Headspace extraction techniques

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... 

Currently, the headspace extraction technique is preferred due to the minimal contamination produced to the injector and column of the gas chromatograph this... 

Static HSGC has been used extensively for the determination of VHC in oils, lipids and fatty foods (Fig. 3.15). Quantitation of VHC by an electron-capture detector provides the means to achieve superb detection limits. In aqueous foods the limit of detection has been reported to be 1 ppb, whereas in fatty foods it has been reported to be in the 10-50 ppm range (Entz and Hollifield, 1982). With a multiple headspace extraction technique the detection limits for VHC in butter are even lower (1-5 ppb) (Uhler and Miller, 1988). The absolute detection limit for perchloroethylene in olive oil has been found to be 1 pg (Van Rillaer and Beernaert, 1989). A method for the determination of VHC in olive oil by packed column HSGC is described in EC Regulation 2568/91 (CEC, 1991). Precision data of a capillary column... 

Analysis of Volatile Organic Compounds by Headspace Extraction Techniques... 

In-tube extraction, automated dynamic headspace extraction technique using a packed syringe needle, injection by thermal desorption in a GC injector. 

Uhler and Miller [39] developed a GC multiple headspace extraction technique for the determination of volatile hydrocarbons in butter [39]. In a related procedure to determine styrene and its dimer in milk, acetone was used to precipitate proteins and extract fat and the residues from the packaging material. Using direct injection of these extracts for GC analysis, detection limits were 0.16 mg/kg for styrene and 0.28 mg/kg for the dimer [40]. 

Solid-phase microextraction eliminates many of the drawbacks of other sample preparation techniques, such as headspace, purge and trap, LLE, SPE, or simultaneous distillation/extraction techniques, including excessive preparation time or extravagant use of high-purity organic solvents. SPME ranks amongst other solvent-free sample preparation methods, notably SBSE (Section 3.5.3) and PT (Section 4.2.2) which essentially operate at room temperature, and DHS (Section 4.2.2),... 

Headspace solid phase microextraction (HS-SPME). With this extraction technique, it is possible to concentrate volatile compounds thus allowing their detection even at trace levels, as in the case of volatile and semi-volatile terpenes in archaeological findings [7,31]. Chapter 10 outlines how resinous materials are investigated using HS-SPME-GC/MS. 

Hcxanc can be determined in biological fluids and tissues and breath using a variety of analytical methods. Representative methods are summarized in Table 6-1. Most methods utilize gas chromatographic (GC) techniques for determination of -hexane. The three methods used for preparation of biological fluids and tissues for analysis are solvent extraction, direct aqueous injection, and headspace extraction. Breath samples are usually collected on adsorbent traps or in sampling bags or canisters prior to analysis by GC. 

A new, fast, sensitive, and solventless extraction technique was developed in order to analyze beer carbonyl compounds. The method was based on solid-phase microextraction with on-fiber derivatization. A derivatization agent, 0-(2,3,4,5,6-pentafluorobenzyl) hydroxylamine (PFBOA), was absorbed onto a divinyl benzene/poly(dimethylsiloxane) 65- xm fiber and exposed to the headspace of a vial with a beer sample. Carbonyl compounds selectively reacted with PFBOA, and the oximes formed were desorbed into a gas chromatograph injection port and quantified by mass spectrometry. This method provided very high reproducibility and linearity When it was used for the analysis of aged beers, nine aldehydes were detected 2-methylpropanal, 2-methylbutanal, 3-methylbutanal, pentanal, hexanal, furfural, methional, phenylacetaldehyde, and (E)-2-nonenal. (107 words)... 

Schoenmakers et al. [72] analyzed two representative commercial rubbers by gas chromatography-mass spectrometry (GC-MS) and detected more than 100 different compounds. The rubbers, mixtures of isobutylene and isoprene, were analyzed after being cryogenically grinded and submitted to two different extraction procedures a Sohxlet extraction with a series of solvents and a static-headspace extraction, which entailed placing the sample in a 20-mL sealed vial in an oven at 110°C for 5,20, or 50 min. Although these are not the conditions to which pharmaceutical products are submitted, the results may give an idea of which compounds could be expected from these materials. Residual monomers, isobutylene in the dimeric or tetrameric form, and compounds derived from the scission of the polymeric chain were found in the extracts. Table 32 presents an overview of the nature of the compounds identified in the headspace and Soxhlet extracts of the polymers. While the liquid-phase extraction was able to extract less volatile compounds, the headspace technique was able to show the presence of compounds with low molecular mass... 

