What is analytical thermal desorption?
Thermal desorption – Preconcentration for gas chromatography
Thermal desorption (TD) is the process of heating a material to release adsorbed compounds from it.
As an analytical method, TD is used as a pre-concentration technique for gas chromatography (GC), making GC compatible with low-concentration analytes that would otherwise be impossible to detect with this method.
Through the use of valving in the flow path, it also allows high-concentration samples to be split, avoiding overload of the GC column and extending the detection limit in this direction too.
How analytical thermal desorption works
Analytical thermal desorption 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 desorbers used just single-stage operation, whereby 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.
However, most modern commercial thermal desorbers accommodate two-stage operation, whereby the gas stream from the sorbent tube is collected on a narrower tube integral to the thermal desorber, called the focusing trap or cold trap.
Heating this focusing trap releases the analytes once again, before injection into the GC. However, this time the analytes are even more concentrated, leading to improved sensitivity and better GC peak shape.
Why choose thermal desorption?
TD offers numerous benefits for the analysis of trace-level volatile and semi-volatile organic compounds (VOCs and SVOCs). These include:
- High sensitivity – Two-stage desorption using sorbent tubes allows concentration enhancements of up to 106, greatly enhancing the detection limit of GC.
- Analytical quality – The highly concentrated plug of vapour that is introduced to the GC from the focusing trap keeps peaks narrow, improving the quality of the analysis.
- 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-C40H82.
- 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 thermal desorption
Thermal desorption was originally used for occupational health monitoring, but applications have since extended to cover a much wider range. The full range of applications is illustrated by the publications listed in Application Note 004. Some of the most important are mentioned below, and are covered in greater detail on the application pages:
For more detail on the background to thermal desorption and information on application scope, see Application Note 012. See also our range of Application Notes (click “Resources” then “TD Application Notes” to see a drop-down list of Application Notes divided by category).