An Introduction to Analytical Thermal Desorption

Thermal Desorption – Preconcentration for Gas Chromatography

Thermal desorption (TD) refers to the process of heating a material in order to release adsorbed compounds from it.

TD, as an analytical method, is used as a pre-concentration technique for gas chromatography (GC), enabling GC to be compatible with low-concentration analytes that would otherwise be impossible to detect with this method.

Through the use of valving in the flow path, TD also allows splitting of high-concentration samples, preventing overload of the GC column and extending the detection limit even in this direction.

How Analytical Thermal Desorption Works

Fundamentally, analytical thermal desorption involves collecting VOCs onto a sorbent, followed by heating this sorbent in a flow of gas in order to release the compounds and concentrate them into a smaller volume.

Early thermal desorbers used only single-stage operation, whereby the volatiles collected on a sorbent tube are discharged by heating the tube in a flow of gas, from where they travel directly into the GC.

However, a two-stage operation is accommodated by most modern commercial thermal desorbers, whereby the gas stream from the sorbent tube is collected on a narrower tube integral to the thermal desorber, referred to as the cold trap or focusing trap.

When this focusing trap is heated, the analytes are released once again before they are injected into the GC. However, this time the analytes are even more concentrated, resulting in better GC peak shape and enhanced sensitivity.

Why Choose Thermal Desorption?

TD provides numerous benefits for the analysis of trace-level volatile and semi-volatile organic compounds (VOCs and SVOCs). These include:

• Analytical quality – The highly concentrated plug of vapor that is introduced to the GC from the focusing trap keeps peaks narrow, enhancing the quality of the analysis
• High sensitivity – Two-stage desorption using sorbent tubes permits concentration enhancements of up to 10⁶, considerably improving the detection limit of GC
• Saving effort and time – By selecting the perfect sampling device, sample preparation is significantly reduced, or even entirely eliminated. 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
• Wide dynamic range – Two-stage desorption and sample splitting means that modern thermal desorbers are capable of handling analyte concentrations ranging from part-per-trillion right up to low-percent levels
• Selective analysis – Optimizing the sorbents and TD protocols enables only the target compounds to be introduced to the GC, eliminating (for instance) residual water
• Sample compatibility – TD can be combined with many different GC sampling procedures, making it possible to sample from a variety of sample types, whether solid, gas, or liquid
• Analyte range – Modern sorbents and TD instrument design enable quantitative analysis of ultra-volatiles such as acetylene, all the way to semi-volatiles such as n-C₄₀H₈₂.

Applications of Thermal Desorption

Originally, TD was used for occupational health monitoring, but applications have since extended to cover a much broader range. The complete range of applications is illustrated by the publications listed in Application Note 004. Some of the most important applications are mentioned below:

• Occupational/workplace health monitoring
• Residual volatiles emitted from materials and products
• Outdoor environmental monitoring
• Aroma profiling of drink and food
• Breath analysis for disease diagnosis
• Defence/homeland security (detection of chemical agents)
• Studies of biological systems, including plant–herbivore interactions

This information has been sourced, reviewed and adapted from materials provided by Markes International Limited.

For more information on this source, please visit Markes International Limited.