A comparative evaluation of the disulfur dinitride process for the visualisation of fingermarks on metal surfaces

Highlights

• Disulfur dinitride compared to other fingermark visualisation methods

• Fingermark recovery rates determined for metals exposed to adverse environments

• Disulfur dinitride develops fingermarks on metals after washing, wiping and heating

• (SN)_x polymer microstructures formed during fingermark visualisation revealed

Abstract

The disulfur dinitride process for fingermark visualisation was first reported a decade ago, with promising results obtained for a range of materials including metals. However, the friction sensitive nature of the material and difficulty of synthesis made routine use difficult. Many of these issues have since been addressed, making equipment and chemicals available to build an understanding of how the effectiveness of disulfur dinitride compares to other fingermark visualisation processes currently used on metal surfaces. This enables more informed advice to be given on selection of processes for treatment of metal items, an area of operational interest that encompasses weapons used in violent crime and the increasing incidence in metal theft. This paper reports a comparative study into the effectiveness of disulfur dinitride, cyanoacrylate fuming, vacuum metal deposition, gun blueing and wet powder suspensions on brass, bronze, copper and stainless steel. Experiments were conducted with the surfaces exposed to a range of environments including long term ageing, water/detergent washing, acetone washing and high temperature exposure. The results indicate that disulfur dinitride is an effective process for fingermark visualisation on metal surfaces, including those exposed to adverse environments, and may offer potential improvements over existing processes for those surfaces. Further work, including pseudo-operational trials, is recommended.

Introduction

Chemical techniques for the visualisation of latent fingermarks have been reported from the mid-1800s [1]. This even pre-dates the introduction of the comparison of fingerprints with crime scene marks as a means of identifying the perpetrators of crime, and the development of techniques to visualise fingermarks on the wide range of surfaces that may be encountered has been a rich area of research from the 1900s onwards [2,3].

The modern forensic practitioner potentially has access to a comprehensive suite of visualisation and imaging processes for the recovery of fingermarks [4], and the selection of the most appropriate processes may be tailored according to knowledge of the type of surface that is present and the environment it has been exposed to. However, the surfaces that may be encountered during criminal investigations may change over time for example recent increases in the recycled content of plastic bags [5] or changes from paper to polymer banknotes [[6], [7], [8]], and there remain surfaces for which fingermark recovery has always been problematic (for example leather [9], fabrics [10] and clingfilm [11]). As a consequence, there is an ongoing requirement to evaluate techniques that could increase the fingermark yield from ‘problem’ surfaces.

One type of surface for which fingermark recovery appears lower than may be expected, is metals. Although metals can be regarded as a ‘non-porous’ surface, they differ from other non-porous surfaces in that most metals and alloys are not chemically inert and chemical interactions can occur between the metal surface and the fingermark deposits [[12], [13], [14]]. These interactions may prove detrimental or beneficial to subsequent fingermark recovery. Although many of the processes recommended [4] for non-porous surfaces are reasonably effective in visualising fingermarks on metals, their effectiveness is often less than their equivalent performance on other non-porous surfaces such as glass, ceramic and polymers.

Metals are a surface of significant operational interest. In recent years there has been a rise in the commercial value of metals and a corresponding increase in the crime of metal theft [15]. There has also been a much-publicised increase in violent crimes [16] with a 42% year on year increase in gun crime leading to 2544 offences recorded in 2016/17, and a 24% increase in knife crime over the same period with over 4000 offences resulting in injury reported in the Metropolitan Police area alone. Metal surfaces associated with these crimes include brass (for fired and unfired cartridge casings) and stainless steel (for knife blades). Any increase in fingermark recovery from such items will contribute to the reduction in crimes of this type.

Cartridge casings have been long recognised as a problematic substrate for fingermark recovery, and the reported success rates from fired cartridges worldwide are very low. A series of studies that attempt to address this issue have been reported from the 1970s onwards, with researchers exploring processes for selectively etching metals and electrodeposition from solution. Nitric acid was proposed as a fuming process for selective etching [17], and the potential of gun blueing as a selective electrodeposition process for steel and copper-based alloys was also recognised [18]. Other researchers have proposed similar etching and electrodeposition processes both for copper-based alloys and other metals such as aluminium, including potassium permanganate [19], acidified hydrogen peroxide [20], palladium deposition [21], aluminium black [22] and cold patination fluid [23].

