## 4 Discussion
### Collection Technique Considerations  
The SPME fibre coating has a large influence on the types of compounds that can be collected. The PDMS/DVB fibre used in the current study is generally used for the collection of volatiles, amines and nitroaromatic compounds with molecular weight range of 50-300. Low molecular weight VOCs (<90 MW) tend to be more efficiently collected on fibres containing carboxen. This explains the low recovery of lighter sulfides and lighter oxygenated compounds using the PDMS/DVB fibre. The fibre is bipolar which extends its range to collect polar and nonpolar compounds. However, targeted alcohols analysis is traditionally performed using a carbowax fibre which also explains the reduced number of alcohols collected using SPME. The wider target range using PDMS/DVB is advantageous due to the number of compound classes typically reported in decomposition odour. Due to the nature of non-target and forensic analyses, widening the range of analytes that can be collected using a single fibre allows for more compounds of interest to be collected while ensuring high throughput. The majority of studies on decomposition odour have used the PDMS/DVB fibre to obtain successful results [1,2,12]; however comparable results have been obtained using a Carboxen/PDMS (CAR/PMDS) fibre [17]. Although sulfur-containing compounds are consistently reported as major constituents of decomposition odour (especially dimethyl disulfide and dimethyl trisulfide) [1,3,6,15,17,18,19], the PDMS/DVB fibre is not suited to the collection of this compound class. This is a significant drawback of this type of analysis, especially during early decomposition where sulfur-containing compounds are prominent. The dual sorbent combination used for the sorbent tubes increased the range of compounds collected. Tenax TA is suitable for C\({}_{7}\)-C\({}_{30}\) and typically targets aromatics, semi-volatiles, nonpolar and polar compounds, while the Carbograph 5 TD increases the ability to collect VOCs in the C\({}_{3/4}\) to C\({}_{67}\) range. The combination of sorbents is often used in decomposition odour analysis to widen the range of analytes that can be collected [3,19,20]. The sorbent combination was suitable for collecting decomposition-specific compounds for most compound classes. Carboxylic acids were not identified in decomposition soil using sorbent tubes but were identified in the soil when analysed using SPME. Many of the carboxylic acids collected with SPME in this study (propanoic acid, 2-methylpropanoic acid, butanoic acid, 2-methylbutanoic acid, 3-methylbutanoic acid, pentanoic acid, and hexanoic acid) have previously been identified in the headspace of decomposed remains using sorbent tubes and other pumped sorbent-based collection techniques [3,15]. Carboxylic acids were not detected in a previous gravesoil study and the researchers suggested that this was likely due to sampling and/or sample preparation [11]. It is notable that many esters of the corresponding acids were identified in the sorbent tube samples. This may have been attributed to the sorbent selection for the sorbent tubes and SPME. However, this could also point to microbial processing causing conversion of carboxylic acids to esters. Esters of C\({}_{4}\) to C\({}_{28}\) carboxylic acids have been identified as markers of shifts in soil microbial community and indicated that non-amended surface soils exhibited a lack of esters [29]. In fact, the overwhelming presence of esters in early decomposition (using both techniques) is not a trend previously documented for decomposition odour in the headspace of remains or in soil. Decomposition of soft tissue introduces a pulse of nutrients and moisture into the soil below the remains that can stimulate microbial metabolism. This can have a direct effect on the proliferation of soil bacteria and microbial community structure, consequently altering bacterial metabolism VOCs and producing large number of esters.