Scent the wrong way

[December 14, 2009]
The undoubted ability of dogs to discriminate between different human scents has assisted many law enforcement officers to locate and apprehend criminals. In many of the subsequent court cases, sufficient evidence is often accumulated that the role of the dogs is not produced as evidence and is not a critical factor. Which is just as well, because the use of canine evidence linking an individual to a particular location is still controversial.

Just like the delays during the recent introduction of DNA evidence in courts, the rules for human scent identification line-ups, as they are called, have not been established. In the USA, human scent evidence has been challenged from various directions, including the uniqueness of the scent and its stability beyond collection.

This led to a Californian court ruling that such evidence is only admissible in court if "the person performing the technique used the correct scientific procedures, the training and experience of the dog and dog handler prove them to be proficient, and the methods used by the dog handler in the case are reliable."

Currently, there are no universal rules governing the collection and storage of human scent. The potential variables include the method of collection, the type of storage container, the storage temperature and the effects of aging on the scent composition. All of these might modify the volatile compounds that are present in the scent of an individual, or introduce new ones as a result of the conditions, hampering the subsequent association of a modified scent to a suspect's scent to the satisfaction of the courts.

As a first step, a triumvirate of scientists in the USA have investigated the effects of storage conditions on the composition of human scent. Kenneth Furton, Davia Hudson and Allison Curran from the International Forensic Research Institute at Florida International University worked with scent from several volunteers.

In the first instance, the effects of the storage containers were evaluated in the absence of scent samples, to see if they were inert or released volatile compounds during storage. Blank gauze was cleaned by supercritical fluid extraction and heat sealed in a variety of containers for up to five weeks before removal and analysis by SPME-GC/MS.

For all containers studied, volatile compounds were transferred from the container to the gauze. The worst performers were heavy duty Kapak pouches and aluminised Kapak pouches which emitted 107 and 88 volatiles, respectively, over five weeks. From a forensic viewpoint, the most disappointing aspect was that some of these compounds were known human scent compounds, such as aldehydes, alkanes and methyl esters. The researchers speculated that the heat sealing process might be the source of many of these volatiles but did not go further to prove it at this stage.

The best results were obtained with a glass vial, which released an average of five compounds over five weeks, only one of which (nonane) was a known human scent component. So, the subsequent tests were conducted with these glass vials.

Scent was self-collected by six volunteers over five days, giving 20 samples per person. They were taken from the palms and forearms using two brands of gauze pads or one brand of sorbent cotton and stored for up to seven weeks in glass vials at room temperature.

Using 3D covariance mapping to compare the GC/MS results, a Dukal brand gauze provided the least variation in scent composition over the seven weeks. The other branded gauze material was the worst performer and the differences were attributed to the different chemistries of the fibre sorbents. For storage at -80°C, the 100% cotton sorbent gave the most variation in composition over time.

When sorbents were stored in glass vials under a UVA/UVB light source, methyl esters, aldehydes and alkanes were detected within three weeks. The new compounds varied from sorbent to sorbent, so would affect the overall scent profile in different ways. However, the team did note that the ratios of the monitored primary odour compounds remained consistent over the test period.

The results were consistent with published work showing that dogs could match odours collected on the same day but their performances deteriorated when they were attempting to match the odours of stored objects to a subject.

The study has shown that human scents should be stored in glass vials without UVA/UVB exposure to minimise changes in composition but further work is required to develop and validate a procedure that will stand up to the closest of scrutiny in a court of law.

Related links:

No comments: