A technique for thermal desorption analyses suitable for thermally-labile, volatile compounds

Authors: Alborn, Hans

Submitted to: Journal of Chemical Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/1/2018
Publication Date: 2/1/2018
Citation: Alborn, H.T. 2018. A technique for thermal desorption analyses suitable for thermally-labile, volatile compounds. Journal of Chemical Ecology. 44(2):103-110. doi:10.1007/s10886-018-0924-6.
DOI: https://doi.org/10.1007/s10886-018-0924-6

Interpretive Summary: Many important agricultural plant and insect interactions are governed by odors released by the plants or insects. There exists a continual need for new or improved methods to collect and identify these odors and then utilize them to help protect crops from insect pests. Scientists at the USDA-ARS Gainesville, FL Chemistry Research Unit are at the forefront of below ground, plant-produced odor signals that affect nematode and insect behavior, research with special instrumental requirements. Here we present the development and evaluation of a new low cost and relatively simple thermal desorption technology where a liquid carbon dioxide-chilled cold trap is easily added as an integral part of an existing gas chromatographic (GC) injector. A GC is an instrument used to separate the individual odor components of a complex mixture of odors. Using temperature gradient-based focusing of odor samples eliminated the need for flash heating, thus resulting in greatly reduced sample degradation when compared to earlier designs and alternative techniques. This technique, that requires very short sampling times can, in combination with in-house designed probes, make it possible to not only study root pests in the laboratory but might also be used for efficient field screening for below ground pest problems before they are visible above ground thus there is a potential for additional agricultural applications. One additional bonus with the new system is that the presence of the cold trap does not affect normal injector performance. Thus in addition to low cost, it does not require a dedicated instrument since it allows on-the-fly switching between injection techniques.

Technical Abstract: Our group has for some time studied below ground plant produced volatile signals affecting nematode and insect behavior. The research requires repeated sampling of intact plant/soil systems in the lab as well as the field with the help of probes to minimize unwanted effects on the systems we are studying. After evaluating commonly utilized adsorption filter or SPME fiber based sample collection and techniques to identify volatile compounds in the sampled air, we found dynamic sampling of small air volumes on Tenax filters followed by thermal desorption sample introduction to be the most suitable analytical technique for our applications. Here we present the development and evaluation of a low cost and relatively simple thermal desorption technique where a liquid CO2 chilled cold trap is added as an integral part of a split less injector. Temperature gradient based focusing and low thermal mass minimizes aerosol formation and eliminates the need for flash heating resulting in low sample degradation comparable to solvent based on-column injections. Additionally, since the presence of the cold trap does not affect normal split-less injections, on-the-fly switching between split-less and thermal desorption mode can be used for external standard quantifications.