Gas-Phase Extraction

Experiments and other empirical work discussed herein were carried out by highly-skilled personnel well abreast of risks and hazards associated with subjecting volatilised materials to microwave energy while within a confined volume. Special care should be exercised if and when attempting to 

This hazard level will be reduced if not totally removed when commercial instrumentation is made available to the laboratory community in the near future. By the same token, when such instrumentation is available even better operating conditions will be available to users as the apparatus and peripherals used therefor will have been designed specifically for that purpose keeping the critical parameters in mind. 

The fact that different chemical species absorb microwave energy to a different extent implies that the thermal energy so-produced and imparted to the surrounding environment will also vary with the chemical species. Hence, for systems that possess inherent non-homogeneous structural characteristics, or that contain different chemical species with different dielectric properties dispersed into a homogeneous environment, it is possible to effect a selective heating of some areas, or components of the systems. 

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Principles and Characteristics Pare et al. [475] have patented another approach to extraction, the Microwave-Assisted Process (MAP ). In MAP the microwaves (2.45 GHz, 500 W) directly heat the material to be extracted, which is immersed in a microwave transparent solvent (such as hexane, benzene or iso-octane). MAP offers a radical change from conventional sample preparation work in the analytical laboratory. The technology was first introduced for liquid-phase extraction but has been extended to gas-phase extraction (headspace analysis). MAP constitutes a relatively new series of technologies that relate to novel methods of enhancing chemistry using microwave energy [476]. 

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]. 

Static headspace may also be carried out by substituting the heating step by a microwave treatment. In this procedure the material is immersed in a solvent that is transparent to microwaves relative to the sample in order to impart most, if not all, of the microwave energy to the sample [208]. Another configuration of MAP gas-phase extraction relates to dynamic headspace sampling. 

On-line supercritical fluid extraction/GC methods combine the ability of liquid solvent extraction to extract efficiently a broad range of analytes with the ability of gas-phase extraction methods to rapidly and efficiently transfer the extracted analytes to the gas chromatograph. The characteristics of supercritical fluids make them ideal for the development of on-line sample extraction/gas chromatographic (SFE-GQ techniques. SFE has the ability to extract many analytes from a variety of matrices with recoveries that rival liquid solvent extraction, but with much shorter extraction times. Additionally, since most supercritical fluids are converted to the gas phase upon depressurization to ambient conditions, SFE has the potential to introduce extracted analytes to the GC in the gas phase. As shown in Fig. 13.8, the required instrumentation to perform direct coupling SFE-GC includes suitable transfer lines and a conventional gas chromatograph [162,163]. 

Air sampling media has been extracted using different solvent systems. Both particulate and gas phase extracts have been extracted using methanoFethyl acetate [99, 100, 102], while other studies have employed petroleum ether/acetone for extraction of the PUF/XAD and dichloromethane for the filter [101]. Generally extracts are filtered prior to analysis. Some researchers have also included a clean-up step. For example, Shoeib, Hamer and Vlahos [101] passed PUF/XAD sample extracts through an alumina column and eluted with dichloromethane in ethyl acetate, prior to the analysis of FTOHs and FSAs. 

Stir bar sorptive extraction Gas-phase extraction Thermal desorption... 

The addition of such a substance prior to extraction is effected only when the added substance is a desirable component of the final product formulation where the extract will be used. Consequently, for liquid-phase extraction, this practice is more useful in industrial process apphcations such as, for example, the use of alcohol for the production of extracts to be used in the confection of liqueurs. In gas-phase extractions, however, this technique is very useful in enhancing the volatihsation of analytes. This has apphcations in the analytical... 

For many years, the traditional sample preparation methods, such as the Soxhlet extraction, were applied. Most of these methods have been used for more than 100 years, and they mostly require large amounts of organic solvents. These methods were tested during those times, and the analysts were familiar with the processes and protocols required. However, the trends in recent years are automation, short extraction times, and reduced organic solvent consumption. Modern approaches in solid sample preparation include microwave-assisted solvent extraction (MASE), pressurized liquid extraction, accelerated solvent extraction (ASE), matrix solid-phase dispersion (MSPD), automated Soxhlet extraction, supercritical fluid extraction (SEE), gas-phase extraction, etc. 

Volatile organic compounds (VOCs), generally have a boding point less than 200°C and a vapor pressure greater than 0.1 Torr at 25°C and atmospheric pressure. Usually a gas-phase extraction by static or dynamic headspace sampling is used to separate VOCs from an aqueous or solid samples for introduction into... 

VFAs Gas chromatography (offline) On-line sampling and gas chromatography Gas phase extraction at pH < 2 Indirectly via titration Fluorescence spectroscopy Near-infrared spectroscopy Brondz (2002) Find et al. (2003) Boe et al. (2008) Molina et al. (2009) Peck and Chynoweth (1992) Lomborg et al. (2009)... 

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