Thermal Desorption GC-MS (TD GC-MS)

The inherent sensitivity of chromatography and mass spectrometry makes the combined techniques ideal for the analysis of smells, odours and/or non odorous volatile organic compounds (VOC) emitted from a wide range of products from food to polymers and pharmaceutical compounds (see residual solvents in medicinal compounds) to clothing and toys. Although Headspace GC-MS has been used for this type of analysis for many years, the technique lacks the sensitivity of Thermal Desorption GC-MS (TD GC-MS).

One of the most common applications for TD GC-MS is the analysis of odours from indoor or outdoor air.

Environmental samples are usually collected in gas bags or directly onto Thermal Desorption tubes. Samples collected in gas bags should be couriered to SGS M-Scan for transfer to Thermal Desorption tubes as soon as possible. An empty gas bag which can be used to detect background components from the bag and act as a travelling blank should accompany the samples. Gas bags should have a volume of at least 1 litre.

If samples are to be collected onto thermal desorption tubes on site, pre conditioned tubes are supplied by SGS M-Scan for the purpose. Sufficient tubes are usually supplied for duplicate sampling and spare tubes are provided as travelling blanks.

For general odour/VOC screening, tubes containing the porous polymer Tenax are used. Details of how to collect the samples are available in Collection of Atmospheric or Gaseous Samples onto Tenax which accompanies tubes despatched by SGS M-Scan.

Under certain circumstances adsorbants other than Tenax may be recommended e.g. for sampling mercaptans. To avoid overloading these tubes, lesser volumes of sample should be collected-please consult SGS M-Scan for details or advice.

Samples are collected onto Thermal Desorption tubes by active pumping using a pre calibrated pump. Intrinsically safe, hand held pumps are available for hire from SGS M-Scan as part of a complete analytical programme.

Once returned to SGS M-Scan the VOCs are desorbed from the Tenax tubes, concentrated on a cryotrap, before being flash desorbed into a Gas Chromatography-Mass Spectrometry (GC-MS) system. The results of odour/VOC analysis are compared against SGS M-Scan’s own database of odour thresholds which contains the concentration of compounds at which 50% of the population would notice their odour. This database is based on the best olfactometry data available worldwide.