What is analytical thermal desorption?
Thermal desorption – Preconcentration of VOCs for gas chromatography
Thermal desorption (TD), sometimes called thermodesorption, is the process of heating a material to release adsorbed compounds from it.
As an analytical technique, TD is used as a preconcentration technique for gas chromatography–mass spectrometry (GC–MS), making it compatible with low-concentration analytes that would otherwise be impossible to detect.
Listen to our founder, Elizabeth Woolfenden, explain the key principles of thermal desorption
How thermal desorption works
TD fundamentally involves collecting VOCs onto a sorbent, and then heating this sorbent in a flow of gas to release the compounds and concentrate them into a smaller volume.
Early thermal desorption units used just single-stage operation. In this mode, the volatiles collected on a sorbent tube are released by heating the tube in a flow of gas, from where they pass directly into the GC–MS.
However, most modern commercial thermal desorbers accommodate two-stage operation. This technique involves passing the gas stream from the sorbent tube into a narrower tube integral to the thermal desorber, which is also packed with sorbent. This is called the focusing trap or cold trap.
Heating this focusing trap releases the analytes once again, before injection into the GC–MS. However, this time the analytes are even more concentrated, leading to improved sensitivity and better GC peak shape.
The operation of modern two-stage thermal desorption.
What are the benefits of thermal desorption?
TD offers numerous benefits for the analysis of trace-level volatile and semi-volatile organic compounds (VOCs and SVOCs) compared to solvent extraction and other sample preparation techniques.
- High sensitivity – Two-stage desorption using sorbent tubes ensures narrow GC peaks, allowing concentration enhancements of up to 106, greatly enhancing detection limits.
- Saving time and effort – By choosing the appropriate sampling device, sample preparation is greatly reduced, or eliminated entirely. This in turn makes it much easier to automate.
- No analytical interference – By adding nothing to the sample before analysis, the sample is not diluted, and analytical interferences (such as solvent artefacts) are eliminated.
- Selective analysis – Optimising the sorbents and TD protocols allows only the compounds of interest to be introduced to the GC, eliminating (for example) residual water.
- Wide dynamic range – Two-stage desorption and sample splitting means that modern thermal desorbers can handle analyte concentrations ranging from part-per-trillion right up to low-percent levels.
- Analyte range – Modern sorbents and TD instrument design allow quantitative analysis of ultra-volatiles such as acetylene, all the way to semi-volatiles such as n-C44H90.
- Sample compatibility – TD can be integrated with a variety of GC sampling procedures, making it possible to sample from a wide range of sample types, whether gas, liquid or solid.
Applications of modern TD instrumentation
Thermal desorption is widely thought of as a technique for environmental air monitoring, and while this remains very important, in recent years the scope of TD has expanded substantially.
Key applications now include:
- Environmental monitoring of air, water and soil
- Monitoring industrial emissions
- Quality control of products and materials
- Aroma-profiling food and drink
- Forensic investigations
- Homeland security
- Breath analysis for disease diagnosis
- Chemical ecology
For more information on the benefits and applications of TD,
download our introductory guide: