Analysis headspace

Headspace analysis avoids the need for solvent extraction for volatile analytes. 

In Chapter 18, we described solvent extraction and solid-phase extraction sample preparation methods, which are applicable to GC analyses as well as others. A convenient way of sampling volatile samples for GC analysis is the technique of head-space analysis. A sample in a sealed vial is equilibrated at a fixed temperature, for example, for 10 min, and the vapor in equilibrium above the sample is sampled and injected into the gas chromatograph. A typical 20-mL glass vial is capped with a silicone rubber septum lined with polytetrafluoroethylene (PTFE). A syringe needle can be inserted to withdraw a 1-mL portion. Or the pressurized vapor is allowed to expand into a 1-mL sample loop at atmospheric pressure, and then an auxiliary carrier gas carries the loop contents to the GC loop injector. Volatile compounds in solid or liquid samples can be determined at parts per million or less. Pharmaceutical tablets can be dissolved in a water-sodium sulfate solution 

Headspace analysis is a method of choice for the determination of volatile compounds in polymers. The principle of the analysis is quite simple. The sample is placed in a container leaving a large headspace, which is filled with an inert gas (sometimes under pressure) that also serves as the GC carrier gas. Under the prevailing equilibrium conditions a proportion of the volatiles in the sample transfers to the gas-filled headspace, which is then withdrawn and analysed by GC. 

Some of the applications of headspace analysis include the determination of vinyl chloride and other impurities in PVC, styrene monomer in PS, methyl methacrylate monomer in polyacrylates, ethylene in polyethylene, acrylonitrile monomer in ABS terpolymers, epichlorohydrin in epoxy resins, and residual solvents in polymers (see next). 

A glass ignition tnbe is supported as shown in a Wade 0.8 cm diameter brass coupling unit, covered with a silicone rubber septum and sealed with a Wade 0.8 cm brass stop-end body. The stop-end body has two 1 mm diameter holes drilled through the cap. The whole unit is placed in a slot in a cylindrical copper block which is heated to 300 C. 

A sample of the polymer (0.25-0.50 g) is placed in an ignition tube and sealed with Wade fittings and a septum, as described previously. The tube is then heated in the copper block under the required conditions of time and temperature. A sample of the headspace gas is withdrawn from the ignition tube into a Hamilton gas-tight hypodermic syringe via the septum and injected into a GC. 

Using this method several PS from different manufacturers were heated at 200 °C for 15 minutes under helium, and the liberated volatiles were examined by GC. All the samples liberated the same range of aromatic hydrocarbons, these differing only in their relative 

The headspace analysis procedure is simple the food is sealed in a container, then brought to the desired temperature and left for a while to establish an equilibrium between volatiles bound to the food matrix and those present in the vapor phase. A given volume of the headspace is withdrawn with a gas syringe and then injected into a gas chromatograph equipped with a suitable separation column (static headspace analysis). Since the water content and an excessively large volume of the sample substantially reduce the separation efficiency of gas chromatography, only the major volatile compounds are indicated by the detector. The static headspace analysis makes an important contribution when the positions of the aroma sub- 

More material is obtained by dynamic head-space analysis or by solid phase microextraction (SPME). In the dynamic procedure the headspace volatiles are flushed out, adsorbed and thus concentrated in a polymer, as outlined in 5.2.1.2. However, it is difficult to obtain a representative sample by this flushing procedure, a sample that would match the original headspace composition. A model system assay (Fig. 5.7) might clarify the problems. Samples (e) and (f) were obtained by adsorption on different polymers. They are different from each other and differ from sample (b), which was obtained 

SPME is based on the partitioning of compounds between a sample and a coated fiber immersed in it. The odorants are first adsorbed onto the fiber (e. g. nonpolar polydimethylsilo-xane or polar polyacrylate) immersed in a liquid food, a food extract or in the headspace above a food sample for a certain period of time. After adsorption is completed, the compounds are thermally desorbed into a GC injector block for further analysis. 

