Continuing on forms of direct analysis from Part 11: Headspace, we are covering thermal desorption in this article.
Thermal desorption is another gas chromatography sample introduction technique and is well suited to trace level analysis of volatile organic compounds. In this technique, a known volume of sample air is passed through a trap comprising a tube containing a powdered adsorbent such as charcoal, tenax etc. Contaminants in the air are trapped in the adsorbent bed. After collection of the sample, the trap is then inserted into the carrier gas path at the inlet to the chromatograph and rapidly heated to release the analytes into the column. Figure 1 shows the thermal desorption unit along with a selection of desorption tubes. However, for the analysis of solid samples, the material to be analysed is placed in the tube over which a gas is passed while the tube is heated. In thermal desorption, the partition can be a dynamic process in which the analyte goes from the gas state and is then moved to the inlet of a gas chromatograph where they are cryofocused (by Peltier cooling, liquid CO2 or liquid N2 [in increasing order of cooling efficiency]). Once the analyte is moved away from the solid sample, the partition coefficient favours the transfer of the analyte away from the solid sample into the gaseous state. With Headspace, the process is typically static, where once the equilibrium coefficient is reached no further analyte is extracted.
In general, thermal desorption can be more sensitive than standard headspace but care must be taken in selecting the correct sorbent to make sure it has the suitable capacity and the ability to trap the required analytes.
Figure 2.22 Thermal desorption apparatus along with a selection of desorption tubes. Reproduced with permission from Marks International Limited
 Zweiben and A.J. Shaw, PDA Journal of Pharmaceutical Science and Technology, 2009, 63, 353.