Analysis Head space

The previous discussion has dealt with liquid injection. There are other more specialised injection techniques which are important for low-level and contaminant analysis such as adsorption of the sample from the vapour phase onto a solid substrate followed by thermal desorption of the adsorbed volatile onto a gas chromatographic column for analysis and head-space analysis. In head-space analysis the sample is allowed to come to thermal equilibrium at a controlled temperature in a sealed vial. The gaseous phase in the vial is sampled and analysed. This technique has two major advantages (i) only the volatiles in the sample are transferred to the column, thus reducing contamination and (ii) the components of interest are usually at relatively high concentration in the vapour as opposed to in the sample (which may be in any physical form, solid, liquid or paste). Quantification is complicated and is best done using standard additions where this is possible. 

1 Anionics. The many approaches in the literature to linear alkylbenzene sulphonate (LAS) analysis include the following, which demonstrate a number of sample preparation and detection options. 

Alcohol sulphates can be readily determined by acid hydrolysis, recovery of the parent alcohols, and separation either as is or after conversion to their trimethylsilyl ether derivatives. Most manufacturers demonstrate such separations in their applications literature. Typical conditions are a 10 m X 0.53 mm i.d. column of methylsilicone programmed from 70 C to 240°C at 5°C per min with helium as carrier and FID. 

The hydrophobe distribution of alkylethoxylated sulphates can be obtained by reaction with 30% HBr in glacial acetic acid at 90°C overnight to give alkyl bromides, followed by separation on a column (6ftx 1/4 in o.d.) of 10% OV-17 on Chromosorb W with temperature programming from 100°C to 250°C at 8°C per minute with helium as carrier gas and FID [5]. 

2 Nonionics. Alcohol ethoxylates and alkylphenol ethoxylates were separated on a fused silica column (30 m X 0.25 mm (0.25 micron film)) of SE-54 using helium as carrier gas and El or Cl (chemical ionisation) (methane) MS detection. Temperature programming was from 70°C (1 min) to 300°C (10 min hold) at 3°C per minute. Ethoxylates up to 6EO units could be detected [6]. 

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Ioffe, B. V. Vitenberg, A. G. Head-Space Analysis and Related Methods in Gas Chromatography. Wiley-Interscience New York, 1982. 

Pretreatment of hair samples also includes an extraction, usually with an alkaline sodium hydroxide solution, followed by cleaning up with LLE with n-hexane/ethyl acetate. Instead of LLE, the employment of SPE is also possible. Furthermore, the solid phase microextraction (SPME) in combination with head-space analysis is usable [104-106]. In the case of using hair samples, possible external contamination (e.g., by passive smoking of Cannabis) has to be considered as false positive result. False positive results can be avoided by washing of the hair samples previous to extraction [107]. Storage of collected samples is another important fact that can cause false results in their content of A9-THC and metabolites [108-110]. 

Steichen RJ. 1976. Modified solution approach for the gas chromatographic determination of residual monomers by head-space analysis. Anal Chem 48 1398-1402. 

Scudamore KA. 1985. Determination of methyl bromide in grain using head- space analysis-method 20. lARC Sci PubI 68 375-380. 

In recent years, the abuse of y-hydroxybutyrate (GHB) has risen dramatically (LeBeau et al., 2000). This metabolite of GABA can be difficult to analyze because of its short half-life and endogenous presence. However, a simple and fast head space analysis using GC-FID with MS confirmation has been published (LeBeau et al., 2000). 

Friant, S.L. and Suffet, I.H. Interachve effects of temperature, salt concentration, and pH on head space analysis for isolahng volatile trace organics in aqueous environmental samples. Anal. Chem., 51(13) 2167-2172, 1979. 

Determination of residual solvents and volatile impurities by head space analysis... 

Which of the following capillary columns would be most suitable for use in the determination of residual solvents by head space analysis (consult Table 11.1) ... 

Another form of head space analysis uses a purge trapping device to trap volatile impurities. In this technique a gas, e.g. helium, is bubbled through the sample which is dissolved in suitable solvent (usually water) and the volatile impurities are thus stripped from the solution and passed in the stream of gas through a polymeric adsorbant where they become trapped and thus concentrated. The stream of gas is then switched so it passes in reverse direction through the polymeric trap, which is heated to desorb the trapped volatiles and the gas stream is then diverted into the GC. This type of procedure is used in environmental analysis to concentrate volatiles in water which are present at low levels. 

The direct gas-chromatographic method is especially suited as an analytical tool for enantiomer analysis when no sample derivatization is required. In the absence of diastereomeric effects between enantiomers ( EE-effect )25, the chiral compound may be investigated in situ, i.e., without isolation and purification using a minute amount of sample, e.g., by head-space analysis with 10 9 g using flame ionization detector (FID) or 1 (F 11 g using selected ion monitoring (GLC-MSSIM). 

Taylor, A.J., Sivasundaram, L.R., Linforth, R.S.T., Surawang, S. (2003) Time-resolved head-space analysis by proton-transfer-reaction mass-spectrometry. In Deibler, K.D., Delwiche, J. (eds) Handbook of Flavor Characterization. Sensory Analysis, Chemistry and Physiology. Dekker, New York, pp 411-422. 

Snow, N.H. and G.C. Slack. 2002. Head-space analysis in modem gas chromatography. Trends Anal. Chem. 21 608-617. 

Direct determination of. analytes in a stream of gas (Head Space Analysis -HSA)... 

S. Romano, J. Renner, and P. Leitner, Gas chromatographic determination of residual ethylene oxide by head space analysis, Anal. Chem., 45 2327-2330. 

Perkin Elmer Corporation, Applications of Gas Chromatographic Head Space Analysis, Technical Note 16/1978. 

Today it is becoming increasingly clear that this business-as-usual second reaction was as misguided as the panicky initial response had been. For gradually but surely, the gas chromatograph—which soon expanded its scope of effectiveness by the successive introduction of capillary columns, the mass spectrometer, and quantitative head-space analysis techniques—has profoundly changed the perfumer s daily work. 

Quantification. Gas Chromatography. In blood head-space analysis, detection limit 18 ng/ml, FID—J. M. Christensen et al, Clinica chim. Acta, 1981,116, 389-395. 

Quantification. Gas Chromatography. In blood detection and identification of volatile compounds using head-space analysis and FID—J. D. Ramsey and R. J. Flanagan, J. Chromat., 1982, 240, 423 4. 

Gas Chromatography-Mass Spectrometry. In serum head-space analysis—A. Zlatkis et aL, J. Chromat., 1974,91,379-383. 

Gas Chromatography. In plasma or urine trichloroethanol, ECD—D. J. Berry, Chromat., 1975,107, 107-114. In blood or urine chloral hydrate, trichloroethanol and trichloroacetic acid, head-space analysis, detection limit 500 ng/ml for chloral hydrate and trichloroethanol, ECD—D. D. Breimer et al., J. Chromat., 1974,88, 55-63. 

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