Thermal desorption instrumentation

Overview of thermal desorption

Thermal desorption (TD) arose out of the need to improve upon conventional sample preparation techniques such as solvent extraction, solid-phase microextraction, purge-and-trap and static headspace. It gives greater sensitivity than these techniques and can be used for a wider range of compound classes ranging in volatility from C₂ to n-C₄₄ hydrocarbons. TD is also applicable to a wide range of sample types – solids (using dynamic headspace, headspace sorptive extraction or direct desorption), liquids (using immersive sorptive extraction) and gases (using pumped sampling, passive sampling, on-line sampling or canisters). It is safer and more environmentally friendly than solvent extraction, is easily automated, easy to validate and complies with key standard methods.

What is thermal desorption?

TD is a versatile preconcentration and injection technique for gas chromatography, which is often used to automate analysis of volatile and semi-volatile organic compounds in air, gas and materials. Modern sorbent technology and sophisticated TD instrumentation can be optimised to capture the widest range of analytes for both target and untargeted analysis whilst ensuring selective purging of interferences such as solvents and water from the analytical system. The adsorbent trapping and thermal desorption process can handle wide dynamic and analyte ranges, allowing for the detection of ultra-volatiles to semi-volatiles in concentrations ranging from part-per-trillion (ppt) to low-percent levels. TD also greatly improves sample throughput and is easily automated and validated, saving time and manpower.

The majority of TD applications use sorbent tubes and a two-stage desorption process to focus the analytes into a narrow, capillary GC-compatible band of gas, and so achieve the maximum concentration enhancement and sensitivity.

The process consists of three steps:

• Sample collection: Up to several hundred litres of vapour is sampled off-line onto a sorbent tube.
• Tube desorption: The sample is heated in a flow of carrier gas and the analytes swept onto an electrically-cooled focusing trap.
• Trap desorption: At the end of tube desorption, the focusing trap heats rapidly in a reverse flow of carrier gas transferring or injecting the analytes into the GC column as a narrow band. Analytes are transferred to the GC in just a few hundred micro-litres of carrier gas providing pre-concentration factors of several orders of magnitude.

Alternatively, volumes of whole air or gas, collected in canisters, bags, or from on-line air/gas streams can be introduced directly to the electrically cooled focusing trap for concentration and desorption/transfer into the GC column.

Sample materials can also be weighed directly into empty TD sample tubes or liners for selective extraction/desorption of retained volatiles in a dynamic headspace process.