Dependence of thermal desorption method for profiling volatile organic compound (VOC) emissions from soil

The study of volatile organic compounds (VOCs) in soil and other complex substrates is a rapidly developing field. VOCs in soils originate from a wide variety of biological sources; bacterial, fungal, mesofaunal, and plant. They are vital to inter- and intra-species interaction and soil health, and therefore offer a potential reactive, functional diagnostic tool to determine soil quality. The standard methodology for untargeted VOC profiling in environmental samples has been headspace solid phase microextraction (HS-SPME), avoiding the need for solvent extraction procedures used in many biological soil tests. However, this technique can suffer from a lack of sensitivity due to competition between individual VOCs on the solid phases used for VOC recovery. Other common techniques used to monitor the VOC fingerprints from soils include high capacity sorptive extraction (HCSE). This study presents a novel SPME-trap-enrichment method using an automated, cryogen-free, focusing and pre-concentration trap method to reduce phase competition and increase sensitivity of analysis. This method was evaluated against single-SPME-trap and HCSE methods for sensitivity and number of compounds identified using a range of induced soil qualities (good, medium and poor). Results showed that SPME-trap-enrichment was able to identify 71% and 7% more compounds than single-SPME-trap and HCSE, respectively, using a software-based data processing approach, as well as increased total peak areas. The VOC profile was substantially affected by the extraction method used. The samples within the ‘poor’ treatment produced a larger number of aromatic, carboxylic acid and sulphur containing compound groups, while the ‘good’ and ‘medium’ groups were largely characterised by ketones and aldehydes. The potential ecological significance of the compounds identified were also explored. Of those tested, SPME-trap-enrichment represented the most robust and sensitive technique for VOC analysis, offering the potential to better elucidate the multifaceted interactions of VOCs within plant-microbial-soil systems.