Fig. 3.5. Two-stage thermal desorption. Secondary desorption. The trap is rapidly heated in a stream of gas and the trapped analytes are transferred to the chromatographic column. The direction of the desorption gas flow is reverse that of the trap gas flow. Parts of the sample are usually split (outlet split) before the GC column.

Adsorbent tube, cooling

There are also some limitations of the technique. Of course, the ana-lytes of interest must be stable on the adsorbent and possible to desorb by heat. The selection of a suitable adsorbent is thereby most critical. TD cannot be used for compounds that are too unstable for conventional GC analysis. Inorganic gases can, with a few exceptions, not be analysed by TD. There is also an upper limit on how non-volatile the compounds can be; the technique is best investigated for VOCs, although also semi-VOCs, at least to some extent, can be analysed (this is partly depending on the analytical equipment). A general disadvantage of TD compared to solvent desorption has been that all sample is consumed in one analysis. It is not possible to re-analyse the sample if something fails during the analysis, or if it is desirable to analyse it under other analytical conditions. If the amount of collected sample is much higher than expected it can result in poor chromatography and an overloaded detector, which makes it impossible to correctly quantify the sample; it would have been necessary to split more of the sample before letting it into the GC. Correspondingly, a lower split can be needed if the collected amount is smaller than expected. Today, however, analytical equipment that can re-collect the sample that has been split after desorption and before entering the column is commercially available. Both the Markes' Unity and the Perkin-Elmer TurboMatrixTM 650 ATD allow sample split re-collection. If one does not have equipment with such facilities, it is wise to take at least duplicate samples, to have a spare sample if the analysis fails or if additional analyses are of interest.

If badge-type samplers (SKC-Ultra) or radial samplers (Radiello) are used, the adsorbent/diffusive sampling cartridge is put in a conventional stainless steel tube for TD before analysis, and analysed as conventional TD samplers. It is, however, important to take desorption time and flow into consideration when setting the parameters for TD of these samples. When performing diffusive sampling on tube-type samplers most of the analytes are adsorbed on the part of the adsorbent bed that is closest to the diffusion end of the tube. During desorption the flow direction through the tubes is reversed in relation to the direction of diffusion. This means that the adsorbed analytes in practice only pass a small part of the adsorbent bed during desorption. On the contrary, when the adsorbent is poured from the SKC-Ultra sampler or the Radiello cartridge is put into the tube for TD, the collected analytes are dispersed across the entire adsorbent surface inside the tube. Therefore, it might be necessary to increase desorption time and/or the desorption flow through the tube, compared with analysis of samples collected by passive sampling on tube-type samplers, to ensure a complete desorption of the analytes.

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