Headspace Analysis - Volatiles

Headspace GLC analysis is a method used to monitor a vapour over a polymeric matrix. It is a very effective technique, but may require more time and effort than direct injection. This method can be performed manually, when a vial containing the monomer is heated, an equilibrium is established, for volatile compounds between the sample and the headspace above it. Because no dissolution step is required, sample viscosity problems and loss of response due to dilution are eliminated. Automated headspace analysis units are available from instrument manufacturers, as well as multiple extraction systems. Any analytically useful headspace method must obey Henry s law  

There are two basic principal methods of headspace analysis in polymers. In one method a solution of the polymer is examined, in the other method solid polymer is examined directly. When working with polymer solutions, headspace equilibrium is more readily obtained than the solid approach and the calibration procedure is simplified. 

In the solid polymer approach the residual monomer is partitioned between the headspace and the polymer phase. The monomer concentration in the head space is determined and the original concentration in the polymer is calculated. Since polymer standards of known monomer content are not readily available, the headspace monomer concentration must be related to the original concentration in the polymer either by assuming 100% diffusion of the constituent into the head space or through equilibrium calculations utilising Henry s Law and the appropriate partition coefficient. Berens [1] determined this coefficient for the vinyl chloride-PVC system and applied this to the determination of vinyl chloride in PVC resin powder. Equilibration of residual vinyl chloride with the headspace occurred within one hour when PVC powder was heated to 90 °C. 

A good example of the solution approach is that of Crompton and Myers [2] who carried out gas chromatographic analyses out on solutions of polystyrene in the presence of internal standards. To avoid interferences in the analysis, it is essential for the solvent and the internal standard to have retention times different from those of the volatile compounds being determined in the polymer. Application of the volatiles apparatus described by Crompton and Myers [2] to a polystyrene provides a rapid means of determining the retention time of the volatile compounds present in the polymer, enabling a suitable solvent and internal standard to be selected for the subsequent quantitative analysis by solution procedures. 

Crompton and Myers [2] used the solid polymer approach in their simple and inexpensive apparatus for liberating both existing volatiles and those produced by thermal degradation from polymers by heating at temperatures up to 300 °C, in the absence of solvent, prior to their examination by gas chromatography. The technique avoids the disadvantages resulting from the use of extraction or solution procedures. 

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Static Headspace Analysis Volatile Priority Pollutants I 517... 

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. 

Headspace analysis has also been used to determine trichloroethylene in water samples. High accuracy and excellent precision were reported when GC/ECD was used to analyze headspace gases over water (Dietz and Singley 1979). Direct injection of water into a portable GC suitable for field use employed an ultraviolet detector (Motwani et al. 1986). While detection was comparable to the more common methods (low ppb), recovery was very low. Solid waste leachates from sanitary landfills have been analyzed for trichloroethylene and other volatile organic compounds (Schultz and Kjeldsen 1986). Detection limits for the procedure, which involves extraction with pentane followed by GC/MS analysis, are in the low-ppb and low-ppm ranges for concentrated and unconcentrated samples, respectively. Accuracy and precision data were not reported. 

Pavlostathis SG, Mathavan GN. 1992. Application of headspace analysis for the determination of volatile organic compounds in contaminated soils. Environ Technol 13 23-33. 

Advantages and disadvantages of HS-GC over regular GC are summarised in Table. 4.26. HS-GC fingerprinting chromatograms obviously include only the volatile components present and do not provide a complete picture of sample composition on the other hand, when solvent extraction is used, all the soluble sample constituents are removed, including also those having no appreciable vapour pressure at the equilibration temperature. Headspace analysis enhances the peaks of volatile trace components. 

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. 

Direct methods of analysis such as distillation [158,167,168], liquid-liquid extraction [ 159,169], headspace analysis [ 170-172], dynamic headspace analysis [157,173-178], and direct injection [179] have been used mainly for specific volatile components. 

In this method volatile organic matter in seawater is concentrated on a Tenax GC solid adsorbent trap and dry-ice trap in series. The trapped organic material is then desorbed and oxidised to carbon dioxide, which is measured with a non-dispersive infrared analyser. A dynamic headspace method was used for the extraction with the assistance of nitrogen purging. Dynamic headspace analysis [184] is an efficient extraction procedure. The efficiency of extraction... 

