2.2.2.2.2. Thermal desorption of dynamic headspace volatiles by Tenax

A second set of thermal desorption techniques combines the sensitivity of thermal desorption with the controlled sampling of dynamic headspace collections. Collection of sample volatiles is similar to other dynamic headspace collection techniques. Unlike SPME, sample peaks collected and separated by thermal desorption can be readily quantified using techniques for solvent-desorbed collection systems (see section 2.2.2.3.). 

Sample headspace is actively drawn by vacuum through a cartridge where volatiles are trapped in the adsorbent packing material. The packing material most commonly used is Tenax TA (replacing Tenax GC), which can be combined with activated carbon to increase capture of both non-polar and polar certain chemicals (Raguso and Pellmyr, 1998). Thermally-desorbed Tenax has a long history of use in honey bee systems and is still used today (Moritz and Crewe, 1991; Schöning et al., 2012).  The cartridge is later (immediately after volatile sampling) inserted into a modified GC injection port (which is larger than the normal syringe needle injection port to accommodate the Tenax filter) and rapidly heated to desorb all of the trapped volatiles from the packing material onto the column (see section 2.2.3.). Refer to sections 2.2.2.3. and 2.2.2.4. for application examples.

Pros: Like SPME, Tenax thermal desorption has a distinct advantage over solvent-desorbed samples in the absence of solvent peaks that may obscure highly-volatile compounds. Typically, samples are analysed immediately after volatile collection.

Cons: One major disadvantage of Tenax is that these thermal desorption methods require significant equipment and expertise compared with SPME and solvent desorption methods.  Tenax collections make use of the volatile collection infrastructure used with solvent-extracted adsorbents as well as a modified injector port. Compared to solvent desorption, thermal desorption is a relatively slow sample injection technique that will lead to peak broadening for very volatile compounds during GC separations. These problems are partially corrected by cryofocusing techniques that use an automatic thermal inject system to rapidly heat and inject sample chemicals onto the column. Because cryotrap methods are advanced techniques, new researchers should use other collection techniques to initially sample volatiles (see sections 2.2.2.2.1 and 2.2.2.3).

 

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