During the scrutinising examination of a crime scene, it is entirely plausible for dozens or more fingerprints and fragments of fingerprints to be recovered. Not at all surprising considering how often we touch endless surfaces in our day-to-day lives. Consider how many people might grasp the handle of a shop door in an average day. If that shop were to become a crime scene, how could one possibly distinguish between prints that had originated on the day of the crime and those deposited weeks or months ago? Is it possible to estimate the age of a fingerprint?

Firstly, a quick review of just what a fingerprint is. We all know fingerprints are a series of unique arches, loops and whorls left behind when we touch a surface. But people may be slightly less sure of what these deposits are actually composed of.

Although the composition of a fingerprint is somewhat complex, 95-99% of the deposit is simply water, which will typically readily evaporate. The remaining 1-5% is an intricate mixture of organic and inorganic compounds ranging from amino acids and fatty acids to trace metals. Chloride, potassium, sodium, calcium, hydrocarbons, sterols – the list goes on. A vast concoction of chemicals emitted through our skin and deposited whenever our fingertips touch a surface.

But what we didn’t know until recently, is that these deposited chemicals gradually move with time, and that this movement can be used to determine how long a fingerprint has been on a particular surface. Researchers from the National Institute of Standards and Technology recently stumbled upon this very fact (Muramoto & Sisco, 2015).

Fingerprint when freshly deposited (left) and after 72 hours (right).
Credit: Muramoto/NIST

Like many discoveries, the research itself was something of an accident. The NIST researchers were initially using analytical techniques to detect trace amounts of illicit substances present in fingerprints. In the process of this investigation, they noticed the movement of chemicals within the fingerprint over time. Fingerprints are made up of ridges and valleys forming unique patterns, the characteristic features that allow investigators to distinguish between prints deposited by different people. These features are imprinted in various chemicals when an individual leaves a print behind. However over time the chemicals composing the fingerprint begin to migrate, moving from the defined ridges of the fingerprint into the valleys, essentially blurring the details of the print.

The researchers focused on particular biomolecules, namely fatty acids such as palmitic acid. By depositing fingerprints on sterile silicon wafers and storing the samples under strictly controlled conditions for a period of time, scientists were able to clearly observe the migration of molecules using a technique known as time-of-flight secondary ion mass spectrometry (TOF-SIMS). After a period of only 1 hour after fingerprint deposition, the friction ridge patterns of the fingerprint were clearly visible with the fatty acid molecules under observation residing along the ridges of the print. However within 24 hours the molecules had diffused into the valleys, blurring the patterns of the fingerprint.

The research thus far has simply been conducted to prove the concept of fingerprint component migration for ageing fingerprints, but further work could investigate time effects on a greater scale and even differences in the migration of different molecules. Although the method is advantageous in that it does not depend on chemical changes in fingerprints, which can be very dependent on individual circumstances, further work would be warranted to establish how environmental differences could affect the rate at which this molecular movement occurs, including temperature and humidity effects as well as those caused by the deposition surface.

As intriguing as this research is, this is not the first time scientists have tried to devise a method of ageing fingerprints using chemistry. In fact, researchers have been attempting to accurately age fingerprints for decades. Research has focussed on the changes in the chemical composition of fingerprints over time. For instance, concentrating on a particular compound, such as cholesterol, and establishing the rate at which the concentration of that compound changes over time (Weyermann et al, 2011). Unfortunately many such studies have found changes in the chemical composition of fingerprints to be too variable and unpredictable, particularly when taking into account the differences between donors and the effects of different conditions. Other studies have attempted to determine the age of a fingerprint based on how well powder adheres to the ridges (Wertheim, 2003), by changes in fluorescence wavelength over time (Duff & Menzel, 1978), and changes in electrostatic charge with time (Watson et al, 2010). A vast array of scenarios have been studied intently.

A method of establishing the age of a deposited fingerprint has been at the forefront of latent print research for a long time, and is likely to continue. Although fascinating advances have been made, scientists are a long way from catching criminals by the age of a fingerprint.

References

Cadd, S. Islam, M. Manson, P. Bleay, S. Fingerprint composition and aging: A literature review. Sci Justice. 2015(55) pp. 219-238.

Duff, J. Menzel, E. Laser assisted thin-layer chromatography and luminescence of fingerprints: an approach to fingerprint age determination. J. Forensic Sci. 1978(23), pp 129-134.

Muramoto, S. Sisco, E. Strategies for Potential Age Dating of Fingerprints through the Diffusion of Sebum Molecules on a Nonporous Surface Analysed Using Time-of-Flight Secondary Ion Mass Spectrometry. Anal Chem. 2015(87) pp. 8035-8038.

National Institute of Standards & Technology. Who, What, When: Determining the Age of Fingerprints. [online] Available: http://www.nist.gov/mml/mmsd/20150824fingerprints.cfm

Watson, P. Prance, R. J. Prance, H. Bearsmore-Rust, S. T. Imaging the time sequence of latent electrostatic fingerprints. Proc. SPIE ‘Optics and photonics for counterterrorism and crime fighting VI, Toulouse, 7838, 783803-1-6, ISBN 9780819483560 (2010).

Wertheim, K. Fingerprint age determination: is there any hope? J. Forensic Identif. 2003(53), pp 42-49.

Weyermann, C. Roux, C. Champod, C. Initial Results on the Composition of Fingerprints and its Evolution as a Function of Time by GC/MS Analysis. J. Forensic Sci. 2011(56), pp 102-108.