Headspace analysis involves examination of the vapours derived from a sample by warming in a pressurized partially filled and sealed container. After equilibration under controlled conditions, the proportions of volatile sample components in the vapours of the headspace are representative of those in the bulk sample. The system, which is usually automated to ensure satisfactory reproducibility, consists of a thermostatically heated compartment in which batches of samples can be equilibrated, and a means of introducing small volumes of the headspace vapours under positive pressure into the carrier-gas stream for injection into the chromatograph (Figure 4.25). The technique is particularly useful for samples that are mixtures of volatile and non-volatile components such as residual monomers in polymers, flavours and perfumes, and solvents or alcohol in blood samples. Sensitivity can be improved by combining headspace analysis with thermal desorption whereby the sample vapours are first passed through an adsorption tube to pre-concentrate them prior to analysis. 

This is an alternative technique to headspace analysis for the identification and determination of volatile organic compounds in water. The sample is purged with an inert gas for a fixed period of time. Volatile compounds are sparged from the sample and collected on a solid sorbent trap—usually activated carbon. The trap is then rapidly heated and the compounds collected and transferred as a plug under a reversed flow of inert gas to an external gas chromatograph. Chromatographic techniques are then used to quantify and identify sample components. 

Headspace analysis (EPA 3810, 5021) also works well for analyzing volatile petroleum constituents in soil. In the test method, the soil is placed in a headspace vial and heated to drive out the volatiles from the sample into the headspace of the sample container. Salts can be added for more efficient release of the volatile compounds into the headspace. Similar to water headspace analysis, the soil headspace technique is useful when heavy oils and high analyte concentrations are present, which can severely contaminate purge-and-trap instrumentation. Detection limits are generally higher for headspace analysis than for purge-and-trap analysis. 

Roberts, D.D., Pollien, P., and Milo, C. Solid-phase microextraction method development for headspace analysis of volatile flavor compounds, /. Agric. Food Chem., 48(6) 2430-2437, 2000. 

Herzfeld D, van der Gun K, Louw R. 1988. Quantitative determination of volatile organochlorine compounds in water by GC-headspace analysis with dibromomethane as an internal standard. Chemosphere 1425-1430. 

Table III. Amounts of volatile compounds (ng/kg) detected in white bread by dynamic headspace analysis CX)NTROL is without and SOYA is with 30 g soya flour addition (see recipe 15)...

Females of the desert spider Agenelopsis aperta emit a volatile pheromone that attracts conspecihc males (Riechert and Singer, 1995). This pheromone was identified as 8-methyl-2-nonanone (1 Fig. 4.1), a previously unknown arthropod semio-chemical. It was found by headspace analysis and abdominal washings of females 2 weeks after their hnal molt, when they become sexually receptive it was absent in females of other age classes. The pheromone attracted males in a three-choice arena system at doses as low as 500 ng (Papke et al., 2001). Another female-specific ketone, 6-methyl-3-heptanone (2), was not attractive. Very low doses of 1 (10-9 mg/ml applied to a hlter paper placed in empty juvenile female webs) also induced courtship behavior in males (Papke et al., 2001). The normal behavioral sequence was followed, except for phases which required input from the female. The ED50 value (mean effective dose) of 1 was 5.5x 10-4 mg/ml hexane. In contrast, ketone 2 only induced a response in some males at unnaturally high concentrations... 

Agelopoulos, N.G. and Pickett, J. A. (1998). Headspace analysis in chemical ecology effects of different sampling methods on ratios of volatile compounds present in headspace samples. Journal of Chemical Ecology 24 1161-1172. 

Massouras, T., Pappa, E. C., and Mallatou, H. (2006). Headspace analysis of volatile flavor compounds of Teleme cheese made from sheep and goat milk. Int.. Dairy Technol. 59, 250-256. 

Theirry, A., Maillard, M. B., and Le Quere, J. L. (1999). Dynamic headspace analysis of Emmental aqueous phase as a method to quantify changes in volatile flavor compounds during ripening. Int. Dairy. 9, 453-463. 

Volatile organic compounds Comparison of solvent extraction, headspace analysis and vapour partitioning, methanol extraction [88]... 

Headspace analysis is the method of choice for determining volatile organic compounds in soil [178-183]. A limitation of this method is the incomplete desorption of the contaminants in soil-water mixtures, but this problem can be overcome through the addition of methanol to the sample [181]. Good recoveries of volatile organic compounds in soils were obtained via thermal vaporisation of the sample followed by Tenax GC trapping and gas chromatography-mass spectrometry. 

Headspace analysis has been employed in the extraction of dithiocarbamate insecticide in vegetables [227]. Other techniques occasionally used are vacuum distillation followed by gas chromatography-mass spectrometry in the determination of volatile organic compounds in leaves, steam distillation in the determination of organochlorine insecticides in fruit and vegetables [229], and water distillation followed by high-performance liquid chromatography in the determination of 2-aminobutane in potatoes [102,230]. 

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