The effects of the surface properties of thermal paper on fingermark visualisation

Abstract

This research addresses the difficulty forensic practitioners have in developing latent fingermarks on thermal paper, e.g. receipts, whereby the standard method either causes blackening of the paper or a large volume of acetone has to be used to wash the receipts to remove the dyes prior to mark enhancement resulting in a loss of printed document evidence. To overcome the blackening issue and to avoid the need to pre-treat the substrate with acetone, Vacuum Metal Deposition (VMD) and iodine methods were explored as potential solutions. The VMD method was selected to be used in this novel way as no solvents are required to develop marks. Iodine fuming has historically been used to develop fingermarks, and although it is included in the UK government manual as a technique, there is no established optimal protocol in the literature. One method used by the Australian Federal Police to enhance marks on thermal paper is an iodine-based solution using an environmentally damaging solvent, AK-225, which has been prohibited from use in the EU and UK due to the potential of the solvent to deplete the ozone layer. This research developed four novel iodine solutions using Solstice PF™ and HFE-7100 as carrier solvents against which iodine fuming was compared. An α-naphthoflavone (benzoflavone) fixing agent was incorporated into two of these novel iodine-based solutions. Both gold/zinc (Au/Zn) and silver/zinc (Ag/Zn) VMD methods were shown to effectively enhance marks on the five thermal paper types tested with Ag/Zn more effective than Au/Zn on three of the five thermal papers used in this study. Of the iodine techniques, the novel iodine in Solstice PF™ with benzoflavone fixing agent was the most effective at enhancing fingermarks on 80 % of these substrates. The effectiveness of all of the techniques was dependent upon the substrate.

To understand the effect of the substrate properties of the papers on the development of latent marks, a range of techniques were used to study the substrate characteristics. Scanning Electron Microscopy and Energy Dispersive X-Ray spectroscopy was used to image the surface of the samples and gain information about the elemental composition of the paper types. From this differentiation of the five thermal papers was possible through analysis of calcium, aluminium and silicon content. By analysing the ratio of these elements within the papers it was apparent that two of the five paper types were shown to be similar to each other but different from the other three papers in regards to the percentage and ratio of these elements present. Fourier Transform Infrared (FTIR) spectroscopy in combination with chemometric principal component analysis resulted in papers being grouped by surface chemistry with two of the five paper types shown to be different from both the other paper types and each other. To gather detailed information about the roughness of the topography of the different paper types at the micron scale, Atomic Force Microscopy (AFM) was used. From this analysis, differentiation and statistically significant grouping of the different paper types was possible however, skew and kurtosis measurements did not allow for differentiation. Tensiometric analysis was used to determine surface wetting and the rate of absorption leading to differentiation of the paper types through a combination of these variables. The successful characterisation of the substrates meant that the interactions between the mark, the substrate and the development techniques can be evaluated. The understanding of the substrate gained through the application of the different analytical techniques aided in understanding how the marks interacted with the thermal papers and the development techniques. This was particularly important in understanding how marks would be absorbed into, and the extent of outward spreading across, the substrate which subsequently increased the understanding the different efficacy of the techniques studied in this research. Whilst it was anticipated that the VMD development methods targeting surface compounds would be most affected by the spreading of a mark, it was shown that this effect was not the same for the Au/Zn and Ag/Zn methods. There were links between the mean initial Optical Contact Angle (OCA) which is indicative of the immediate spreading of water and water-soluble mark components after deposition, the presence of aluminosilicates and the efficacy of the two VMD methods. A low (approximately 75 °) mean initial OCA in combination with aluminosilicates leading to Ag/Zn being more effective than Au/Zn however, a high initial OCA (~90 °) meant that Au/Zn was the more effective of the two VMD techniques regardless of whether aluminosilicates were present or not. The value of determining the silicon, aluminium and calcium content of the papers was also shown for understanding the efficacy of the iodine-based methods. For example, the absence of aluminosilicates meant that the Solstice PF™ with fixing agent method was more effective in developing marks to a potentially identifiable standard (approaching 50 %). In addition to studying thermal papers, this work will help in the development of a protocol whereby the surface characterisation of a substrate can be used to inform and understand fingermark visualisation on complex substrates.

Date of Award	19 Jun 2024
Original language	English
Awarding Institution	Abertay University
Sponsors	West Technology Systems Ltd
Supervisor	Ben Jones (Supervisor), Keith Sturrock (Supervisor), Joanna Fraser (Supervisor) & Kevin Farrugia (Supervisor)