Static headspace, SPME

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. 

M. E. Miller, J. D. Stuart, Comparison of gas sampled and SPME sampled static headspace for the determination of volatile flavor components, Anal. Chem., 71, 23 27 (1999). 

Due to the volatility of some of the compounds present in food, it is very important to utilize cryogenic cooling when the sample is introduced onto the GC column. This helps to prevent the loss of low-molecular weight volatiles and also tends to focus volatiles on the initial portion of the column, thus allowing for improved separation and quantification. The use of a film thickness of 1.0 mm will also aid in the retention of the aforementioned compounds. In the static headspace procedure, the 4-min pressurization step is also crucial, in that equal pressures between the sample vials and the GC must be attained to ensure reproducible sample injections. Forboth the static and SPME procedures, heating the samples for 30 min prior to injection is important to ensure proper equilibration between the sample and the head-space. 

Because SPME extracts compounds selectively, the response to each compound must be calibrated for quantification. A specific compound can be quantified by using three GC peak area values from solvent injection, static headspace (gas-tight syringe), and SPME. The solvent injection is used to quantify the GC peak area response of a compound. This is used to quantify the amount of the compound in the headspace. The SPME response is then compared to the quantified static headspace extraction. These three stages are necessary because a known gas-phase concentration of most aroma compounds at low levels is not readily produced. A headspace of unknown concentration is thus produced and quantified with the solvent injection. Calibration must be conducted independently for each fiber and must include each compound to be quantified. 

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. 

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. 

Most of the static headspace methods determine the partition coefficient by quantifying volatile concentration above a sample by gas-chromatography. The vapour phase calibration method (VPC) uses an external vapour standard for calibration. One must assure that the pure component is completely vaporized before injection. A widely employed alternative is the Liquid calibration static headspace (LC-SH) method (YoiWey et al. 1991 Nedjma 1997). A third approach uses HS-SPME. SPME may be used to determine partition coefficients if short sampling times are applied the process must only sample the headspace and not disrupt the equilibrium (Jung and Ebeler 2003). This method has become very popular to study the effect of wine macromolecules on the liquid-vapor equilibrium, (Whiton and Zoecklein 2000 Escalona et al. 2002 Hartmann et al. 2002 Aronson and Ebeler 2004). 

Because SPME is a static extraction technique, the need for a large surface area is no longer as critical as in SPE. Smooth liquid coatings can be used that avoid plugging. Also, by sampling from headspace, SPME can extract analytes from highly complex matrices such as sludge. 

SPME can also be used to extract target analytes from food and drug samples. Thus, it has been employed for the extraction of caffeine from coffee and tea [225], and for that of volatile impurities in drugs. Headspace SPME has also been tested for flavour analysis in foods. Thus, the SPME/GC/TOF-MS tandem was successfully used for the rapid analysis of volatile flavour compounds in apple fruit. The sample (300-450 g of apple) was subjected to static headspace sampling for 4 6 h in order to allow the volatiles... 

Figure 5 shows the dynamic response of each sensor in the array as it senses the volatiles desorbed from the fibre. The general profile reflects a combination of factors inherent within the measurement system namely desorption characteristics of the volatile species from the SPME fibre, diffusion of the volatiles through the static headspace between the fibre and the sensors and the response characteristics of the sensors to the volatiles. Different portions of the general response profiles are potentially of use in terms of resolving differences between samples and thus classifying sample types. 

Flores Menendes, J. C., Fernandez Sanchez, M. L., Femmdez Martinez, E., Sanchez Uria, J. E., and Sanz-Medel, A., Static Headspace versus head space SPME(HS SPME) for the determination of volatile organochlorine compounds in landhll leachates hy gaschromatography, Talanta, 63, 809-814, 2004. 

Several others techniques dealing with the injection problems have been developed. Among them the solid-phase microextraction method (SPME) and the full evaporation technique must be mentioned. According to Camarasu, the SPME technique seems to be very promising for RS determination in pharmaceuticals, with much better sensitivity than the static headspace technique. 

Fig. 2. Static headspace sampling of insect volatiles with a SPME device.

Fig. 1 Comparison of dynamic, static, and SPME headspace sampling, (a) Dynamic headspace sampling uses a sorbent or cold trap to concentrate volatile analytes before analysis by the GC. (b) Static headspace sampling uses direct transfer of a volume of gas from the headspace above the heated sample vial directly to the GC for analysis. Injection designs are illustrated in Fig. 2. (c) SPME headspace sampling uses a fiber support with solid-phase coating. The fiber is placed in the headspace and reaches equilibrium with the headspace volatile analytes. The SPME fiber is transferred by means of a syringe and thermally desorbed in the injector of the GC for analysis.

Residual solvent Headspace SPME (PDMS/DVB) Gas-tight SPME (PDMS/DVB) Static headspace... 

This term can be applied to a number of techniques including static headspace, purge and trap, and thermal desorption. All of these techniques involve the extraction of a gaseous component from a solid or liquid sample. Static headspace is usually a one step technique where the componait of interest is extracted from the sample held in a closed vessel usually at elevated temperatures and then injected onto the GC. Purge and trap is a multistep technique where the compound of interest is extracted into a matrix and then thermally desorbed onto the GC for analysis. SPME used in conjunction with GC analysis could be considered a purge and trap technique. In thermal desorption, the sample is heated r idly and isolated using a cryogenic trap with the compounds isolated thermally desorbed. 

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