Thermal Desorption

Thermal desorption (TD) is a sampling technique used with GCs that concentrates volatile organic compounds (VOCs) and semi-volatile organic compounds (SVOCs) in order to improve sensitivity and provide better peak shapes on instrument chromatograms. Normally, these compounds would be extremely difficult or impossible to distinguish on instrument readouts through ordinary analysis, but TD allows users to analyze these compounds more easily in a streamlined process. TD offers several advantages over other sample preparation techniques due to its speed and automation. Most modern TD instruments can be integrated with GC or GC/MS systems to run continuously to reduce waste and improve sample throughput.

The basic principle of TD relies on the higher vapor pressure of VOCs. Depending on the application, soil, foods, polymers or other materials containing VOCs are placed in stainless steel or glass TD sample tubes. While sample tubes are commonly used for the technique, several sampling alternatives exist, including direct sampling from the atmosphere. First, the sample is heated to high temperatures, causing the volatile compounds to separate from the sample. An inert carrier gas, such as nitrogen, moves the volatile compounds to a sorbent tube to be collected and concentrated. This sorbent tube is then heated to release the volatile compounds into the inlet of a GC. This process is known as a single-stage operation, and has been in use since the 1970s when the technique was first utilized for GC.

Unlike older TD instruments that utilize a single-stage operation, most modern TD instruments utilize a two-stage operation. In a two-stage operation, instead of leading directly to the GC, the sorbent tube leads to a narrower focusing/cold trap to further concentrate the desired compounds. This trap can be packed or unpacked with sorbent and is often cooled below room temperature. The trap is then heated while a stream of carrier gas moves the collected compounds into the GC for analysis. This second step improves the sensitivity and peak shapes. Newer TD systems also allow users to collect the volatile compounds from an outlet just before the GC inlet for later analysis under different conditions.

The most common applications for TD involve intaking a set volume of air into a TD sample tube for analysis. Applications include occupational health monitoring, toxic emissions from manufacturing processes, and vehicle and environmental air quality testing. While air monitoring and testing is the most common application for TD, many other applications exist, ranging from flavors and fragrances testing to residual chemical testing in pharmaceuticals.

PerkinElmer is a leading vendor for TD, and has been since the introduction of the technique in the 1970s. PerkinElmer’s TurboMatrix line of TD instruments comes in a variety of configurations to meet users’ needs. Markes is another prominent vendor of TD instruments with its UNITY and TD100 instruments. The company’s partnership with Agilent Technologies allows its TD instruments to easily integrate with Agilent’s line of GC systems. Shimadzu is also a top vendor for TD, with its new TD-30 series of TD systems released earlier this year, which can be equipped with up to 120 samples for large-volume automated analysis. Prices for TD vary, as sample tubes can be a few hundred dollars while instruments can cost a few thousand dollars.

Growth in demand for the TD market should follow a similar course as GC and GC/MS, both of which are expected to increase in the low to mid-single digits over the next several years. The largest area of growth will come from the applied sector for its use in environmental testing and applications in the food and beverage industry.

TD at a Glance: 

Leading Vendors

  • Markes
  • Shimadzu
  • PerkinElmer

Largest Markets

  • Environmental Testing
  • Cosmetics
  • Agriculture and Food

Instrument Price

  • $4,000–$35,000
< | >