Volatile organic compounds headspace sampling

Volatilization Loss of volatile organic compounds Proper sampling techniques, headspace-free sample containers, cold storage... 

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. 

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. 

Various sample enrichment techniques are used to isolate volatile organic compounds from mammalian secretions and excretions. The dynamic headspace stripping of volatiles from collected material with purified inert gas and trapping of the volatile compounds on a porous polymer as described by Novotny [3], have been adapted by other workers to concentrate volatiles from various mammalian secretions [4-6]. It is risky to use activated charcoal as an adsorbent in the traps that are used in these methods because of the selective adsorption of compounds with different polarities and molecular sizes on different types of activated charcoal. Due to the high catalytic activity of activated charcoal, thermal conversion can occur if thermal desorption is used to recover the trapped material from such a trap. 

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. 

Various other workers have reported on the determination of volatile organic compounds in soils [186,187] and landfill soils [188]. Soil fumigants such as methyl bromide have also been determined by this technique [189]. Trifluoroacetic acid is a breakdown product of hydrofluorocarbons and hydrochlorofluorocarbon refrigerant products in the atmosphere and, as such, due to the known toxicity of trifluoroacetic acid, it is important to be able to determine it in the atmosphere, water and in soil from an environmental point of view [190]. In this method the trifluoroacetic acid is extracted from the soil sample by sulfuric acid and methanol, which is then followed by the derivatisation of it to the methyl ester. The highly volatile methyl ester is then analysed with a recovery of 87% using headspace gas chromatography. Levels of trifluoroacetic acid in soil down to 0.2 ng/g can be determined by the procedure. 

Kawata et al. [ 128] have described the effects of headspace conditions on recoveries of volatile organic compounds from sediments and soils. Hewitt [129] compared three vapour partitioning headspace and three solvent extraction methods for the preparation of soil samples for volatile organic carbon determination in soils. Methanol extraction was the most efficient method of spiked volatile organic carbon recovery, which depended on the soil organic carbon content, the octanol-water partitioning coefficients of analytes and the extraction time. 

James and Stack [131] found that solid-phase microextraction is an effective technique for determining volatile organic compounds in landfill sites. The headspace above the sample was sampled. 

Fig.8 FT-IR gas analysis system utilizing a 1/2 turn coiled HWG and a headspace sample vial. VOC volatile organic compound [46]...

Static headspace extraction is also known as equilibrium headspace extraction or simply as headspace. It is one of the most common techniques for the quantitative and qualitative analysis of volatile organic compounds from a variety of matrices. This technique has been available for over 30 years [9], so the instrumentation is both mature and reliable. With the current availability of computer-controlled instrumentation, automated analysis with accurate control of all instrument parameters has become routine. The method of extraction is straightforward A sample, either solid or liquid, is placed in a headspace autosampler (HSAS) vial, typically 10 or 20 mL, and the volatile analytes diffuse into the headspace of the vial as shown in Figure 4.1. Once the concentration of the analyte in the headspace of the vial reaches equilibrium with the concentration in the sample matrix, a portion of the headspace is swept into a gas chromatograph for analysis. This can be done by either manual injection as shown in Figure 4.1 or by use of an autosampler. 

Headspace-GC-MS analysis is useful for the determination of volatile compounds in samples that are difficult to analyze by conventional chromatographic means, e.g., when the matrix is too complex or contains substances that seriously interfere with the analysis or even damage the column. Peak area for equilibrium headspace gas chromatography depends on, e.g., sample volume and the partition coefficient of the compound of interest between the gas phase and matrix. The need to include the partition coefficient and thus the sample matrix into the calibration procedure causes serious problems with certain sample types, for which no calibration sample can be prepared. These problems can, however, be handled with multiple headspace extraction (MHE) [118]. Headspace-GC-MS has been used for studying the volatile organic compounds in polymers [119]. The degradation products of starch/polyethylene blends [120] and PHB [121] have also been identified. 

In this technique, an aliquot of sample (10 ml) is placed in a septum vial (20 ml) to a maximum of 50% capacity. The vial is spiked with surrogates and then heated for a moderate period ( 30 min) to create an equilibrium for volatile organic compounds between the air phase (headspace) and the water. The headspace is sampled (20—100 pi) with an airtight syringe and injected into a GC. Analytes are similar to... 

Antoine et al. 1986 Ashley et al. 1992, 1994 Michael et al. 1980). Recent improvements in the method have resulted in excellent sensitivity (300 ppt) and acceptable precision and accuracy (Ashley et al. 1992, 1994). The purge-and-trap method has also been used to analyze breast milk for other volatile organic compounds and could be used for analyzing benzene in breast milk (Michael et al. 1980). For headspace analysis, the samples are placed in a special vial, and the gas generated above the liquid sample under equilibrium conditions is analyzed (Gruenke et al. 1986 Pekari et al. 1989). 

Examples of the application of headspace extraction are flavors in food products, volatile organic compounds in soils, and residual solvents in pharmaceutical products [33, 34]. The main advantages of headspace extraction are minimal sample preparation and the possibility for direct introduction of headspace gas into the gas chromatograph. 

Uehori and co-workers (1987) developed a retention index in GC to screen and quantify volatile organic compounds in blood. A dynamic headspace analyzer and GC/FID with retention indices were employed for the detection of 1,1-dichloroethane at nanogram levels. Uehori and co-workers noted that this method is simple, reliable and requires little or no sample preparation. 

Reinert KH, Hunter JV, Sabatino T. 1983. Dynamic heated headspace analyses of volatile organic compounds present in fish tissue samples. J Agric Food Chem 31 1057-1060. 

To reiterate, the choice of coating thickness is important in the analysis of organic analytes. For example, thick coatings (100 pm) are useful for volatile organic compounds because they have low values of K thus, more capacity is obtained for the isolation. An example method is EPA Method 624 for volatiles (Supelco). The sample may be analyzed either in the headspace or in the solution. When the headspace method is used, salt is added to the sample to the saturation point. This step helps to drive the compounds into the vapor phase. 

D.J. Tranthim-Fryer, R.C. Hansson, K.W. Norman, Headspace/solid-phase microexlraction/gas chromatography-mass spectrometry a screening technique for the recovery and identification of volatile organic compounds (VOC s) in postmortem blood and viscera samples, J. Forensic Sci., 46, 934-946 (2001). 

It is important that soil samples are taken in appropriate sample bottles/ containers. Samples for volatile organic compounds should be taken in sealed glass vials, filled to the top to minimise any headspace, to ensure that volatile components cannot escape. Also special precautions have to be taken for other labile and unstable parameters. Prior to commencing a site investigation, risk assessors should liaise with their analytical laboratory to establish what sample containers should be used. 

For GC analysis, HS is a preconcentration technique particularly suitable for the sampling of volatile organic compounds in air, water, and solids. Few reports have been published on the use of static headspace in the analysis of free amines in aqueous samples because of the high polarity and solubility in water of these compounds." In one experiment," static headspace preconcentration was developed for the gas chromatographic analysis of aliphatic amines in aqueous samples. A liquid-gas ratio of 1, an incubation temperature of 80°C (15 min), a pH of 13.7, and a mixture of salts (NaCl and K2SO4) at saturation concentration gave a maximal headspace amine concentration (Table 11.4). 

Determination of Priority Pollutant Volatile Organic Compounds (VOCs) in Wastewater Comparison of Sample Preparation Methods— pLLE versus Statis Headspace Sampling... 

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