Sample preparation techniques headspace extraction

Isolation of the products from complex matrixes (e.g. polymer and water, air, or soil) is often a demanding task. In the process of stability testing (10 days at 40 °C, 1 h at reflux temperature) of selected plastic additives (DEHA, DEHP and Irganox 1076) in EU aqueous simulants, the additive samples after exposure were simply extracted from the aqueous simulants with hexane [63]. A sonication step was necessary to ensure maximum extraction of control samples. Albertsson et al. developed several sample preparation techniques using headspace-GC-MS [64], LLE [65] and SPE [66-68]. A practical guide to LLE is available [3]. 

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),... 

Miniaturisation of scientific instruments, following on from size reduction of electronic devices, has recently been hyped up in analytical chemistry (Tables 10.19 and 10.20). Typical examples of miniaturisation in sample preparation techniques are micro liquid-liquid extraction (in-vial extraction), ambient static headspace and disc cartridge SPE, solid-phase microextraction (SPME) and stir bar sorptive extraction (SBSE). A main driving force for miniaturisation is the possibility to use MS detection. Also, standard laboratory instrumentation such as GC, HPLC [88] and MS is being miniaturised. Miniaturisation of the LC system is compulsory, because the pressure to decrease solvent usage continues. Quite obviously, compact detectors, such as ECD, LIF, UV (and preferably also MS), are welcome. 

In 2003, Smith reviewed newer sample preparation techniques, including pressurized liquid extraction, solid phase microextractions, membrane extraction, and headspace analysis. Most of these techniques aim to reduce the amount of sample and solvent required for efficient extraction. 

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-... 

For a description of the various sample preparation techniques, such as, solid-phase extraction (SPE), solid-phase microextraction (SPME), headspace and purge and trap (P T), dynamic membrane extraction (DMA) and the different detection methods, the reader is directed to the other chapters of this book and especially the chapter on sample preparation methods. 

Determination of Priority Pollutant Volatile Organic Compounds (VOCs) in Wastewater using Micro-Liquid-Liquid Extraction (pLLE) and Static Headspace (HS) Sample Preparation Techniques Combined with Gas Chromatography... 

Solid-phase microextraction (SPME) is a solvent-free sample preparation technique. The volume of the extraction phase is very small compared to the sample volume. The extraction is not exhaustive, but is based on equilibrium between the sample and the extraction phase, which is located on a fiber. SPME involves an adsorption step of the analyte, from a gas headspace or in a liquid sample (direct immersion), and a desorption step, which often is coupled directly with injection in the analytical system. Although SPME is mainly used in combination with GC, it has also been automated for HPLC. Eigure 9.10 shows a schematic representation of an SPME device. 

This is a simplified representation where n is the amount of analyte extracted, Kfs is the distribution coefficient of the analyte between stationary phase and sample, Vf is the volume of the stationary phase, and Q is the start concentration of the analyte in the sample. The sample volume is not of importance in SPME, since it relies on the equilibrium (nonexhaustive) and the fact that the volume of the stationary phase is very small compared to the sample volume. This makes SPME a good sample preparation technique for in the field sampling. Note that the equilibrium is obtained much faster when performing headspace sampling compared to immersion sampling. This is due to the faster movement of the analytes in the gas phase compared to that in the liquid phase. 

While liquid-liquid, headspace, and sorbent-based extractions are perhaps the most commonly nsed and pnbhshed sample preparation techniqnes for GC, there are numerous additional techniques to consider. While we do not attempt to fully describe every technique that has ever been nsed, the techniques described below are certainly of importance in the arsenal of sample preparation techniques for GC. These include supercritical-fluid extraction, accelerated solvent extraction, microwave-assisted extraction, pyrolysis, thermal desorption, and membrane-based extractions, pins comments on antomation and derivatization. 

Solid-phase microextraction (SPME) is a recent sample preparation technique for trace analysis by GC (12). It is a simple, solvent-free method that uses a polar or nonpolar coated fused-silica fiber to directly extract analytes from various matrices (usually aqueous). It can be used in a headspace mode as well. After the fiber is removed from the sample, it is transferred to the heated inlet of a chromatographic system and the analytes are thermally desorbed for analysis. The technique works well for the analysis of trace analytes in water or urine. It has been applied in the field of forensic science in the analysis of fire debris, explosives, and drugs in biological fluids (13-15). 

Figure 3 GC/MS chromatograms of tomato volatiles obtained by three different sample preparation techniques (A) dynamic headspace on Tenax (B) headspace SPME (PDMS) and (C) Freon liquid-liquid extraction. Peak identification is as follows (1) 3-methylbuta-nal (2) l-penten-3-one (3) hexanal (4) (Z)-3-hexenal (5) 2-methyl butanol (6) (E)-2-hexenal (7) ( )-2-heptenal (8) 6-methyl-5-hepten-2-one (9) (Z)-3-hexenol (10) 2-isobu-tylthiazole (11) phenylacetaldehyde (12) methyl salicylate (13) geranylacetone (14) 2-phenylethanol (15) 3-ionone (IS) 2-octanone internal standard.

Figure 4 GC/MS chromatograms of pickle volatiles obtained by three different sample preparation techniques (A) dynamic times headspace on Tenax with a 30-m X 0.25-inm-i.d. DB-5 column, film thickness = 1 [tm (B) sohd-phase extraction (with cartridges) with a 30-m X 0.25-mm-i.d. FFAP column, film thickness = 0.25 im and (C) headspace SPME (75- xm Carboxen/PDMS fiber) using same analytical column as in the dynamic headspace method. Peak identities appear in Table 4.

SPME is a rapid, solventless extraction/concentration technique that affords significantly lower detection levels for higher molecular weight/higher boiling point compounds than static headspace. Its many advantages over other sample preparation techniques for flavor, fragrance, and odor analysis have been pointed out in numerous chapters in this book. 

Liquid (solvent) extraction is not the only way of sample preparation, but stands along with various forms of heat extraction (headspace, thermal desorption, pyrolysis, etc.) and with laser desorption techniques. 

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... 

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. 

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. 

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]. 

SPME is a patented sample preparation method for GC applications (32-36). The solvent-free technique was developed in 1989 by Janusz Pawliszyn (http. /Avww.science.uwaterloo.ca/ -janusz/spme.html) at the University of Waterloo in Ontario, Canada, and a manual device made by Supelco, Inc. has been available since 1993. In 1996, Varian Associates, Inc., constructed the first SPME autosampler. SPME involves exposing a fused silica fiber that has been coated with a non-volatile polymer to a sample or its headspace. The absorbed analytes are thermally desorbed in the injector of a gas chromatograph for separation and quantification. The fiber is mounted in a syringe-like holder which protects the fiber during storage and I netration of septa on the sample vial and in the GC injector. This device is operated like an ordinary GC syringe for sampling and injection. The extraction principle can be described as an equilibrium process in which the analyte partitions between the fiber and the aqueous phase. 

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