Sorbent Thermal Desorption

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Contributors: Wolfgang Horn, Catherine H. Stephens
=== Overview ===
Technique: Sorbent thermal desorption (S-TDS), or TDS
Formal name: Sorbent Thermal Desorption
Summary description of this technique: For the detection of VOCs in the air sampling on sorbent tubes is very efficient. The tubes were analysed with a combination of a TD-System with GC/MS (link to GC-MS).
Sample image of the data: This is an image depicting how this technique works. It is a sample introduction method for gas chromatography-mass spectrometry. The data generated using this method will look the same as that generated using the GC-MS technique.
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===Details===
:What this techniques measures: Sorbent Thermal Desorption measures volatile organic compounds that are present in the air of display cases the surrounding air in the buildings or outdoor air. The VOCs were sampled on adsorption tubes filled with one or several specific adsorptive media. The loaded tubes were then thermally desorbed and the released compounds are typically separated via gas chromatography and detected with a mass spectrometer. The most used adsorbent is Tenax TA® which is suitable for VOCs that elutes on a specific column between n-Hexane and n-Hexadecane. Up to semi volatile organic compounds (SVOCs) that eluting comparable to Docosane Tenax TA® is usable as well. Nevertheless, combinations of other adsorbents can enlarge the spectrum or is better suited for specific compounds. but many other VOCs. The main advantage of TDS is the low detection limit of about 1 µg/m³ for many VOC-compounds. 
:Limitations of this technique: Compounds like formaldehyde or acetaldehyde couldn’t be measured with DTS systems. For very sensitive fragile organic compounds raise of temperature can be problematic and these compounds couldn’t be measured with DTS. 
:Can/how can this technique be made quantitative?: This technique is described in many standards and was used for labelling procedures. The nature of labelling is the quantitative analyse of the emitted compounds from materials. Knowledge of the air volume that is drawn through the adsorbent is necessary for the quantitative calculation of concentrations in air. The results were compared to standards of the compounds that were also measured with the GC-system.
===Samples===
:Phases it can be used to examine (gas, liquid, solid): Only the air near the interesting object or from a special place or room were used.
:Is this technique non-destructive? Non-destructive. Only air is drawn through adsorbents. A very small effect is that the small amount of air is cleaned. 
:How invasive is this technique? Typically it is not invasive. If air samples are taken from display cases the air exchange in the cases could be higher than normal. 
:Minimum size of sample necessary to use this technique? The procedure of thermal desorption need a small amount of air. Volumes between 1 and 5 litres are typical amounts. 
:Time to run one experiment? Sampling will last in maximum 1 hour. The analytical procedure that follows needs approximately 2 hours. 
:Sample preparation methods: Methods are described in ISO, ASTM – standards or EPA methods (see references below).
=== Applications ===
:Examples of how this technique is used in the field? Lots of health-related evaluations of indoor air concentrations of VOCs were done. Nearly all labelling procedures, 
:Risks associated with using this technique? There are no risks for the samples investigated. Only air around or near the material is needed
=== Budgetary Considerations ===
:Approximate cost to purchase equipment for this technique? A typical TD-GC-MS combination costs about $200,000 USD (2022). Thermal desorption tubes costs approximately $100 USD (2022) but it can be used several times (50 to 100 times). 
:Annual cost to maintain or run? For TD-GC-MS you need helium as the carrier gas. For one GC you need 3 gas cylinders per year (~$3000 USD); Standards for comparison ($1000 – 10000 USD); new TD-tubes; repair or annual service of the analytical equipment (up to $10,000 USD) [2022 values] 
:Sample analysis costs? Typically one analysis costs $200 USD (2022 values). But you must add also the personal costs for analysing and reporting the data. 
:Time it may take to get results from a contract laboratory? Single samples need in minimum one day for reporting.
=== Case Studies ===
=== Additional Information ===
:Complementary Techniques: Some of the relevant organic compounds that can be found in indoor or display case air are aldehydes (e. g. formaldehyde, acetaldehyde) and carbonic acids (acetic acid or formic acid). These compounds couldn’t been measured with TD-GC-MS. So you need for the complete spectrum two further analytical methods. (These include analyses for the presence of Formaldehyde And/or for formic and acetic acid in air; a VDI standard exists based on an IC method.”) 
:Variations of this technique: SPME
=== References Resources, databases, publications ===
* ASTM D6196-15e1; 2018 - Standard Practice for Choosing Sorbents, Sampling Parameters and Thermal Desorption Analytical Conditions for Monitoring Volatile Organic Chemicals in Air
* EPA Method 8260D (SW-846): Volatile Organic Compounds by Gas Chromatography-Mass Spectrometry (GC/MS)
* ISO 16000-6:2021: Indoor air — Part 6: Determination of organic compounds (VVOC, VOC, SVOC) in indoor and test chamber air by active sampling on sorbent tubes, thermal desorption and gas chromatography using MS or MS FID