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

Abstract

Thermal desorption is a readily automated gas extraction technology based on standard gas chromatography (GC) parameters and providing an efficient, high-sensitivity alternative to conventional solvent extraction. The process of thermal desorption involves the extraction of volatile or semi-volatile organic compounds from a sorbent 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 systems 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 efficiency of the thermal desorption process, which in turn determines the ultimate sensitivity and reliability of a thermal desorption-based analytical method. These factors include retention efficiency (during sampling/focusing), desorption efficiency, artefacts, band broadening and analyte/system stability. This paper describes optimisation of all of these parameters during the development and validation of thermal desorption procedures. Examples of optimum method performance in terms of precision, linearity and sensitivity are presented. A novel approach to overcoming the traditional one-shot limitation 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 alternative real-time detection systems, which can be combined with thermal desorption to extend its utility for indoor air research, are also described.