Comparative studies have also been conducted by different research groups, looking at these processes singly and in sequence with other processes such as cyanoacrylate fuming [[24], [25], [26], [27]]. In general, the results from these studies indicate that palladium deposition and gun blueing tend to give the best results, and the best sequences include these processes used in sequence after cyanoacrylate fuming. However, focused studies into the impact of the firing process into fingermark recovery [28] confirm that the high temperatures, abrasion and deposition of propellant residue all reduce the chances of recovering fingermarks, and this is in accord with the low operational recovery rates observed on fired ammunition.

It has been noted that on copper-based alloys, residual corrosion may be left on the surface by the interaction between the fingermark and the metal, and this ‘signature’ may be persistent even when marks are washed or rubbed away. The first technique to utilise this effect was the Scanning Kelvin Probe [29,30] where a fine (μm scale) vibrating gold probe is scanned across the metal surface and a map of Volta potential is recorded. The corrosion initiated by eccrine constituents, and the thin insulating layer of sebaceous constituents on the surface both produce different Volta potentials to the bare metal surface and enable fingermarks to be visualised.

The same effect was utilised by Bond [13,14,31], who employed heating to enhance the corrosion signature, followed by electrostatic powdering (charging the casing and applying a toner powder) to reveal fingermarks. Although this process was commercialised, it ultimately proved difficult to reproduce the laboratory results on commercial equipment. Bond also conducted studies into the corrosion mechanisms occurring between the fingermark and the metal surface [14] and proposed that the main reaction was the action of chlorides to produce metal hydroxides and hydrochloric acid, although a subsequent study by Wightman and O'Connor [12] indicated this may not be the case and that other mechanisms, such as reduction of oxides by organic species in the fingermark, may also be operating.

Another discovery that is thought to utilise this corrosion signature to visualise fingermarks on brass and copper-based alloys is disulfur dinitride (S₂N₂) [[32], [33], [34]]. The traditional process begins by decomposing the material tetrasulfur tetranitride (S₄N₄), to the active and volatile S₂N₂ compound, under vacuum, where it is selectively polymerised to (SN)_x on the fingermark ridges, Fig. 1.

Promising results have been obtained on brass using this process, and it was also observed that the process continued to visualise marks on surfaces that had been wiped clean [35].

In contrast to brass and copper-based alloys, there has been less focus on processes tailored towards the visualisation of fingermarks on stainless steel surfaces. This is partly because these surfaces are more chemically inert and existing processes such as cyanoacrylate fuming are reasonably effective. Increases in knife crime make this surface of more operational interest however, and there has been some work undertaken to develop electropolymerisation processes that are effective on this type of surface [36,37]. S₂N₂ has also been observed to develop marks on stainless steel, although the development mode in this case is predominantly deposition of blue (SN)_x on the background with the ridges remaining paler.

Since the first published papers into the S₂N₂ process, research has predominantly focused on making the process more practical and safer to use by moving away from the utilisation of friction sensitive S₄N₄ as well as in reducing processing times to those comparable with cyanoacrylate treatment. A practical solution utilising a safe to handle precursor that is rapidly decomposed to S₂N₂ within the safe confines of the vacuum chamber has now been developed, and this forms the emphasis of the current work.

The study reported in this paper is designed to evaluate the effectiveness of S₂N₂ on a range of operationally significant metal surfaces (including brass–used for cartridge casings, bronze and copper–associated with metal theft, and stainless steel–associated with knife crime). The performance of S₂N₂ was compared to a range of existing processes that can be used on metal surfaces, which are listed below, together with the rationale for their selection:

• Superglue (cyanoacrylate) fuming–effective on non-porous surfaces, and also involves visualisation of fingermarks by polymerisation

• Vacuum metal deposition–a process that is known to be highly sensitive on non-porous surfaces, including those that have been wetted or exposed to high temperature [38]

• Gun blueing (used on copper-based samples only)–a selective deposition process optimised for copper-based alloys

• Powder suspensions (used on stainless steel only)–an effective process on non-porous surfaces, capable of developing marks on surfaces that have been wetted, and found to give good results on stainless steel but not on copper-based alloys [39]

The objective of the comparative study was to obtain an initial understanding of the performance of S₂N₂ relative to existing processes to inform if and when it could be recommended for fingermark visualisation on metal surfaces. Generating such information is also a requirement for developing validation libraries for any novel process prior to its acceptance and implementation on operational casework.