Headspace sampling, analytical method Applications

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. 

Preinjection sample preparation is not a chromatographic issue per se. Nevertheless, it is an important consideration in the successful application of a complete analytical process. Nerin et al. reviewed sample treatment techniques applicable to polymer extract analysis, including headspace methods, supercritical fluid extraction, and solid phase microextraction. 

Several papers investigated the use of SPME for VFA analysis in wastewater and in air. Briefly, a fiber is exposed to the sample headspace or inserted directly into the sample. Analytes adsorb onto the fiber and are subsequently desorbed at high temperatures in the GC injection port. SPME is a solvent-free technique which introduces less potential contaminants into the GC compared to direct injections. SPME is also rapid since no further sample preparation steps are required. It may be used for routine analysis provided that the specific autosampler required for this method is available and that the optimized method conditions are suitable for autosampler application. Further information on principles and other applications of this technique can be found elsewhere. " " Parameters which have been optimized for VFA analysis are fiber coating, fiber exposure time, sample temperature, sample pH, sample agitation, potential salt addition, and desorption parameters. Surrogate standards employed for VFA analysis were 2-ethylbutyric acids for GC/FID or GC/MS and C-labeled organic acids for GC/MS. The method was optimized using standards in deionized water and only a few wastewater samples were analyzed as examples. 

Solid phase microextraction (SPME) was introduced by Arthur and Pawliszyn over 20 years ago [44]. It is a straightforward, solvent-free, and fast sample extraction method. SPME has become a widely used technique in many areas of analytical chemistry, such as food analysis, environmental sampling, forensics/toxicology, and biological analysis. Recent reviews have been published showing the latest development of this versatile extraction method [45 8]. SPME is based on the partition of the analyte between the extraction phase and the matrix. The technique can be used to monitor analytes in liquid samples or in the headspace and is basically compatible with HPLC and CE, but most applications are made by GC. As indicated by its name, it is not an exhaustive extraction technique and only a fraction of the target analyte is actually extracted. The quantity of analyte extracted is proportional to its concentration, as long as the equilibrium between the analyte in the fiber and the sample is reached. It provides linear results for wide concentrations of analytes (typically from levels of parts per million to parts per billion). 

Because liquid and headspace sampUng methods differ in kinetics, the two approaches are complementary. Equilibrium is attained more rapidly in headspace SPME than in direct-immersion SPME, because there is no liquid to hinder diffusion of the analyte onto the stationary phase. For a given sampling time, immersion SPME is more sensitive than HS-SPME for analytes predominantly present in the liquid. The reverse is true for analytes that are primarily in the headspace. Several additional factors can affect SPME and do influence the choice between immersion and headspace sampling [997]. Overall extraction with HS-SPME is apt to be lower than in direct-immersion because transfer of analytes from the sample to the gas phase seldom is quantitative. HS-SPME was compared with PT [998] and HS-GC-MS [954,999]. Application of HS-SPME eliminates many problems of other headspace techniques and extends headspace sampling to less volatile compounds due to the concentration effect at the fibre coating. Thermal desorption... 

Using immersion and headspace sampling, we show in Figs. 8 through 12 that the SPME method can also be used for fragrance analysis and is applicable to a wide variety of sample types. With the aid of SPME technique, analytical chemists will be freed from the complex and time-consuming classic sample cleanup and preparation procedures that are currently used. 

The principles behind MAP liquid-phase and gas-phase extractions are fundamentally similar and rely on the use of microwaves to selectively apply energy to a matrix rather than to the environment surrounding it. MAP gas-phase extractions (MAP-HS) give better sensitivity than the conventional static headspace extraction method. MAP-HS may also be applied in dynamic applications. This allows the application of a prolonged, low-power irradiation, or of a multi-pulse irradiation of the sample, thus providing a means to extract all of the volatile analytes from the matrix [477]. 

The methods that generally are used to remove volatile organic chemicals (VOCs) from biological samples for analysis are applicable to chlorobenzene. These include headspace analysis, purge-and-trap (gas stripping) collection from aqueous solutions or slurry samples, solvent extraction, and direct collection on resins. Headspace analysis offers speed, simplicity, and good reproducibility for a particular type of sample. However, partitioning of the analyte between the headspace and the sample matrix is dependent upon the nature of the matrix and must be determined separately for different kinds of matrices (Walters 1986). 

In the next sections, the mean applications of static headspace gas chromatography will be described, with an emphasis in methods that could be performed without extensive modification of the equipment commonly present in the forensic toxicology laboratories, in any case, analytical considerations will be discussed, from sampling, materials and reactants needed, analysis, to interpretation of results, method validation and the importance of these test in the legal media, will be reviewed. 

The equipment used for headspace analysis in its simplest form may comprise a vial of suitable volume with a septum turn closure through which the sample may be drawn using a gas-tight syringe. With the use of a means of temperature control, this method of sampling headspace volatiles can be effective for simple applications where a qualitative or semiquantitative analytical screening procedure is required. 

This chapter describes the application of SPME to develop a rapid, sensitive analytical technique for analyzing the volatile compounds found in the headspace of rice. A key requirement of this methodology is the heating of the sample in order to produce sufficient amounts of analytes to be successfully analyzed. Analysis of less than 1-g samples of milled or brown rice kernels is possible. The key odorant in fragrant rice is 2-AP, and recoveries are enhanced by the addition of water. However, the recovery of other compounds may be suppressed by the addition of water, as observed with the internal standard TMP. This method is readily amenable for the analysis of additional compounds once they have been identified as having an impact on the sensory quality of rice. 

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