Exploding a Myth? Measuring the Water Content in Freshly-Deposited Fingermarks

Ze'ev Porat, Chemistry Division, NRCN, Israel
Or Keisar, Chemistry Division, NRCN, Israel
Yair Cohen, Chemistry Division, NRCN, Israel
Yacov Finkelstein, Chemistry Division, NRCN, Israel
Natalie Kostirya, Chemistry Division, NRCN, Israel
Roey Ben-David, Chemistry Division, NRCN, Israel
Albert Danon, Chemistry Division, NRCN, Israel
Joseph Almog, The Hebrew University, Jerusalem, Israel

It has long been accepted that the average water content of deposited fingermarks is in the range 98-99%. This assumption probably originated from early literature reports that the water content in eccrine sweat (the sweat glands which are located on the palms) is about 98%. Based on published analytical data and theoretical considerations, Kent(1) has recently challenged the above assessment, claiming that the initial average water content of a fingerprint, soon after deposition, probably amounts to 20% at most. We were challenged by Kent’s proposition and decided to measure the water content in freshly deposited fingermarks by using quartz crystal microbalance (QCM). These measurements probe changes in the resonance frequency of a piezoelectric quartz crystal which can be translated to mass changes using the Sauerbrey equation. The measurements started right after the deposition. The continuous increase in the frequency indicates the mass loss due to water evaporation until a constant level was observed. The measurements were performed under controlled conditions: the resonator was located above a flat heat source inside a closed chamber. The temperature was kept around 40℃ and both the temperature and the relative humidity were monitored. The fingermarks were collected from several volunteers with and without preliminary cleaning procedure (hand washing). In order to verify that the weight loss represented only the evaporation of water, we used the thermally programmed desorption technique hyphenated with mass-spectrometer (TPD-MS) to scan the masses of the evaporated species upon heating several fingermarks up to 300℃. First, mass scans along the 1-100 m/z mass range were taken between room temperature (RT) and 40 °C, for which practically only m/z =18 was observed, indicating the sole desorption of water without any loss of any other materials, e.g. organic substances. Once the sole desorption of water was verified, TPD-MS measurements were taken by individually monitoring the intensity of m/z=18 (H2O) vs. T during a linear ramping of the fingerprint sample temperature from RT to 40 °C. Quantification of the amount of water desorption was provided by calibrating the TPD-MS apparatus using CuSO4·5H2O crystals.

The results for “natural fingermarks” (without pre-washing) showed a water content around 25% whereas after hand washing they were in the range of 40-70%, probably due to the higher content of sebum in natural prints.

References
1. Water content of latent fingerprints- dispelling the myth, Terry Kent, Forensic Science International 266 (2016) 134-138