Review : Optimising Analytical Performance and Extending the Application Range of Thermal Desorption for Indoor Air Monitoring

Abstract

Thermal desorption is a readily automated gas extrac tion technology based on standard gas chromatography (GC) parameters and providing an efficient, high-sensi tivity alternative to conventional solvent extraction. The process of thermal desorption involves the extraction of volatile or semi-volatile organic compounds from a sor bent or material by heating the sample in a flow of inert gas. The extracted analytes are then transferred in the flow of carrier gas to the analyser (typically GC or GC- MS) as a small, discreet and concentrated volume of vapour. In effect the thermal desorber becomes a multi purpose, stand-alone GC injector. Concentration factors as high as 105-106 can be achieved using modern sys tems with analytes collected from several tens, even hundreds, of litres of air being delivered to the analyser in as little as 200 μl of gas. Though inherently simple, many factors contribute to the performance and efficien cy of the thermal desorption process, which in turn deter mines the ultimate sensitivity and reliability of a thermal desorption-based analytical method. These factors in clude retention efficiency (during sampling/focusing), desorption efficiency, artefacts, band broadening and analyte/system stability. This paper describes optimisa tion of all of these parameters during the development and validation of thermal desorption procedures. Exam ples of optimum method performance in terms of preci sion, linearity and sensitivity are presented. A novel approach to overcoming the traditional one-shot limita tion of thermal desorption is also described. Examples are presented which illustrate the range of indoor air- related applications for optimised thermal desorption- GC(-MS). New specialist sampling apparatus and alter native real-time detection systems, which can be com bined with thermal desorption to extend its utility for indoor air research, are also described.