Development of Standard Operating Procedures for the detection of Volatile Organic Compounds associated with the decomposition process

Decomposing meat produces a characteristic profile of Volatile Organic Compounds (VOCs) that changes as a function of time. This has been considered by the forensic community as a possible means of estimating Post Mortem Interval (PMI) and thus could be of importance in criminal investigations.

Previous work has focussed on obtaining VOC profiles for the decomposition stage and the key chemicals identified include alcohols, aldehydes, acids and sulphur compounds including Dimethyl disulphide (DMDS) and Dimethyl trisulphide (DMTS). The type and levels of these VOCs change throughout the decomposition of mammalian cadavers and are of considerable interest in the forensic community in a range of areas including ‘time profiling’ as a means of estimating PMI, locating clandestine graves and to assist in the training of Victim Recovery (VR) dogs. VOCs can accumulate in the air surrounding a cadaver (the ‘headspace’) and the current research focussed on methods to collect, detect and identify these.

This research was carried out using Thermal Desorption Gas Chromatography Mass Spectrometry (TD-GC-MS) and Headspace Gas Chromatography Mass Spectrometry (HS-GC-MS).TD-GC-MS uses an adsorbent tube and pump system to concentrate a sample and HS-GC-MS uses a sealed vial system to introduce and analyse solid or liquid without the requirement for extensive sample preparation. GC-MS was used for this study because of its ability to combine separation and identification, which is particularly appropriate for complex matrices, such as those seen in the decomposition processes.

A combination of pure chemicals and pork were used to develop methodology for TD-GC-MS and HS-GC-MS where the sampling parameters (such as mass of meat, size on container, presence of insects, type of substrate) were investigated in detail. No Certified Reference Material (CRM) was available so a standard mix of chemicals was devised based on a literature review and the results of preliminary experiments. For pork studies, it was found that optimum results were obtained using 500 g pork samples placed on sand as a substrate and contained within clip lock plastic boxes. The results confirmed those of other workers that the accessibility of insects to the meat plays an essential role for a realistic decomposition profile.

A key aim of this project was the development of Standard Operating Procedures (SOPs) for the detection of VOCs produced in the headspace above a cadaver during its decomposition. SOPs are a way of reproducing methodology of an, often complex, but routine activity. The SOPs were developed as an iterative process throughout the project and then further tested and validated using repeatability studies and intra-laboratory evaluation. Validation studies involved repeatability studies by the author and two competent analysts. It was shown that the SOPs fulfilled their basic requirement in terms of ease of use and suitability for detecting VOCs associated with decomposition.

The robustness of these SOPs was successfully demonstrated using chemically similar systems in the phyto-kingdom. The robustness was evaluated by applying them, after some adaptation to the sample collection procedure, to plants that mimic decomposition by producing chemicals that attract insects. The presence of DMDS and DMTS was detected in plants known to attract blowflies which are of key entomological importance in decomposition events. The research was extended to carnivorous plants particularly the lifecycle of Sarraceniaflava, which has not been previously studied in this way. In addition to the work on plants, a short study on a commercial odour reduction system was undertaken which further confirmed the scope of the SOPs. The SOPs were found to be applicable to these systems and the robustness was shown by being able to withstand minor modifications while still being fit for purpose.

Overall, limitations were seen in terms of repeatability for the technique and due to the complexity of the decomposition matrix. However, the SOPs showed great promise to be used as a starting point for forensic practitioners to test their own methodology and instrumentation for decomposition studies.