The detection of volatile and semi-volatile organic compounds (VOCs and SVOCs) is an important aspect of forensic science, as it can provide unequivocal evidence for the use or presence of incriminating materials.
A key factor when deciding on the methods used for forensic analysis is the minimisation of sample preparation. This is important because, as far as the legal system is concerned, it means that there is less risk of compromising data and thus of evidence being challenged in court.
A particular issue affecting some forensic applications is the high degree of reactivity of certain compounds, such as explosives. These can be very sensitive to any slight deterioration in the inertness of the analytical system, and so demand reliably high instrument performance.
What Markes can offer
Unlike some other analysis methods such as solvent extraction, thermal desorption inherently avoids the need for extensive sample manipulation – and in many cases no sample preparation is needed at all.
The use of thermal desorption in forensic analysis now covers a range of applications, including:
- Analysis of arson debris to determine whether or not an accelerant was used.
- Detection of drugs, for example in house dust or on bank notes.
- Analysis of explosives, whether in buildings or vehicles suspected of being used to store or transport arms, or in the ‘fingerprinting’ of spent shotgun cartridges.
- Identification of volatiles released from corpses, to help establish the time of death and events leading up to it.
Thermal desorption sampling methods used in forensic analysis
- Direct desorption of small samples is very simple, and involves simply placing a small (milligram) quantity of the sample in an empty thermal desorption (TD) sorbent tube. This is then heated to release the vapours, which are transferred to the focusing trap of a UNITY-xr or TD100-xr thermal desorber.
- Analysis of bulk materials uses Markes’ Micro-Chamber/Thermal Extractor. Gram-quantities of a material are placed in one of the microchambers, which are warmed while a flow of is gas applied. The vapours released are then collected on a sorbent tube for analysis by TD–GC–MS. A similar protocol can be used with the nylon bags used to collect forensic samples such as arson debris. If necessary, the bag can be heated gently to release the vapours, which are collected onto a sorbent tube using an ACTI-VOC pump or Easy-VOC grab-sampler.
- Pumped-tube sampling of air streams can be carried out using the ACTI-VOC pump, with analysis using Markes’ UNITY-xr or TD100-xr thermal desorbers.
When analysing reactive compounds such as explosives, the inertness of Markes’ equipment ensures that good results are obtained – indeed, advances pioneered by Markes mean that two-stage thermal desorption of trace-level explosives such as trinitrotoluene (TNT) is now considered routine.
- For examples of the application of thermal desorption to forensic investigation, see Application Note 058.
- For a paper citing the use of the UNITY thermal desorber to ascertain whether a fire victim may have been alive at the time of a fire, see: K. Pahor, G. Olson and S.L. Forbes, Post-mortem detection of gasoline residues in lung tissue and heart blood of fire victims, International Journal of Legal Medicine, 2013, published online.
- For a paper describing the use of UNITY and BenchTOF time-of-flight mass spectrometer to comprehensively profile the volatiles emnanting from decaying pigs, see: M. Statheropoulos et al., Combined chemical and optical methods for monitoring the early decay stages of surrogate human models, Forensic Science International, 2011, 210: 154–163.
- For a paper describing the use of the UNITY-ULTRA system for monitoring the decay of phenoxyethanol and phthalates in ballpoint pen ink, see: C. Berger-Karin, U. Hendriks and J. Geyer-Lippman, Comparison of natural and artificial aging of ballpoint inks, Journal of Forensic Science, 2008, 53: 989–992.