Breath sampling is a valuable approach for obtaining information used for disease diagnosis, monitoring of metabolic stress and personal exposure to chemicals. Breath samples can be collected using a number of techniques, which can then be used with thermal desorption to extract and concentrate the VOCs, maximising sensitivity for analysis of trace-level target compounds while minimising interferences.
Direct desorption/thermal extraction
Direct desorption is a variation on the principle of dynamic headspace that can be used to rapidly screen small quantities of solid or semi-solid materials. Small samples are placed directly into empty TD tubes or via tube liners. These are then heated in a stream of inert gas to sweep volatiles directly from the sample matrix onto the focusing trap of the thermal desorption instrument, for subsequent analysis by GC.
Microchambers are compact, stand-alone test chamber units that allow rapid sampling of vapour-phase organic compounds from a product or material, complementing small-scale chamber testing by third-party laboratories. Microchamber sampling is a type of dynamic headspace sampling and a powerful tool for increasing laboratory productivity. The technique is usually used in combination with sorbent tubes and off-line thermal desorption–GC–MS.
Sorptive extraction is an equilibrium process involving interaction between a sorbent and a sample, either by immersion in the sample or through contact in the headspace above the sample. It is commonly used as an alternative to liquid–liquid extraction (LLE) or solid-phase extraction (SPE). Sorptive extraction is a convenient approach for complex samples and for monitoring persistent organic pollutants in foods, beverages and other products derived from natural sources.
Solid-phase microextraction (SPME) uses a small fiber coated with absorptive phase(s), to extract volatiles in either headspace or immersive modes. SPME extraction is an equilibrium process dependent on the partitioning of analytes between the sample matrix (either liquid, solid or headspace) and the absorptive phase. Analytes that become absorbed into the SPME phase are subsequently desorbed using a heated injector, before transfer to the GC column.
Standards & calibration
Calibration of the complete analytical process is of paramount importance when conducting quantitative studies. For TD analysis, additional considerations should be made regarding the loading of standards to be representative of samples, with a variety of tools available to support this.
Headspace (HS) has been widely used for quantitative evaluation of VOCs in solid and liquid samples for many years. Samples are placed into vials, sealed and incubated. Once equilibrium is reached VOCs partition into the headspace at levels that are proportional to their concentration in the original liquid or solid sample. A precise aliquot of headspace vapour is then transferred/injected directly into the GC (traditional methods) or via an electrically-cooled focusing trap for enhanced performance.