TO-17a Thermal Desorption - VOC Analysis

What is it?

Active air sampling is a method of determining airborne concentrations of volatile contaminants using an active sampling medium, such as a thermal desorption tube.  Air samples are collected by actively pumping air through the sampling tube with a small pump at a known flow rate. Contaminants in the air are selectively captured by the sampling medium and stored within the tube.

Sampling times for active sampling can vary from as low as a few minutes to a number of hours.  After the sampling period, the tube is capped and returned to the laboratory for analysis where the contaminants are extracted and analyzed by heating the tube and injecting the effluent onto a GC column, typically connected to a mass spectrometer.  Thermal desorption tubes are used almost exclusively for VOC analysis, but can be custom-tailored to a variety of VOCs from standard hydrocarbons, to oxygenated or halogenated hydrocarbons, to aromatics and amines.


Thermal desorption tubes

Thermal desorption tubes are packed with adsorbent materials that trap VOCs.  There are a variety of adsorbents used in the field of thermal desorption.  Selection of the proper adsorbent(s) is critical to maximizing the efficiency of the thermal desorption process (sampling & analysis), and thereby ensuring the best possible results.  When selecting the correct thermal desorption tube, the analyst must consider the specific analyte or group of analytes of interest.  An ideal thermal desorption tube will capture and retain compounds of interest for the entire sampling period, while subsequently allowing total desorption of the analytes without thermal decomposition.  Airzone utilizes a variety of thermal desorption tubes that allow for the capture of a wide range of VOCs, from tubes designed for lighter, more volatile compounds (Carbotrap 217, n-C3 to n-C12) to tubes designed for heavier, less volatile compounds (Carbotrap 300, n-C3 to n-C30).

Benefits of Thermal desorption tubes

  • Sorbent selection allows for customization of tubes for specific projects.
  • Detection limits enhanced by a factor of 102 – 104, making thermal desorption an ideal option for both indoor and outdoor air monitoring and OH&S studies.
  • Better desorption efficiencies (~95%) for all VOCs compared to most solvent extraction methods (30-80%).
  • No manual sample preparation reduces time and cost per analysis.
  • Reduced costs of consumable materials.
  • Reusable tubes – Thermal desorption tubes are reusable 100-200 times.
  • No solvent disposal costs and associated overhead expense.
  • No masking of peaks of interest by the solvent (especially important with MS).
  • No introduction of solvent artifacts/impurities.

Detection Limits

Thermal desorption offers much better sensitivity than solvent extraction methods, by a factor of 102 – 104. However, as with passive sampling, detection limits are influenced by the sampling strategy (i.e., the exposure time and sampling flow rate). The desorption efficiency, which is compound- and adsorbent-specific, also plays a role in determining detection limits. Environmental and analytical factors such as relative humidity, blank levels and measurement sensitivity also affect detection limits as they do for other sampling devices.  In most cases, thermal desorption systems can accurately monitor down to ppb/ppt levels.