Headspace limitations

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). 

Table 4.25 lists the main characteristics of headspace sampling. In HS-GC sample preparation is very often limited to placing a sample in a vial. Sample extraction, clean-up and preconcentration are not necessary. Elimination of solvents in preparing samples for GC has several benefits ... 

For odour analysis the most valuable detectors are FID, AED and MSD. NPD and ECD detectors are very sensitive, but limited in use to nitrogen or halogen containing compounds TCD and FUR detectors are not sensitive enough for odour analysis by direct headspace autosampling. 

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... 

GC/FPD has been used to measure hydrogen sulfide, free disulfide, and dissolved metal sulfide complexes in water (Radford-Knoery and Cutter 1993). Hydrogen sulfide was measured in the headspace of the sample (100 mL) with a detection limit of 0.6 pmol/L. A detection limit of 0.2 pmol/L was obtained for total dissolved sulfide. This method allows for the determination of the concentration of free sulfide that is in equilibrium with hydrogen sulfide. Complexed sulfide can be estimated from the difference between total dissolved sulfide and free sulfide. 

Figure 10.7 Total ion current chromatograms obtained after headspace SPME for (a) incense from Mount Athos and (b) B. papyrifera olibanum. Peak labels correspond to compound identification given in Table 10.3. The occurrence of the following biomarkers of B. papyrifera olibanum in the incense from Mount Athos are a clear indication of its botanical origin n octanol (18), n octylacetate (40), incensole (127), incensole acetate (129), incensole oxide (130) and incensole oxide acetate (131). Artefacts. Reproduced from S. Hamm, J. Bleton,). Connan, A. Tchapla, Phytochemistry, 66, 1499 1514. Copyright 2005 Elsevier Limited...

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. 

Few well characterized, validated methods are available for the determination of w-hexane in blood. A purge-and-trap method for volatiles has been developed and validated by researchers at the Centers for Disease Control and Prevention (CDC) (Ashley et al. 1992, 1994). Extension of the method to include /7-hexane should be possible. Current analytical methods utilize capillary GC columns and MS detection to provide the sensitivity and selectivity required for the analysis. Detection limits are in the low ppb range (Brugnone et al. 1991 Schuberth 1994). Headspace extraction followed by GC analysis has also been utilized for the determination of /7-hexanc in blood (Brugnone et al. 1991 Michael et al. 1980 Schuberth 1994) however, very little performance data are available. 

Some methods are available for determining -hexane in urine and tissues. A modified dynamic headspace extraction method for urine, mother s milk, and adipose tissue has been reported (Michael et al. 1980). Volatiles swept from the sample are analyzed by capillary GC/FID. Acceptable recovery was reported for model compounds detection limits were not reported (Michael et al. 1980). A solvent extraction procedure utilizing isotope dilution followed by GC/MS analysis has been reported for tissues (White et al. 1979). Recovery was good (104%) and detection limits are approximately 100 ng/mL (White etal. 1979). 

Dynamic headspace-extraction stripping and purge-and-trap methodology are used most often for determination of M-hcxanc in water and hazardous wastes. Dynamic headspace extraction techniques have been applied to water samples (Roberts and Burton 1994) and sediment (Bianchi et al. 1991). Detection limits of 0.5 g/L were reported for lake water (Roberts and Burton 1994) and 20 ng/kg (ppt) for sediment (Bianchi et al. 1991). Supercritical fluid extraction (SFE) is a relatively new technique that has been applied to -hcxane in soil (Yang et al. 1995). Membrane extraction of M-hexane from water samples has been developed to provide online, continuous monitoring (Wong et al. 1995 Xu and Mitra... 

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). 

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