Sample preparation static headspace extraction

In static headspace extraction, sample preparation for liquid samples is usually quite simple—most often, the sample can just be transferred to the headspace sample vial and sealed immediately following collection of sample to minimize storage and handling losses [13],... 

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... 

For a compound to contribute to the aroma of a food, the compound must have odor activity and volatilize from the food into the head-space at a concentration above its detection threshold. Since aroma compounds are usually present in a headspace at levels too low to be detected by GC, headspace extraction also requires concentration. SPME headspace extraction lends itself to aroma analysis, since it selectively extracts and concentrates compounds in the headspace. Some other methods used for sample preparation for aroma analysis include purge-and-trap or porous polymer extraction, static headspace extraction, and solvent extraction. A comparison of these methods is summarized in Table Gl.6.2. 

The ease of initial sample preparation is one of the clear advantages of static headspace extraction. Often, for qualitative analysis, the sample can be placed directly into the headspace vial and analyzed with no additional... 

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. 

One of the main advantages of static headspace extraction is how easy the sample preparation is in the case of qualitative analysis, it suffices to place the sample in a vial and seal it with a PTFE septum and an aluminum lid however, for quantitative analysis, it is necessary to xmderstand and optimize the effects of the matrix, in order to obtain good sensitivity and, above all, accuracy. 

Several factors must be optimized in a static headspace extraction in order to obtain a method with the desired extraction sensitivity, reproducibility and efficiency. These factors include the volume of the used vial, the temperature and pressure levels, and how the sample is to be prepared. 

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. 

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]. 

Heat extraction techniques for solid sample preparation in GC are static and dynamic headspace analysis (SHS, DHS, HS-SPME and HSSE), thermal desorption (TD-GC, TD-GC-MS), pyrolysis and thermochromatography. Nomenclature is not unambiguous as to DHS, TD and PT. The terminology purge-and-trap is usually preferred for the simplest dynamic technique in which it is not necessary to subject the sample to either solvents or elevated temperatures. Scheme 2.7 shows the family of headspace sampling techniques. Headspace sorptive extraction (HSSE) and HS-SPME represent high capacity static headspace. 

One of the most elegant possibilities for instrumental sample preparation and sample transfer for GC-MS systems is the use of the headspace technique (Figure 2.9). Here all the frequently expensive steps, such as extraction of the sample, clean-up and concentration are dispensed with. Using the headspace technique, the volatile substances in the sample are separated from the matrix. The latter is not volatile under the conditions of the analysis. The tightly closed sample vessels, which, for example, are used for the static headspace procedure. 

Additional advantages of the static headspace technique include relatively low cost per analysis, simple sample preparation, and the elimination of reagents. Because the sample analytes are not extracted from the sample material using a solvent, there is no need to deal with solvent reduction by evaporation either into the air, with its concerns about pollution, or by recondensing. 

The analysis of volatiles is generally accomplished by an extraction step, followed by concentration, chromatographic separation, and subsequent detection. Well-established methods of analysis include solvent extraction, static and dynamic headspace sampling, steam distillation with continuous solvent extraction, and supercritical fluid extraction. An overview of sample preparation methods is provided by Teranishi (2). The chromatographic profile will vary depending upon the method of sample preparation employed, and it is not uncommon to produce artifacts during this step (3,4). Thermally labile compounds may decompose in the heated zones of instruments to produce a chromatographic profile that is not truly representative of the sample. 

The aim of GC-0 techniques in food aroma research is to determine the relative odor potency of compounds present in the aroma extract. This method gives the order of priority for identification and thus indicates the chemical origin of olfactory differences (7). The value of the results obtained by GC-O depends directly on the effort invested in sample preparation and analytical conditions. Analysis of an aroma extract by dilution techniques (AEDA, Charm) combined with static headspace GC-O provides a complete characterization of the qualitative aroma composition of a food. However, this is only the first step in understanding the complex aroma of a food. 

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. 

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