Direct thermal desorption system

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. 

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). 

J. J. Manura, Direct thermal analysis using the short path thermal desorption system. Short Path Thermal Desorption Note, No. 5 (1991). 

The GEiM-lOOO low-temperature thermal desorption unit is an ex situ technology that treats soils contaminated with volatile organic compounds (VOCs). This process involves a countercurrent drum, pulse-jet baghouse, and a catalytic oxidizer mounted on a single portable trailer. As the soil is heated in the GEM-1000 unit, contaminants are vaporized. The contaminants are then directed to the system s catalytic oxidizer, which is designed to convert virtually all of the VOCs to carbon dioxide and water vapor. The oxidizer contains approximately 4.9 ft of noble metal catalyst and can destroy between 95 and 99% of the hydrocarbons when operating between 600 and 1250°F. 

When the extracted analytes are to be retained directly on the chromatographic column or at the retention interface, their insertion can be accomplished in various ways, namely (a) by injection into the column, whether directly (SFC, GC) or with the aid of a cooling system (GC, HPLC) (b) by split-splitless injection (SFC, GC) (c) by using a programmed temperature vaporizer (GC) or (d) by injection into a cold trap and subsequent thermal desorption (GC) or elution (HPLC). 

Samples collected in this way can be desorbed using a solvent which provides a solution for conventional injection on any GC or GC-MS. Thermal desorption of specially designed traps directly onto a GC with the appropriate injection system is more sensitive, and very volatile materials are not obscured by large solvent peaks. However, there is the possibility of thermal degradation of the sample and the entire sample is used at once. 

Over the past few years it has often been observed that the photochemical behaviour of adsorbed molecules is distinctly different to that of their gas phase counterparts. Even direct dissociations of molecules physisorbed on insulator substrates were found to have different dynamics to the analagous gas phase reaction, and exhibited a dependence on the coverage. This needs to be understood. For adsorbed molecules a new kind of "dissocation" is possible, namely desorption, Photolytic (non thermal) desorption has been reported from all kinds of substrate. On metal surfaces it is often found that the quantum yield for a direct photodissociation reaction is much lower than in the isolated molecule. This must be accounted for. Finally, the observation which has stimulated a great deal of research in surface photochemistry, photolysis is observable at energies where the gas phase molecules are transparent. It turns out that all of these interrelated effects can be interpreted by a delicate interplay of excitation mechanism and transient quenching. The fine details of course depend on particular adsorbate-substrate systems, which are described in section 4. 

Figure 25.11 Selected hydrogen thermal desorption traces obtained from a bimetallic Cu—Ru surface (Cu coverage = 0.7 monolayers on a Ru(0001) surface) as a function of adsorption temperature The top curve (a) was obtained after the system had received a saturation exposure at 100 K curve (b) Hj desorption trace after a saturation exposure at 230 K. The dashed line indicates the direct superposition of (a) onto (b). The bottom curve (c) represents the difference (b) — (a) and, hence, is equal to the amount of hydrogen spilled over from Ru to Cu sites at 230 K. After Goodman and Peden [88].

In the following sections, we will first consider metal—silicon and metal/III—V compound systems, covering electronic and chemical interface effects. We will then present a general treatment of the adsorption-desorption models which have been proposed, including some discussion of the interpretation of thermal desorption spectra, and finally we will discuss to what extent the two sets of data can be reconciled. We will not be concerned with metal films in device fabrication, nor the formation of additional phases by heat treatment. We shall only deal with interactions and interface chemistry which is directly relevant to adsorption-desorption behaviour. 

Often interference effects from either solvents [74] or other components in sample matrices can cause significant problems especially with direct injection of such solutions. Headspace analysis has been shown to be of great value for residual solvent analysis in drug substance [75] and dmg product [76] because the drag itself is not introduced into the system. Similarly, residual solvent analysis in pharmaceuticals using thermal desorption [77] and solid phase microexttaction (SPME) [78] has been shown to be of benefit. For more con ilex matrices such as... 

The combination of a SPME device with a pyrolysis gas chromatographic (GC) IMS system improved the limit of detection of tributylphosphate (TBP, which served as a simulant) in water by a factor of 20 compared to the same system without the SPME device. SPME fibers were also used to sample headspace vapors of several types of nerve agents, and the fibers were introduced directly into a modified ESI source for subsequent detection by IMS and mass spectrometry (MS). A SPME-IMS system, with thermal desorption, was also used to screen soil samples for precursor and degradation products of CWAs, and it was found that fibers of polydimethyl-siloxane (PDMS) were superior to PDMS-divinylbenzene fibers. ... 

Direct detection by IMS of TMA in chicken meat juice with excellent detection limits of 0.6 0.2 ng using partial least squares (PLS) and fuzzy rule-building expert system (FuRES) was also reported. A combination of thermal desorption with GC... 

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