Optimising the analysis of trace volatile and semi-volatile organic chemicals for monitoring odorous emissions from materials

Thermal desorption (TD) is a readily automated gas extraction technology based on standard gas chromatography parameters and providing an efficient, high-sensitivity alternative to convential solvent extraction. It is applied in single stage form to whole-air samples (canisters, bags, air streams) or in two stage form to organic analytes collected on sorbent tubes. TD involves the extraction of volatile or semi-volatile organic compounds from a sorbent by heating the sample, rapidly, in a flow of inert gas. Under the influence of heat and gas flow, analytes 'thermally desorb' from the sorbent into the gas stream and are transferred to the analyser as a small (100-200 microliter), discreet and concentrated volume of vapour. Overall concentration enhancement factors as high as 10^4 (single stage) or 10^6 (two stage) can be obtained using thermal desorption thus allowing conventional laboratory gas chromatography (GC) or GC-mass spectrometry (GC-MS) technology and/or alternative real-time vapour detectors (enose sensor arrays, process MS, etc) to measure compounds that were originally at ppb or ppt concentrations. Given the sensitivity of the human nose to some classes of organic compound at sub-ppb levels - particularly oxygenated compounds and those containing nitrogen or sulphur - thermal desorption is invariably required as part of the analytical procedure. TD-GC-MS(-enose) methodes provide the only viable analytical option to olfactometry. Through inherently simple, many factors contribute to the performance and efficiency of the TD process which in turn determines the ultimate sensitivity and reliability of a TD-based analytical method. These inertness, band broadening and analyte/system stability. This paper describes optimisation of all of these factors during the development and validation of thermal desorption procedures for odorous compounds including reactive species. Examples of optimum method performance in terms of precision, linearity, sensitivity and efficiency are presented. A novel approach to method validation via repeat analysis is also described. New technology enhancements which allow the fixing of retention times of thermally desorbed compounds (with or without additional confirmatory MS data) are also presented. These allow confident measurement of trace-level, odorous, target analytes in worst-case sample matrices - i.e. amongst complex mixtures of higher concentration organics.