SPME is a sample-preparation technique based on absorption that is useful for extraction and concentration of analytes either by submersion in a liquid phase or exposure to a gaseous phase (Belardi and Pawliszyn, 1989 Arthur et al., 1992). Following exposure of the fiber to the sample, absorbed analytes can be thermally desorbed in a conventional GC injection port. The fiber behaves as a liquid solvent that selectively extracts analytes, with more polar fibers having a greater affinity for polar analytes. Headspace extraction from equilibrium is based on partition coefficients of individual compounds between the food and headspace and between the headspace and the fiber coat-... 

Steele et al. [55] used a dynamic heated headspace purge and trap extraction technique with selected ion monitoring capillary GC-MS to measure styrene monomer levels in amounts down to 2 ng/g in tomatoes and milk. 

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. 

The four most common approaches to quantitative HSGC calibration are classical external standard, internal standard, standard addition, and multiple headspace extraction (MHE). The choice of technique depends on the type of sample being analyzed. 

Multiple headspace extraction (MHE) is used to find the total peak area of an analyte in an exhaustive headspace extraction, which allows the analyst to determine the total amount of analyte present in the sample. This technique, along with the mathematical models behind it, was originally presented by McAuliffe [17] and Suzuki et al. [18]. Kolb and Ettre have an in-depth presentation of the mathematics of MHE in their book [15], and the reader is encouraged to reference that work for further information on the mathematical model. 

There are many techniques available for the preparation of volatile analytes prior to instrumental analysis. In this chapter the major techniques, leading primarily to gas chromatographic analysis, have been explored. It is seen that the classical techniques purge and trap, static headspace extraction, and liquid-liquid extraction still have important roles in chemical analysis of all sample types. New techniques, such as SPME and membrane extraction, offer promise as the needs for automation, field sampling, and solvent reduction increase. For whatever problems may confront the analyst, there is an appropriate technique available the main analytical difficulty may lie in choosing the most appropriate one. 

A substantial amount of information on volatiles can be obtained with less than 30 g of each of these samples in a direct DHS/GC/MS analysis. DHS operation sweeps volatile flavors from the surface of food samples in a similar way as we sniff for the volatile flavors of a food. DHS does not require high sampling temperature or solvent for extraction and may be considered as a lcw-artifact arcma sampling technique. The concentrating effect of DHS provides better sensitivity than static headspace sampling. Techniques such as GC-coupled aroma perception and GC/MS identification can be used to complement other approaches in improvement of flavor quality of a variety of products. 

A number of techniques can be used to isolate analytes from water. The technique used will depend on the volatility of the analyte. Volatile compounds (i.e., more volatile than n-C12) can be analyzed using Purge and Trap techniques or by Headspace analysis. Semivolatile compounds are extracted using liquid—liquid or solid phase extraction techniques. 

Maurer has reviewed the application of LC-MS and LC-MS/MS to the detection of alkaloids in human biofluids [14]. Extraction techniques include liquid-liquid extraction relying upon the ionization of alkaloids in aqueous acid, solid phase extraction (SPE) in which alkaloids are cleaned up and concentrated from the biomatrix by adsorption and subsequent elution from a small cartridge of solid phase adsorbent, and solid-phase microextraction (SPME), in which analytes are adsorbed directly from the matrix or the headspace above the heated matrix onto a fine fiber of adsorbent on fused silica. The latter process is more commonly used with GC-M S but is finding increasing use with LC-MS. 

The third alternative is the use of a Solid Phase Microextraction fibre (SPME) to collect the volatiles in the headspace. The technique is clean, very easy to use and provides a good concentration of many volatiles on the headspaces of wine. Because of this, it has been the technique of choice in some recent works (Marti et al. 2003 Fan and Qian 2005 Gurbuz et al. 2006 Tat et al. 2007). Nevertheless, the use of this technique is not exempt from problems either. On the one hand the technique does not provide an extract, and on the other hand it is quite difficult to optimize and validate and therefore to assess the reliability of the results. It should be concluded that, although the technique is appealing, more research is needed in order to establish its advantages and drawbacks. 

Headspace Extraction Headspace (HS) extraction is a well-known method of sample preparation and is frequently used in many laboratories, especially in industrial applications. It involves a partitioning equilibrium between the gas phase and a sample (liquid or solid). In this technique, an aliquot of gas phase is sampled into GC. There are two types of analysis, static and d3Uiamic. In the static version, when the equilibrium is reached, the gas phase is injected into GC. In dynamic analysis, the volatiles are exhaustively extracted by the stream of gas. However, matrix effects result in decreased sensitivity for certain substances, especially polar and hydrophilic samples. A comprehensive book describing HS techniques was presented by Kolb [31]. 

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