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Environmental
The use of active sampling onto glass tubes packed with charcoal, followed by
carbon disulfide (CS
2
) extraction and gas chromatography (GC) analysis, was
developed as an air monitoring method for vapor-phase organic compounds
(VOCs) in the 1970s. The approach is still used today for some personal expo-
sure assessment (occupational hygiene) applications and stack emission test-
ing, but is fundamentally limited with respect to detection limits. Thermal des-
orption (TD) is a complementary gas extraction technique whereby sorbent
tubes (Figure 1) are heated in a flow of carrier gas. Trapped vapors desorb from
the sample tubes into the gas stream and are transferred into the GC/MS for
analysis. Here, we summarize the key advantages of thermal desorption versus
solvent extraction.
Sensitivity & Reproducibility
Solvent extraction of charcoal tubes requires at least 1 or 2ml of CS
2
followed
by injection of only 1-2µl of extract into the GC/MS, resulting in a 1000-fold
dilution of the sample right at the start of the process. Conversely, thermal
desorption allows complete transfer of all target analytes to the analytical sys-
tem, with no dilution or solvent interference. Detection limits offered by thermal
desorption methods facilitate ambient monitoring at ppt/ppb levels as well as
higher ppm (and%-level) concentrations. In addition to high sensitivity, thermal
desorption is highy reproducible, offering efficiency greater than 95%, regard-
less of ambient conditions and the nature of the target analytes. By comparison,
results from solvent desorption tubes may be highly variable.
Passive Sampling Option
While thermal desorption tubes are used extensively for active air sampling,
they are also compatible with low-cost passive sampling. Passive samplers
eliminate the requirement for personal monitoring pumps making them much
less heavy/intrusive. Instead of a pump, each tube is simply fitted with a diffu-
sion cap at the sampling end.
Repeat Analysis & Method Compliance
The historical advantages of solvent desorption tubes over thermal desorption, such as multiple sample injection and method
compliance, no longer hold true. Since the advent of the SecureTD-Q™ thermal desoption unit, quantitative re-collection of split
flow during both tube and trap desorption is possible. The utility of quantitative sample re-collection for repeat TD-GC/MS
analysis has recently been recognized in standard methods as an aid to TD method/data validation.
1
Well-validated thermal des-
orption methods for many applications are now available from all the major international standards agencies. Key examples
include: EN ISO 16017, ISO 16000-6, ASTM D-6196, US EPA Method TO-17, NIOSH 2549, MDHS 72, 80, etc. (UK) and EN
14662.
Conclusion
Thermal desorption technology offers several significant advantages over conventional solvent extraction. TD systems offer better
sensitivity, desorption efficiency, and reproducibility compared to charcoal/CS
2
systems. Additionally, tubes may be used for both
passive and active sampling without modification. These benefits, in combination with SecureTD-Q™ technology, which allows
repeat analysis, make thermal desorption an excellent choice for many air monitoring applications.
References
1. ASTM D6196-03
Enhancing Air Monitoring Methods with Thermal Desorption
Advantages Over Solvent Extraction Tubes
By Liz Woolfenden, Director, Markes International, UK, and Irene DeGraff, Product Marketing Manager
• Accurately monitor down to ppb/ppt levels.
• Use thermal desorption tubes for either active or passive sampling, without modification.
• Compliant with air sampling methods.
Figure 1
A selection of thermal desorption
air sampling tubes from Restek’s new line.