Fading fingerprints

The discovery of fingerprints at a crime scene is always a major piece of evidence that the police are delighted to find as it places an individual directly at the incident. However, what it does not do is confirm that a suspect was present at the time of the crime and this uncertainty often represents a key argument in the case for the defence. So, the fingerprint can lose its relevance unless the time of contact can be established.

In order to determine the age of a fingerprint, it is essential to know its initial composition but that is not a trivial operation. Published studies have shown that the composition of fingerprints is so variable that it is almost impossible to make any predictions of their age. However, undeterred by these observations, a research team has undertaken a detailed study on fingerprint composition and its evolution with time.

Celine Weyermann and Christophe Champod from the Forensic Science Institute at the University of Lausanne, Switzerland and Claude Roux from the Centre for Forensic Science at the University of Technology, Sydney, Australia focused on the initial fingerprint composition in an attempt to establish some commonalities between those collected from different people.

Fingerprints are more complex than they seem at first sight, with contributions not only from the fingers but also from four other sources. Eccrine secretions from the hands are supplemented with apocrine sweat to release a mixture of inorganic and organic compounds that contains sodium chloride, urea, amino acids, proteins and lipids.

Sebaceous gland secretions are also common fingerprint components, due to frequent touching of the face and hair, and these contribute such fat-soluble compounds as fatty acids, wax esters, glycerides, squalene, sterols and sterol esters. Epidermal substances and external contaminants make up the fingerprint blend.

Fingerprints from three male and three female donors aged 25-35 years were impressed onto various surfaces and stored for up to 30 days in the dark in the absence of air flow under normal temperature and humidity. They were extracted with dichloromethane containing anthracene as an internal standard for GC/MS analysis under electron ionisation.

The compounds present were identified by matching their mass spectra with those from the NIST database and the amount of each was estimated from the peak areas of major ions in their spectra.

All fingerprints contained squalene, which is a steroid precursor, and cholesterol, in addition to squalene derivatives, fatty acids and wax esters. The female donors showed more compounds in their fingerprints, which were attributed to compounds from skin lotions and perfumes. Disregarding octyl methoxycinnamate, which dominated the fingerprints of one female, squalene was always the most abundant component.

However, the mean peak areas of squalene from all of the donors varied markedly, from 1-11 µg absolute weight, and those of cholesterol followed the same inter-individual pattern. This inconsistency is a serious problem when attempting to derive general rules for the fingerprint composition.

Even comparing the fingerprints of one individual there was a wide variation in the levels of squalene and cholesterol, with relative standard deviations of 50% for both compounds in prints from the same day and 71 and 79% for squalene and cholesterol, respectively, taken on different days.

This uncertainty was reduced to about 50% by calculating the peak area ratios of each compound to that of the internal standard, and was lowered even further when the squalene-to-cholesterol peak area ratios were calculated. These values were still regarded as too high by the researchers, who declared that the donor-dependent initial composition should be taken into account when aging of the fingerprints is being studied.

Fingerprints were taken from microfilters, glass inserts, glass, paper and PVDF, revealing a surface effect with porous surfaces yielding greater amounts of fingerprint residues than non-porous ones. This effect was slightly reduced once again by considering the relative peak areas of squalene and cholesterol.

Having examined the initial composition of the fingerprints, the team studied their aging on glass and microfilters. Most compounds did not degrade significantly over the period studied, with the notable exception of squalene. Its mass spectral peak disappeared quickly on glass and was undetectable after seven days but was more stable on the microfilters, degrading at a slower rate and remaining detectable after 30 days.

The cholesterol peak decreased far more slowly than squalene on glass, showing no change after one day, and was unchanged on the microfilters.

While showing promise, these initial studies have revealed the difficulties associated with measuring the age of human fingerprints, due to the variable composition of the prints as they are laid down.

The researchers suggested that alternative methods such as desorption electrospray ionisation mass spectrometry should be explored to try and identify more compounds that, taken with squalene and cholesterol, could provide a panel of target compounds that show low intra-donor variability at the time the prints were made.

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.