Thermal desorption, analytical method Applications

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]... 

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. 

Thermal Desorption Thermal desorption is an alternative GC inlet system particularly used for VOC analysis. However, the analytes subjected to thermal desorption must be thermally stable to achieve successful analysis. Otherwise, decomposition occurs. This technique is mainly used for determination of volatiles in the air. Such a methodology requires sample collection onto sohd sorbents, then desorption of analytes and GC analysis. Traditionally, activated charcoal was used as a sorbent followed by extraction with carbon disulfide. However, solvent desorption involves re-dilution of the VOCs, thus partially negating the enrichment effect. Therefore, the sampling method is to pump a sample of gas (air) through the sorbent tube containing certain sorbents in order to concentrate the VOC. Afterwards, the sample tube is placed in thermal desorber oven and the analytes are released from the sorbent by application of high temperature and a flow of carrier gas. Additionally, desorbed compounds are refocused in a cold trap and then released into the GC column. Such a two-step thermal desorption process provides a narrow chromatographic band at the head of the column. 

The most common extraction techniques for semivolatile and nonvolatile compounds from solid samples that can be coupled on-line with chromatography are liquid-solid extractions enhanced by microwaves, ultrasound sonication or with elevated temperature and pressures, and extraction with supercritical fluid. Elevated temperatures and the associated high mass-transfer rates are often essential when the goal is quantitative and reproducible extraction. In the case of volatile compounds, the sample pretreatment is typically easier, and solvent-free extraction methods, such as head-space extraction and thermal desorption/extraction cmi be applied. In on-line systems, the extraction can be performed in either static or dynamic mode, as long as the extraction system allows the on-line transfer of the extract to the chromatographic system. Most applications utilize dynamic extraction. However, dynamic extraction is advantageous in many respects, since the analytes are removed as soon as they are transferred from the sample to the extractant (solvent, fluid or gas) and the sample is continuously exposed to fresh solvent favouring further transfer of analytes from the sample matrix to the solvent. 

Thermal desorption from the SPE cartridge is a further possibility [77,122]. In this approach, the sample is introduced at a controlled speed into the packed liner of a PTV injector set to a low temperature with the water eliminated via the split vent. Salts and involatile polar material are rinsed from the sorbent with water and the sorbent dried by purging with a high carrier gas flow rate. The trapped analytes are subsequently desorbed in the splitless mode by rapidly heating the PTV to the injection temperature. The most commonly used sorbents are Tenax and Carbofrit. The method is restricted to a narrow range of applications by the low breakthrough volume of polar analytes on the... 

Where thermal desorption is inadequate to remove an analyte from the surface, a laser beam can be directed, focused or unfocused, against a solid, and compounds on surfaces of solids can be vaporized and ionized at ambient pressure in air. In one application of laser-based IMS to environmental analyses, soils contaminated with petroleum products were assayed for PAHs. In this, a laser was used to irradiate soil, vaporizing PAHs into the gas phase. This provided a direct, fast, extraction-free method for soil analyses. 

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... 

In the calibration of thermal desorption tubes, the same conditions should predominate as in sample collection. Methods such as the liquid application of a calibration solution to the adsorption materials or the comparison with direct injections have been shown to be unsatisfactory. Alternatively, Certified Reference Standards (CRSs) are available (e.g. Markes Int., UK). CRS tubes are recommended in many key standard methods (e.g. US EPA Method TO-17) for auditing purposes and as a means of establishing analytical quality control. CRS tubes are often certified traceable to primary standards, have a minimum shelf life of typically 6 months. They are available ready for use with concentrations... 

Most SPME applications involve GC. Following extraction, the analytes are thermally desorbed in the chromatograph injector. More recently, the scope of apphcation has been extended to nonvolatile and thermally unstable compounds by coupling SPME to LC. Desorption is performed at an appropriate interface consisting of a standard six-way HPLC injector with a special fiber-desorption chamber used instead of the sample loop. A different approach to SPME-LC called in-tube SPME has also been developed, it uses an open tubular fused-silica capillary column instead of the typical SPME fiber. This latter method has been used for the determination of carbamate pesticides in water with relative standard deviations of... 

Gas chromatography is a physical separation method in which the components in a mixture are selectively distributed between the mobile phase, which is an inert carrier gas, and a stationary phase, which is present as a coating of either column packing particles or the inner column wall. The chromatographic process occurs as a result of repeated sorption/desorption steps during the movement of the analytes along the stationary phase by the carrier gas. The separation is due to the differences in distribution coefficients of the individual components in the mixture. Being a gas-phase separation method, GC requires the analytes to be volatilized prior to their separation. As such, the application of GC is limited to components with sufficient volatility and thermal stability. 

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