary concentration of the sample. 

Gas Chromatographic Methods. Gas chromatographic methods may be used for measuring volatile oxidation products. Static headspace, dynamic headspace, or direct injection methods may be employed. Specific aldehydes may be measured as indicators for oxidative stability of oils and fats. Thus, propanal is an and as indicator for stability of omega-3 fatty acids, whereas hexanal is best for following the oxidative stability of omega-6 fatty acids. 

A dynamic headspace gas-partitioning method has been used to assess the effect of dissolved organic carbon on the value of H for mirex (Yin and Hassett 1986), and with a gas chromatographic detection system for analysis, this is applicable to native water samples containing mirex. 

Gas chromatography Gas chromatographic methods for cyanide usually involve using headspace techniques to detect hydrogen cyanide with the use of a nitrogen-phosphorus detector (NPD) or electron capture detector (BCD). Total cyanides can be analyzed in this way after conversion to hydrogen cyanide. 

This second edition offers new material on methods of sensory detection (nasal through the nose) or (retronasal through the mouth and back of the oral cavity), different flavor release phenomena in the headspace versus the mouth, and matrix in flavor release from oils compared to emulsion systems. Advanced gas chromatographic methods are included, such as solid phase microextraction for the volatile analyses in foods and vegetable oils, gas chromatography-olfactometry, and aroma extraction dilution analyses. 

It is not easy to determine which factors play the greatest role in obtaining good accuracy and precision. One must consider the assumptions inherent in the theory as well as the chemical, mechanical, and instrumental parameters. In general, gas chromatographic methods agree within 1-5% with other physicochemical methods. For example, Hussam and Carr (22) showed that in the measurement of vapor/liquid equilibria via headspace GC, complex thermodynamic and analytical correction factors were needed. These often came from other experimental measurements that were not necessarily accurately known. Another source of significant error can be in determination of the mass of stationary phase contained within the column (59). Other sources of error include measurement of holdup time (60), flowrate, sample mass, response factors, peak area, or baseline fidelity. 

Principles and Characteristics Solution headspace gas chromatographic sampling has a counterpart in a solvent-free, direct method for the rapid determination of volatile components in solid samples. Volatile and semi-volatile components can be desorbed directly from sample matrices or from sorbent or cryogenic traps without any significant sample preparation. 

Lipid oxidation was monitored through analysis of the hydroperoxide content measured via the ferric thiocyanate method [64]. Oil was extracted from an aqueous emulsion prepared from the microcapsules using isooctane/2-propanol 50 50 (v/v). Headspace gas chromatographic analysis of secondary lipid oxidation products was performed in the majority of trials, but since correlation of propanal as a key compound was always very good with the hydroperoxide content, data are not reported in this chapter. 

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. 

For the GC method, the generated carbon disulfide is analysed using a flame photomeric detector in the sulfur mode. The acid decomposition is carried out in a sealed glass container at 80 °C, and an aliquot of the headspace is injected into a gas chromatograph. " ... 

Corwen [58] used this method for the analysis of ketones and aldehydes in seawater. Halocarbons were similarly separated from environmental samples by Kaiser and Oliver [59]. There have been many other applications of the technique [60-69]. The major advantage of the headspace method is simplicity in handling the materials. At most, only one chemical, the salt used in the salting-out procedure, needs to be added and in most cases the headspace gas can be injected directly into a gas chromatograph or carbon analyser. On the other hand the concentration of organic materials present is limited by the volume of seawater in the sample bottle. This is very much a batch process. 

Of the three general methods, the last seems to be the most practical. Theoretically, with high enough concentrations of hydrocarbons, the first method, the headspace analysis, should be both the most accurate and the easiest to calibrate. Operationally, it leaves much to be desired both because of the problems of sensitivity and those of the accommodation of the larger molecules in water. The second method, vacuum degassing, requires much more equipment than the third method and requires that large amounts of water vapor be removed before the sample is injected into the gas chromatograph. The last method is so much less complicated that even with its calibration problems it has been adopted almost universally. 

The volatile substances were extracted from portions of 0. lg hair using solid-phase micro extraction (SPME). The method uses a fibre coated with an adsorbent that can extract organic compounds from the headspace above the sample. Extracted compounds are desorbed upon exposure of the SPME fibre in the heated injector port of a gas chromatograph (GC). 

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