Thermal desorption encompasses two primary functions: sample collection/concentration and transfer to a detector.
The use of adsorbents and large sample volumes to collect vapor phase compounds, (such as VOC's in air or residual components from solids), facilitates accurate analysis even when sample levels are very low.
Organics are concentrated on sorbent media while the sample matrix, air or water, is discarded. In much the same way as a vacuum cleaner filters dust from air as it sweeps over a surface, Dynatherm instruments capture compounds of interest on adsorbent material packed in glass cartridges. Collected compounds are then introduced into another analytical instrument, typically a gas chromatograph, where they are separated, measured, and identified.
Thermal desorption uses heat, (instead of solvent extraction), to release organic compounds from the adsorbent and transfer the entire collected sample to a gas chromatograph for analysis. This eliminates extraction time, (typically 12-24 hours), a solvent peak in the chromatogram (which can mask compounds of interest), and waste solvent disposal, an increasing expense in many labs.
The process is flexible and convenient. For example, an investigator equipped with a battery-powered sampler can quickly collect liters of air on a sorbent tube and bring it to the lab for analysis. Alternatively, the thermal desorber/GC instruments can be mobilized for direct source sampling in the field.
When samples are collected in areas of high humidity, two-stage sorbent trapping and thermal desorption effectively eliminate water interferences and enhance analysis of polar compounds in the sample stream. (Other collection techniques, such as canister sampling, must limit sampling volumes to control the amount of water transferred to the GC, which tends to lower sensitivity as well as restrict recovery of polar compounds.)