Adsorptive Enrichment and Thermal Desorption

Relative strengths of some commonly used sorbents (Markes 2012a). = Graphitized carbon blacks. 

Halogenated narcotics Vinyl chloride Ethylene oxide Styrene 

High-boiling, non-polar substances Halogenated narcotics Ethylene oxide 

Thermally stable materials offering a wide range of sorbent strengths have been developed over the last 30 or 40 years for use in TD apphcations. These include standard carbon blacks (Carbographs and Carbopacks/traps), carbonised molecular sieves such as the Carboxen series and modified carbon blacks (Carbograph 5 TD and Carbopack X) which offer much of the strength of carbonized molecular sieves but with minimal water retention. All are now in widespread use as sorbents in thermal desorption tubes (Betz, et al., 1989). 

Like Tenax, GCBs also have a low affinity for water. These adsorption materials can be dried with a dry gas stream, for example, the GC carrier gas (in the direction of adsorption ), without significant loss of material (dry purge). 

---

K. Dettmer and W. Engewald. Absorbent Materials Commonly Used in Air Analysis for Adsorptive Enrichment and Thermal Desorption of Volatile Organic Compounds, Anal. BioanaL Chem. 2002,373, 490. 

Dettmer, K., Knobloch, T., Engewald, W. Stability of reactive low boiling hydrocarbons on carbon based adsorbents typically used for adsorptive enrichment and thermal desorption. Fresenius J. Anal. Chem. 366, 70-78 (2000)... 

Dettmer K, Engewald W (2002) Adsorbent materials commonly used in air analysis for adsorptive enrichment and thermal desorption of volatile organic compounds. Anal Bioanal Chem 373(6) 490-500. doi 10.1007/s00216-002-1352-5... 

Various sample enrichment techniques are used to isolate volatile organic compounds from mammalian secretions and excretions. The dynamic headspace stripping of volatiles from collected material with purified inert gas and trapping of the volatile compounds on a porous polymer as described by Novotny [3], have been adapted by other workers to concentrate volatiles from various mammalian secretions [4-6]. It is risky to use activated charcoal as an adsorbent in the traps that are used in these methods because of the selective adsorption of compounds with different polarities and molecular sizes on different types of activated charcoal. Due to the high catalytic activity of activated charcoal, thermal conversion can occur if thermal desorption is used to recover the trapped material from such a trap. 

---