Thermal Desorption (a.k.a. ‘Analytical’ Thermal Desorption) is a sample introduction technique for GCMS where compounds of interest that are adsorbed or absorbed into a solid (a ‘sorbent’ or other solid material of interest) are released from that solid using heat and an inert gas flow.

Once released, these compounds can be either recaptured or ‘focused’ onto a secondary trap for re-concentration and rapid injection into a GCMS, or passed directly, more slowly, to an instrument such as a SIFT-MS that analyzes the chemicals released in real time.

What's TD?

Sorbents are placed into tubes that are used to capture and concentrate compounds of interest from many liters of air that are pumped through them.  Any compounds of interest trapped onto the sorbents and then re-released later, with heat and flow, by the TD.  When this happens, the compounds of interest are in a concentrated form, thousands of times more concentrated than in the original air.  In this way TD can be used for trapping and pre-concentrating air or gas samples for sensitive detection by other instruments.

Thermal Desorption (a.k.a. ‘Analytical’ Thermal Desorption) is a sample introduction technique for GCMS where compounds of interest that are adsorbed or absorbed into a solid (a ‘sorbent’ or other solid material of interest) are released from that solid using heat and an inert gas flow.

Once released, these compounds can be either recaptured or ‘focused’ onto a secondary trap for re-concentration and rapid injection into a GCMS, or passed directly, more slowly, to an instrument such as a SIFT-MS that analyzes the chemicals released in real time.

Why TD?

Sorbents are placed into tubes that are used to capture and concentrate compounds of interest from many liters of air that are pumped through them.  Any compounds of interest trapped onto the sorbents and then re-released later, with heat and flow, by the TD.  When this happens, the compounds of interest are in a concentrated form, thousands of times more concentrated than in the original air.  In this way TD can be used for trapping and pre-concentrating air or gas samples for sensitive detection by other instruments.

GERSTEL TDU with Multiple Tubes from GERSTEL, Inc. on Vimeo.

TD is not limited to gas samples though.  Solid samples, such as plastics used in automobile parts or polymers that make up beverage bottles, can be analyzed directly by TD.  Chemicals on the surface of the sample are “thermally extracted” in this way and analyzed for many purposes, such as predicting off-odors or flavors from these solids.  Liquids can also be analyzed by exposing a specifically chosen solid (like silicone) to a liquid of interest (such as green tea or whiskey) and then thermally extracting the solid as above.

In all cases, TD serves as a way to capture and concentrate trace amounts of compounds found in solids, liquids, and gases.

How does TD work?

TD is a two stage process.  In the first step, a thermal desorption tube containing the sample is lowered into the thermal desorber by a multifunctional robotic autosampler.  Once in in place, the tube is leak checked and then heat and flow are applied to the tube to drive off compounds of interest.

The released compounds pass directly into a focusing trap placed just below the tube.  This transfer can occur with a split flow of gas (do dilute the sample) or without any split flow at all (‘splitless’).  Inside the trap, compounds are re-captured and focused either onto a glass substrate at low temperatures (for non-target analysis of unknowns) or onto sorbents in the focusing trap (chosen a ahead of time to match targeted, known compounds of interest).  This process occurs over a few minutes.

In the second stage, the focusing trap is rapidly heated, with flow, and the focused analytes are passed directly onto the GC column for separation and later identification and detection with a mass spectrometer (or directly into a SIFT-MS, where detection is immediate).  During this process, the flow out of the trap can be split, with a portion discarded to reduce the concentration to the detector, or without a split (‘splitless’) to maximize the mass of sample sent to the detector.

TD35plus_det_2-666x1024 What is Thermal Desorption?TD done this way has many practical benefits:

  • The direct to trap, direct to column/detector design is the simplest possible way to move the analytes through the system, making troubleshooting easier, reducing maintenance costs, and minimizing the opportunity for loss of analytes in the process (or carry-over from other samples). With this ‘glass to glass’ approach, there is no possibility to introduce interferences from the instrument
  • trapping can be done non-selectively with low temperatures or cryogens for non-targeted analysis of unknowns, or, the trap can use pre-chosen sorbents for selective, targeted analysis
  • The splits in the system are multiplicative: using a 100:1 split at each stage results in an overall reduction of signal by 10000:1!  Alternatively, both stages can be splitless.  In this way the dynamic range of the instrument is maximized, allowing for the analysis of very dilute or very concentrated samples
  • As mentioned above, almost any type of sample is amenable to TD: solids, liquids, and gases.  This maximizes the use and investment in the detection system by making the overall system very flexible
  • Even further, the use of a robotic autosampler used is also incredibly flexible, allowing sampler preparation for thermal desorption to be performed without human intervention, and providing access to techniques beyond TD (such as dynamic headspace, TF-SPME, and pyrolysis).

What is TD used for?

TD is used in many different applications.  Some common examples are

  • The analysis of flavors and fragrances used in foods and consumer products
  • Measuring off-gassing of materials used in homes, offices, and cars (the ‘new car smell’)
  • Examining foods directly for authenticity or for understanding flavor or odor complaints
  • Detecting dangerous chemicals for law enforcement or military applications
  • Determining the quality of indoor or outdoor air, and making sure they meet legal requirements
  • Making sure that packaging for drugs or plastics in medical implants are safe