Sample thermal extraction

This is a relatively new technique that is used for PCBs and other nonpolar, volatile and semi-volatile organic compounds. Typically, a small aliquot of soil sample (0.5-20 g) is used for the extraction. Soil samples are extracted with one or more organic solvents using microwave energy at elevated temperature (100-115 °C) and pressure (50-175 psi). This method uses less solvent and takes significantly less time than Soxhlet extraction but is limited to thermally stable compounds. 

Selective extractions are not only of interest to solvent extraction, but also to thermal extractions. For example, selective in situ detection of polymer additives is possible using laser mass spectrometry, notably UV laser desorption/MS [561]. The proper matching of extraction technique to a sample determines the success of the operation and enhances the precision and accuracy of the analysis. 

SFE-GC-MS is particularly useful for (semi)volatile analysis of thermo-labile compounds, which degrade at the higher temperatures used for HS-GC-MS. Vreuls et al. [303] have reported in-vial liquid-liquid extraction with subsequent large-volume on-column injection into GC-MS for the determination of organics in water samples. Automated in-vial LLE-GC-MS requires no sample preparation steps such as filtration or solvent evaporation. On-line SPE-GC-MS has been reported [304], Smart et al. [305] used thermal extraction-gas chromatography-ion trap mass spectrometry (TE-GC-MS) for direct analysis of TLC spots. Scraped-off material was gradually heated, and the analytes were thermally extracted. This thermal desorption method is milder than laser desorption, and allows analysis without extensive decomposition. 

Because no pretreatment of the samples was carried out, the peaks present in the total ion current trace reflect components generated by pyrolysis of primary compounds ( real pyrolysis products ) and components that are present as such in the sample and simply evaporate ( free products ). If desired these two types of products may be differentiated using wires with a Curie temperature of 358°C [36], It was demonstrated in separate analyses (not shown here) that most compounds were not generated by pyrolysis but were present as such in the sample and thermally extracted . Compounds 1-8 and 10-17, 27, 37, 38, 54 and 65 were only present in pyrolysis gas... 

Fig. 11.4 shows the total ion current trace and some mass chromatograms obtained by flash evaporation pyrolysis gas chromatography-mass spectrometric analysis of the polluted sediment sample. All compounds present in this complex mixture were not listed. A selection was made to exemplify several aspects of the screening approach. The peak number correspond with the numbers in Table 11.1. Identifications were based on the same criteria as mentioned above. Although several components were shown to be real pyrolysis products, all the compounds are present as such in the sample and resulted from simple thermal extraction from the wire. This was shown in separate analyses using ferromagnetic wires with a Curie temperature of 358°C. 

Although widely used, solvent extraction procedures have been demonstrated as sensitive to such variables as the content of humic matter and moisture within samples. Supercritical fluid extraction appears to be a more robust procedure. Thermal extraction procedures are sensitive to the size of the soil sample in some cases since the technique can result in cracking higher-molecular-weight... 

Seven Argonne Premium coal samples ranging from lignite to low volatile bituminous in rank were analyzed by Pyrolysis-Field Ionization Mass Spectrometry (Py-FIMS) in order to determine the existence and structural nature of a thermally extractable "mobile phase". In addition, Curie-point Pyrolysis-Low Voltage Mass Spectrometry (Py-LVMS) was employed to demonstrate the importance of mild oxidation on the thermally extractable mobile phase components. 

This method is based on the partitioning of compounds between a sample and a coated fibre immersed in it [16-18]. The volatiles and other compounds are first adsorbed onto the fibre immersed in a liquid sample, an extract, or in the headspace above a sample for a certain period of time. After adsorption is complete, the compounds are thermally desorbed into a GC injector block for further analysis. Particularly in food applications, headspace SPME is preferred to avoid possible contamination of the headspace system by non-volatile food components [16]. 

Stir-bar sorptive extraction (SBSE) is carried out using a commercially available glass stir bar (Twister, from Gerstel GmbH) coated with polydimethylsiloxane (PDMS). A special thermal desorption unit is necessary to introduce the extract into a GC. It can be applied to headspace extraction, but is intended for stirring liquid samples for extraction. The same coatings used for SPME can be used for SBSE, and thus similar selectivity should be observed. 

Thermal decomposition of tetramethylammonium or trimethylanilinium salts was described for phenytoin methylation prior to its GC analysis in plasma [556] and diethyl-stilbestrol in biological fluids [557], Usually a liquid or homogenized sample is extracted into toluene, the respective tetrasubstituted ammonium hydroxide is added and the liquid phase is injected at 260—290°C. 

The thermal extraction (calcination) of cetyltrimethyl ammonium bromide, which is one of the surfactant templates, from a pure silica mesophase to form MCM41 has been studied using SCTA (Sample Controlled Thermal Analysis) by the Marseilles Group. 

Chapter 9 deals with the extraction of volatile compounds from the atmosphere. Particular emphasis is placed here on the methods of thermal desorption and purge-and-trap. Chapter 10 focuses on the methods used to pre-concentrate samples after extraction. In this situation, particular attention is paid to two common approaches, namely rotary evaporation and gas blow-down , although details of two other methods are also provided. 

Thermal extraction techniques are usually performed in conjunction with gas chromatography. Petroleum hydrocarbons can be thermally desorbed from soil matrices at elevated temperatures. The eluting compounds are trapped in an absorbent phase such as Tenax and subsequently desorbed directly onto the column of the gas chromatograph. Whilst this technique is regarded as the closest to producing a real TPH value (C4-C35) it suffers from low sample size requirements (typically 1-10 mg) and is unlikely to be representative of the whole sample. Nevertheless, it can be used as a quick qualitative screening analysis. 

