Measuring the Water Content in Freshly-Deposited Fingerprints

Or keisar, Chemistry Division, NRCN, Beer sheva, Israel (poratze@post.bgu.ac.il)
Yair Cohen, Chemistry Division, NRCN, Beer sheva, Israel
Yacov Finkelstein, Chemistry Division, NRCN, Beer sheva, Israel
Natalie Kostirya, Chemistry Division, NRCN, Beer sheva, Israel
Roey Ben-David, Chemistry Division, NRCN, Beer sheva, Israel
Albert Danon, Chemistry Division, NRCN, Beer sheva, Israel
Ze’ev Porat, Chemistry Division, NRCN, Beer sheva, Israel
Joseph Almog, The Hebrew University, Jerusalem, Israel

It has long been accepted that the average water content of deposited fingermarks is 98% or higher. This convention, which was based on literature reports that the water content in eccrine sweat is about 98%, was recently challenged by Kent(1) who claimed that the initial average water content of freshly-deposited fingermarks probably amounts to 20% at most. To check this assessment, we measured the water content in fingermarks by quartz crystal microbalance (QCM). This technique probes changes in the resonance frequency of a piezoelectric quartz crystal which can be translated to mass changes using the Sauerbrey equation. Fingermarks were collected from several volunteers with and without a preliminary hand washing, and the measurements were performed under controlled temperatures (35-40 °C) inside a closed chamber. Data acquisition started right after deposition and the continuous frequency increment, indicating mass loss, was recorded until achieving a steady frequency value. In order to determine the composition of the evaporated species, temperature programmed desorption mass spectrometry (TPD-MS) was utilized. First, mass scans along the 1-100 m/z mass range were taken between room temperature (RT) and 300 °C, for which practically only m/z =18 was observed, indicating the sole desorption of water without any loss of 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 300 °C. Quantification of the amount of water desorption was provided by calibrating the TPD-MS apparatus using CuSO4·5H2O crystals.
Preliminary results for “natural fingermarks” (without pre-washing) showed a water content in the range of 20-50% whereas after hand washing they were in the range of 40-70%. The source of this difference is being investigated.

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