Determining the order of deposition of natural latent fingerprints and laser printed ink using chemical mapping with secondary ion mass spectrometry

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Abstract

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) chemical mapping was used to investigate the order of deposition of natural latent fingerprints and laser printed ink on paper. This feasibility study shows that sodium, potassium and C3H5 positive ions were particularly abundant endogenous components of the natural fingerprints and also present in the paper examined, but were mostly absent in the laser printed ink. Mapping of these ions enables the observation of friction ridges from latent prints on the ink surface, only when a fingerprint was deposited above the layer of ink. As a demonstration of proof of concept, blind testing of 21 samples from three donors resulted in a 100% success rate. The sensitivity of this technique was investigated within this trial through the examination of up to fifth depletion fingerprints and ageing of up to 28 days. Migration of fingerprint and paper components to the ink surface, although observed with increased ageing time, was not found to compromise determination of the deposition sequence.

Introduction

The development of a scientific understanding of fingerprint evidence is critical in ensuring continued confidence in forensics and validity of investigative methodologies [1], [2], [3], [4], [5]. Until recently, long-used forensic science techniques have been employed to provide evidence in criminal trials without the reliability being questioned; however, high profile cases and publicised errors have resulted in an increased scrutiny of the discipline [6], [7]. This is especially the case with latent fingerprint examination, where interpretation of fingerprint evidence is heavily dependent on the skill and expertise of latent print examiners [8]. Consequently, blind proficiency tests are now being recommended in favour of reference to experience or training [9]. Detection of fingerprints on absorbent materials such as paper is further complicated by the heterogeneous chemistry and morphology of this porous material, as well as by the rapid absorption of components from the surface into the paper, which occurs within seconds after deposition [10]. Penetration and lateral absorption of fingermarks also depend on the chemical components of a deposited print, which varies in relation to the person, their emotional state, food consumption and grooming regime. Studies have shown that depth of penetration of fingermarks varies with types of paper, with a good correlation between penetration depth and quality of chemically developed prints [11], [12], [13], [14], [15]. Establishing the depth of penetration of prints into surfaces, as well as order of layers in, for instance, overprinting with text or images, can provide considerable assistance to crime-scene investigations by providing an insight into the history of documents for forensic studies. In cases such as fraud or counterfeiting it can be imperative to know whether a fingerprint has been deposited before or after the paper is printed with compromising material, and therefore be able to assess whether a suspect is associated with the printed evidence. This is currently impossible to determine because existing, commonly implemented development techniques using ninhydrin or its analogues utilise a solution that permeates through an entire document and therefore cannot be used to provide information on the deposition sequence of the fingerprint and ink [10], [16].

To date several instrumental methods with varying levels of sensitivity and practicability have been employed by forensic practitioners and researchers to determine the presence and characterise chemical composition of fingerprints on surfaces. Examples of these include matrix assisted laser desorption ionisation (MALDI) [17], laser desorption instruments as well as emerging ambient ionisation methods such as desorption electrospray ionisation (DESI) [18] and direct analysis in real time (DART) mass spectrometry [19]. These, however, have limited imaging resolution capabilities and are generally partially destructive. Additional techniques being explored include the less destructive and complimentary attenuated reflection Fourier transform infra-red spectroscopy (ATR-FTIR) [20], [21], secondary ion mass spectrometry (SIMS) [22], and X-ray photoelectron spectroscopy (XPS) [23].

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) in particular is one of the most sensitive of all laboratory surface analytical techniques, showing potential in the examination of fingerprints for analysis of constituent metals and contaminants present [22], [24], [25], [26]. ToF-SIMS provides detailed chemical and spatial information of the top few nanometres of a surface, and has shown its ability to capture images and extract chemical data in a minimally invasive manner, with only the first two monolayers being ablated off a surface [27], [28], [29]. This implies that the integrity of the print is not entirely compromised, and should potentially allow for additional extraction of information from the fingerprint, an important factor in forensic investigations. MeV-SIMS, a technique in development that utilises an MeV primary beam and operates in atmospheric conditions, has also demonstrated clear imaging of fingerprints and depth profiling capabilities. Exogenous components of moisturiser-doped fingerprints were mapped in sequenced fingermarks and pen ink marks on paper, giving depth information where other imaging techniques had provided limited results [18], [30].

This paper presents proof of concept and a systematic methodology for the use of ToF-SIMS chemical mapping as a reliable technique in establishing the order of sequencing of laser printed ink and natural, latent fingerprints on paper. As no grooming procedure was utilised to collect fingerprints, samples were not intentionally biased to eccrine or sebaceous deposits, nor doped with personal care products. This study therefore represents the first systematic examination wherein endogenous components of the fingermark are used as elemental and molecular markers. Variation of these markers with donor and presence or migration in depleted and aged fingermarks is also assessed in relation to discrimination of fingermarks deposited above or below a printed layer.

Section snippets

SEM-EDX analysis and sample preparation

Analysis of 80gsm (Lyreco Premium) A4 printing paper and a 14 day-old sample of ink was conducted in a Zeiss Supra 35VP field emission scanning electron microscope (FEG-SEM), coupled with an Oxford Instruments Inca energy dispersive X-ray analyser (EDX). The black ink sample was printed using an HP LaserJet P2055 and a Longbow toner cartridge with factory default settings of fast resolution 1200 dpi. The SEM-EDX was operated under high vacuum conditions and samples were mounted on aluminium

Results and discussion

Fig. 1(a) shows an SEM image of paper and fused laser-printed ink on the paper surface. Paper fibres, voids and filler aggregates are visible at low magnification, with the crystalline structure of the filler and variation in surface texture of fibres becoming more discernable at increasing magnifications. EDX spectra (not presented) indicated the presence of calcium, consistent with filler aggregate composition of calcium carbonate, used during paper manufacture in both ground and precipitated

Conclusions

This paper demonstrates the potential of ToF-SIMS chemical mapping to be effective in providing chronological sequencing information of natural fingerprints on laser printed paper. The technique enables the identification of whether a fingerprint has been deposited before or after the paper has been printed. Here, mapping of selected endogenous elements and molecular species in the latent fingerprint, particularly the sodium, potassium and C3H5 positive ions, reveals the friction ridge pattern

Acknowledgments

This project is supported by funding from The Leverhulme Trust (RPG-138). The study design, analysis, interpretation and reporting were conducted by the author(s) with no involvement from The Trust. This research has been approved by Brunel University Research Ethics Committee.

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