SBSE has many similarities to SPME, as it is also a solventless sample preparation technique and it uses similar sorbents (based on PDMS). SBSE was first described by Baltussen and co-workers in 1999 (78). In SBSE, an aqueous sample is extracted by stirring for a certain time with a PDMS-coated stir-bar. The stir-bar is therea r removed from the sample and the absorbed compounds are then either thermally desorbed and analyzed by GC-MS, or desorbed by means of a liquid for interfacing to a LC system. Heat-desorption gives higher sensitivity while liquid desorption provides higher selectivity. 

This overview is focused on the on-line coupling of pressurized liquid extraction (PLE), microwave-assisted extraction (MAE), supercritical fluid extraction (SEE) and sonication-assisted extraction (SAE) with liquid and gas chromatography for the analysis of solid agricultural and food samples. In addition, head-space techniques and direct thermal extraction are discussed. 

In the sample preparation of semi- and nonvolatile compounds, solvent extraction is typically used for extracting the analytes of interest from a sample matrix. For volatile analytes, head-space or thermal extraction are good alternatives to solvent-based techniques. Several novel extraction systems that utilize elevated temperatures of pressures in the extraction have been developed particularly for solvent-based extraction methods. These new methods typically are much faster and often more selective than older methods and consume smaller amounts of organic solvents and reagents. Commercially available systems with the ability to heat and pressurize liquids include pressurized liquid extraction (PLE), microwave-assisted extinction (MAE) and supercritical fluid extraction (SFE). Also sonication-assisted extraction (SAE) has given promising results. 

The major breakthrough that transformed metal chelate GC into a useful analytical technique was the introduction of fluorinated beta-diketone ligands, which formed complexes of greater volatility and thermal stability. Trifluoroacetylacetone (l,l,l-trifluoro-2,4-pentanedione—HTFA) and hexafluoro-acetylacetone (l,l,l,5,5,5-hexafluoro-2,4,-pentanedione—HHFA) are the fluorinated ligands most frequently employed. HTFA extended the range of metals that may be gas chromatographed with little or no evidence of decomposition to include Ga3+, In3+, Sc3+, Rh3+ and V4+. An example of a recent application is the analysis for beryllium in ambient air particulates. After filter sampling and extraction/chelation, packed column GC with electron capture detection allowed ppm level beryllium quantitation in collected particulates which corresponded to levels of 2-20 x 10 5 pg/m3 in the sampled air. 

SPME consists of two steps extraction and desorption. In the extraction process the fiber is immersed into the sample with a syringe, vigorous stirring is applied, and the organic micropollutants are retained in the fiber depending on their distribution coefficients. Then, using the holder, the fiber is transferred to the analytical instrument for desorption, separation and quantification. The method has been automated and commercial systems are available which will extract, agitate, and inject the sample into a GC system. In HPLC the sample is extracted directly into the eluent stream rather than thermally desorbed." ... 

The standard apparatus used for thermal desorption is equally suitable for thermal extraction. Thermal extraction is used for the analysis of volatile compounds in solid samples of low moisture content (e.g. plant materials, soil, polymers, etc). In this case, a few milligrams of sample in place of the sorbent contained in a standard desorption tube is heated to a temperature below its melting or decomposition temperature. The volatile compounds released from the sample are accumulated and transferred to the column in the same way as for thermal desorption. 

Headspace and thermal desorption are thermal extraction methods which can be directly connected to gas chromatography and do not need additional sample preparation. Usually both methods are applied for the determination of volatile compounds in air and water. Only few applications are known for the direct treatment of soil samples. The investigations for analysis of phenylarsenic compounds were carried out with an Headspace Sampler HS40 (Perkin-Elmer Inc.) and a Thermal desorption system TDS 2 (Gerstel GmbH, Germany). 

Thermal desorption Method development was done with sample amounts of 50-100 mg. The use of larger sample amounts can overload the injection system and the capillary column. Drying of the sample prior to thermal extraction is necessary because problems with humidity content of the soil and other matrix effects can happen. Variations of the parameters desorption time and desorption temperature lead to an optimized method. The calibration curve in the range from 1-100 ppm shows a good linearity with relative standard deviations of about 5-7% (Fig. 11). These values are higher compared to liquid extraction due to the inhomogeneity of the soil samples. An additional homogenization of the samples reduced the relative standard deviation to about 2-3%. 

Tomaszewaska and Hebert (2003) reported a method for the analysis of ( , -dimethyl hydrogen phosphorothioate (0,5-DMPT) in urine. 0,5-DMFT is a specific metabolite of methamidophos, The urine sample was extracted with a CIS column, and the sample was lyophilized at low temperature to prevent loss of highly volatile and thermally unstable metabolite (0,5-DMPT). The lyophilized residue was derivattzed using MTBSTFA and 1% terl-butyl-dimethylchlorosilane in acetonitrile. After filtration, the derivatized product was analyzed with GC/FPD (pulse FPD) in the phosphorus mode. The limit of detection for the method is reported as 0.004 ppm, with a mean recovery of 108%. 

In the last decade there has been an increasing demand for new extraction techniques, amenable to automation, with shortened extraction times and reduced organic solvent consumption, to prevent pollution and reduce the cost of sample preparation. Driven by these goals, advances in microwave extraction have resulted several techniques such as microwave-assisted solvent extraction (MASE) [32, 36-39], vacuum microwave hydrodistillation (VMHD) [40, 41], microwave hydrodistillation (MWHD) [42, 43], compressed air microwave distillation (CAMD) [44], microwave headspace (MHS) [5], and solvent-free microwave hydrodistillation (SEME) [45, 46]. Table 22.3 summarizes the most common microwave extraction techniques for plant matrices and lists their advantages and drawbacks. Over the years procedures based on microwave extraction have replaced some of the conventional processes and other thermal extraction techniques that have been used for decades in chemical laboratories. 

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