Particularly in food applications headspace SPME is preferred to avoid possible contamination of the headspace system by non-volatile food components. Also SPME analysis is quite sensitive to experimental conditions. In addition to the stationary phase, sample, volume concentration of odorants, sample matrix and uniformity as well as temperature and time of the adsorption and desorption processes influence the yield. In quantitative SPME analysis these influences are eliminated by the use of labelled internal standards (cf. 5.2.6.1). 

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Vreugdenhil A J and Butler I S 1998 Investigation of MMT adsorption on soils by diffuse reflectance infrared spectroscopy DRIFTS and headspace analysis gas-phase infrared spectroscopy HAGIS Appl. Organomet. Chem. 

Figure 10. Headspace analysis of poly foil pouches...

Blood Thermally decarboxylated subjected to static headspace analysis GC/ECD (for metabolite trichloroacetic acid) 2 ppb 101-109 Ziglio et al. 1984... 

Sample in sealed vial subjected to static headspace analysis... 

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. 

Analysis of soils and sediments is typically performed with aqueous extraction followed by headspace analysis or the purge-and-trap methods described above. Comparison of these two methods has found them equally suited for on-site analysis of soils (Hewitt et al. 1992). The major limitation of headspace analysis has been incomplete desorption of trichloroethylene from the soil matrix, although this was shown to be alleviated by methanol extraction (Pavlostathis and Mathavan 1992). 

Boekhold AJ, Van der Schee HA, Kaandorp BH. 1989. [Rapid gas-chromatographic determination of trichloroethylene and/or tetrachloroethylene in lettuce by direct headspace analysis.] Z Lebensm Unters Forsch 189 550-553. (German)... 

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

Figure 8.27 A, apparatus for dynaaic headspace analysis of urine with sorbent trapping. B, gas phase stripping apparatus (purge-and-trap).

H. Hachenberg and A. P. Schmidt, "Gas chromatographic Headspace Analysis", Heyden, l4>ndon, UK, 1977. 

B. V. Ioffe and A. G. Viltenberg, "Headspace Analysis and Related Methods in Gas Chromatography", Hiley, New York, NY, 1984. 

Principles and Characteristics Pare et al. [475] have patented another approach to extraction, the Microwave-Assisted Process (MAP ). In MAP the microwaves (2.45 GHz, 500 W) directly heat the material to be extracted, which is immersed in a microwave transparent solvent (such as hexane, benzene or iso-octane). MAP offers a radical change from conventional sample preparation work in the analytical laboratory. The technology was first introduced for liquid-phase extraction but has been extended to gas-phase extraction (headspace analysis). MAP constitutes a relatively new series of technologies that relate to novel methods of enhancing chemistry using microwave energy [476]. 

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. 

Headspace analysis was reviewed [132], including the development of the PT technique [211]. Applied HS-GC has been dealt with in several monographs [207,212],... 

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. 

Principles and Characteristics Extraction or dissolution methods are usually followed by a separation technique prior to subsequent analysis or detection. While coupling of a sample preparation and a chromatographic separation technique is well established (Section 7.1), hyphenation to spectroscopic analysis is more novel and limited. By elimination of the chromatographic column from the sequence precol-umn-column-postcolumn, essentially a chemical sensor remains which ensures short total analysis times (1-2 min). Examples are headspace analysis via a sampling valve or direct injection of vapours into a mass spectrometer (TD-MS see also Section 6.4). In... 

H. Hachenberg and A.P. Schmidt, Gas Chromatographic Headspace Analysis, Heyden, London (1977). 

Johnson B., Headspace analysis and shelf life, Cereal Foods World 1997 42 752-754. 

Garbarini D, Lion L (1985) Evaluation of sorptive partitioning of nonionic pollutants in closed systems by headspace analysis. Environ Sci Technol 19 1122-1128... 

Perlinger, J.A., Eisenreich, S J., Capel, P.D. (1993) Apphcation of headspace analysis to the study of sorption of hydrophobic organic chemicals to a-Al203. Environ. Sci. Technol. 27, 928-937. 

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