Thermal desorption, analytical method

Although on-line sample preparation cannot be regarded as being traditional multidimensional chromatography, the principles of the latter have been employed in the development of many on-line sample preparation techniques, including supercritical fluid extraction (SFE)-GC, SPME, thermal desorption and other on-line extraction methods. As with multidimensional chromatography, the principle is to obtain a portion of the required selectivity by using an additional separation device prior to the main analytical column. 

For the charcoal, XAD, and PUF adsorbents discussed above, solvent extraction techniques have been developed for the removal and concentration of trapped analytes. Although thermal desorption has been used with Tenax-GC in some specialized air sampling situations [primarily with sampling volatile organic compounds (EPA, Method TO-17 )], this approach is not a viable alternative to solvent extraction for the charcoal, XAD, and PUF adsorbents. The polystyrene and PUF adsorbents are thermally unstable and the charcoal chemisorption bonding is more easily broken by... 

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. 

Thermal desorption mass spectrometry is a rapid technique for the determination of oil in soils and sediments [9]. This method exhibited lower analytical variance compared to Soxhlet extraction, i.e. followed by conventional analysis. The analysis time for wet soil samples was about 20min. 

In either case, the use of a DEP allows to extend the temperature range for evaporation. In addition, it reduces thermal degradation as a result of heating the analyte faster than its thermal decomposition usually proceeds, and therefore expands the range of applications for El and Cl to some extent. Whatsoever, employing direct exposure probes is by far no replacement of real desorption ionization methods. [52,53]... 

Although a variety of methods are available for determination of 1,4-dichlorobenzene in blood, few are well characterized and validated. A method has been developed which utilizes headspace purge followed by thermal desorption of the trapped, purged analytes. 1,4-Dichlorobenzene is then determined by capillary GC/MS (Michael et al. 1980 Pellizzari et al. 1985). Recovery is very good (>85%) and detection limits are in the low-ppb range for model compounds (Michael et al. 1980 Pellizzari et al. 1985). Performance data are not available for 1,4-dichlorobenzene. A sensitive and reliable method for identification and quantitation of 1,4-dichlorobenzene in samples of whole blood has been developed by Ashley and coworkers at the Centers for Disease Control and Prevention (CDC) (Ashley et al. 1992). 

These liner exchange systems make feasible yet another analysis mode direct thermal desorption (DTD). Here the liner or an insert is packed with the solid sample. The liner exchange system can then be used in place of a conventional autosampler. The liner is automatically inserted into the PTV and the volatiles thermally desorbed onto the column. Some analysts may feel uneasy about such desorption from the solid phase how does one know that all of the volatile analytes have been released from the sample crystal lattice However, where applicable, this approach may not be as difficult to validate as one might imagine. For instance, the PTV can be cooled after the analyte transfer, and then, at the end of the chromatographic temperature programme, reheated to repeat the process. Ideally all of the analyte should transfer in the first cycle and none in the second, demonstrating that complete desorption occurs in the method. 

Others. Attempts were made to develop general methods for mer-captans, amines, alcoholamines, and nitroalkanes. However, results were not satisfactory. We did not use a single collection medium for amines. The media used for various amines included silica gel with and without SO, a porous polymer with thermal desorption, and a bubbler. The preferred method is collection of silica gel followed by acidification of the sample with dilute HC1 immediately after collection. This method should be successful for ammonia and most aliphatic amines. Ion chromatography was used successfully for the analysis of methyl amine. This analytical method may be... 

MS operation is based on magnetic and electric fields that exert forces on charged ions in a vacuum. Therefore, a compound must be charged or ionized in the source to be introduced in the gas phase into the vacuum system of the MS. This is easily attainable for gaseous or heat-volatile samples. However, many thermally labile analytes may decompose upon heating. Such samples require either desorption or desolvation methods if they are to be analyzed by MS. Although ionization and desorption/desolvation are usually separate processes, the term ionization method is commonly used to refer to both ionization and desorption or desolvation methods. 

A direct mass spectrometric method for simultaneous detection of five benzimidazoles including levamisole, thiabendazole, mebendazole, fenbendazole, and febantel in sheep milk was reported (377). The method, which involves injection of crude milk extracts and selection and collision of the most abundant ionic species obtained under electron impact ionization, was highly sensitive and rapid. Another direct mass spectrometric approach for rapid and quantitative determination of phenothiazine in milk was also described (323). This method involves an extraction step using a Cig microcolumn disc, followed by thermal desorption of the analyte from the disc directly into an ion trap mass spectrometer. 

Solid-Phase Microextraction. Solid-phase microextraction (SPME), used as a sample introduction technique for high speed gc, utilizes small-diameter fused-silica fibers coated with polymeric stationary phase for sample extraction and concentration (33). The trapped analyte can be liberated by thermal desorption. By using a specially designed dedicated injector, the desorption process can be shortened to a fraction of a second, producing an injection band narrow enough for high speed gc. A modified system has been investigated for the analysis of volatile compounds listed in EPA Method 624. Separation of all 28 compounds by ion trap mass spectrometric detector is achieved in less than 150 seconds. 

These semi-preparative methods are useful where identification is required but for quantitative and comparative analytical purposes much more rapid sampling techniques, such as automated headspace and solid phase microextraction (SPME), may be preferred. Both of these techniques give similar results for most volatiles. In the former, the vapour above a heated sample is removed by a syringe or gas flushing and injected onto a GC column, either directly or after trapping on a suitable absorbent and thermal desorption. In SPME, the vapour is absorbed on to a suitable bonded medium on a special needle and then injected into the gas chromatogram. 

The mechanism involved in desorption of materials from surfaces by this method is not well understood. One of these mechanisms is the thermal desorption because heating the gas helps desorption of some analytes. However, the successful analysis by DART of analytes having little or no vapour pressure indicates that other processes occur. The transfer of energy to the surface by metastable atoms and molecules has been proposed as the mechanism to facilitate desorption and ionization of these analytes. 

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