What is Thermal Desorption?
What is Thermal Desorption?
Thermal desorption (TD) is a powerful sample introduction technique for gas chromatography (GC) and GC-MS. Instead of dissolving compounds in a solvent, TD heats samples to release volatile and semi-volatile organic compounds (VOCs and SVOCs), which are then carried directly into the analytical system. This solvent-free approach provides very low detection limits, excellent recovery, and compatibility with a wide range of sample types.
Why Thermal Desorption?
The thermal desorption process is the most efficient way to determine volatile and semi-volatile compounds (VOCs and SVOCs) in air. These compounds can be captured at concentration levels thousands of times higher than in the original air. Most compounds are stable and tightly enough bound to the sorbent, so sampling can be done remotely with no special precautions needed to transport samples to a laboratory for analysis.
TD is not limited to gas samples, though. Compounds can be thermally extracted directly from a wide range of solid samples, such as plastics used in automobile parts, beverage bottles, or electronic components or more porous materials such as paper, cloth, and carpet. These thermally extracted compounds can help to determine the cause of off-odors or flavors, electronic system failure, or a forensic problem.
Analytes can also be extracted from liquids either by selective evaporation of the liquid matrix, followed by thermal extraction of the analytes from the resulting matrix, or by using a water-compatible sorbent to extract analytes from a liquid, followed by thermal desorption of the sorbent. The most used example of this is stir bar sorptive extraction (SBSE), sold as the GERSTEL Twister®.
- Low Detection Limits: TD enables the detection of trace-level VOCs and SVOCs, down to parts per trillion (ppt).
- Solvent-Free: Eliminates the need for solvent extraction, reducing background noise, simplifying workflows, and lowering operating costs.
- Broad Applicability: Works for solids, liquids, and gases, including air monitoring, product testing, forensic analysis, and environmental samples.
- Automation Ready: TD integrates seamlessly with autosamplers and automated workflows for high-throughput labs.
How does Thermal Desorption work?
1. Sample Collection – Compounds are trapped on a sorbent-packed tube, or the sample is directly heated in a blank tube. Samples can come from air, materials, or headspace.
2. Desorption Step – The tube is heated under a flow of inert gas, releasing VOCs and SVOCs into the carrier stream.
3. Focusing Trap – Compounds are concentrated in a cooled trap to sharpen peaks and improve resolution.
4. GC Injection – The trap is rapidly heated, transferring analytes in a narrow band to the GC or GC-MS system for separation and detection.
5. Split/No-Split Options – Analytes can be directed fully into the system or split for quantification flexibility.
The sample is collected by passing air through a tube containing the sorbent. Once sampling is complete, the tube is placed in the thermal desorber either manually or by a multifunctional robotic autosampler. The thermal desorber is then sealed, and heat and gas flow are applied to desorb the compounds from the sorbent gently.
The released compounds pass directly into a focusing trap placed just below the tube. This transfer can occur with a split flow (to prevent column overload) or without a split flow (to achieve the lowest detection limits). The focusing trap can be held at a low temperature to trap compounds on an inert surface (such as glass beads or quartz wool) or at higher temperatures using additional sorbents.
Trapping at low temperature is preferred since all compounds are trapped, and there is no doubt that any compounds crucial to the analysis are missed. Using sorbents in the focusing trap is acceptable when the analytes to be determined are known (targeted analysis) and standards are available to ensure proper trapping and transfer to the GC column is occurring.
After the compounds have been trapped, the focusing trap is rapidly heated, and analytes are passed directly onto the GC column for separation, followed by detection with a mass spectrometer. As with the transfer from the thermal desorber, transfer can be performed in a split or splitless mode to dilute further or concentrate the analytes before they reach the GC column.
TD done this way has many practical benefits:
- The direct to trap – direct to column design eliminates the need for valves and transfer lines providing the most direct inert path for the analytes to pass through the system. This eliminates sample loss and carry over as well as reducing maintenance costs.
- Trapping can be done non-selectively at low temperatures for determination of unknowns (non-targeted analysis) or the trap can use sorbents for method specified compound determination (targeted analysis).
- Split flow in the system is 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 flexibility maximizes the use and investment with a single system being able satisfy a wide range of analytical needs.
- The use of a robotic autosampler adds to the systems utility, allowing sample preparation for thermal desorption to be performed without human intervention, and providing access to techniques beyond TD (such as dynamic headspace, Twister, TF-SPME, and pyrolysis).
Applications of Thermal Desorption
Thermal desorption is widely used across industries:
- Environmental Monitoring – Ambient air, soil gas, stack emissions, and microplastics studies.
- Food & Flavor – Aroma and off-odor analysis, flavor profiling, packaging migration studies.
- Forensic & Toxicology – Fire debris, explosives, chemical residues, and toxic gases.
- Material Testing – VOC emissions from consumer products, building materials, and polymers.
- Occupational Safety – Workplace exposure monitoring for hazardous substances.
Thermal Desorption vs. Other Techniques
- Solvent Extraction vs TD: TD avoids solvent contamination, providing cleaner chromatograms and easier automation.
- SPME vs TD: Solid-phase microextraction requires matching specific phases to analytes of interest. Small phase volume does not provide detection limits comparable to TD.
- SPME Arrow vs TD: SPME Arrow requires matching specific phases to analytes of interest. Although phase volume is larger than SPME, it still does not provide detection limits comparable to TD.
FAQ
Air, gases, solids, and liquids that contain VOCs or SVOCs.
Tenax® TA, Carbograph™, Carboxen®, and multi-bed tubes depending on target analytes.
Down to ppt levels with optimized trapping and cryogen-free focusing.