An assessment of the effectiveness of 5-methylthioninhydrin within dual action reagents for latent fingerprint development on paper substrates

Abstract

A critical investigation of 5-methylthioninhydrin (5MTN) is presented as a ‘dual action’ formulation component for the development of latent finger marks on paper substrates. Preparation of a dual action reagent was performed by combining proportions of 5MTN and zinc chloride (ZnCl₂) in a pre-mixed solution. Developed prints (deposited on filter paper substrates) could be subsequently visualised in both colour and fluorescence modes. Finger mark quality was graded using a quartered print approach for a number of reagent compositions to deliver an optimised formulation recipe. To fully appraise 5MTN in comparison to currently employed chemistries, this reagent was evaluated against three alternative amino acid selective reagents, ninhydrin, 1,8-diazafluorenone (DFO) and 1,2-indandione/ZnCl₂. Six common paper types were used for this purpose and split depletion finger marks from six donors were collected. Finger mark sets were also left for two days or two weeks to show the effect of ageing on development quality. For the first time, it was shown that 5MTN/ZnCl₂ is effective as a ‘dual action’ reagent under the United Kingdom climate conditions. However, results presented herein show that the existing recommended chemistries and the 1,2 indandione/ZnCl₂ process are all more effective than this new latent finger mark enhancement reagent. In a preliminary sequencing study, we show the effectiveness of the existing DFO–ninhydrin sequence over dual action reagents.

Introduction

The development of finger marks on porous surfaces was revolutionised by the introduction of reagents that react with amino acids. Although ninhydrin was first synthesised in the early 20th century [1], it was not until the 1950s that Oden and von Hofsten [2] noted its potential as a technique for fingerprint development. Testing showed ninhydrin to be more effective than the iodine and silver nitrate techniques and the reagent rapidly became the technique of choice for porous surfaces in particular. Progressive refinements were made to the ninhydrin formulation and processing conditions to make it more effective [3], [4], [5], and the potential for sequential treatment with metal salts was investigated for altering reaction product colour and for producing fluorescent reaction products [6]. Post treatment of ninhydrin with zinc salts was used operationally in the United Kingdom as a means of producing fluorescent marks, detecting additional marks after the ninhydrin treatment alone. The sequential post-treatment approach was superseded mainly by the development of 1,8-diazafluorenone (DFO) [7], which produced marks that were strongly fluorescent under green light. Some amino acid-rich finger marks developed with DFO may be seen as a pinkish coloured product [7]. It is suggested that the DFO method is performed in sequence prior to ninhydrin, since it is more sensitive, and facilitates the visualisation of more ridge detail. However, ninhydrin is usually the method of choice for the treatment of volume crime articles over DFO as it is more affordable, requires shorter development time, and does not necessitate fluorescence examination.

Over the past number of years, many researchers have focused on synthesising new compounds with improved performance over ninhydrin, DFO, or the DFO-ninhydrin sequence. One approach to this goal was the synthesis and comparison of ninhydrin derivatives [8], [9], [10], [11]. In some cases, such derivatives developed differently coloured fingerprints and provided a better contrast with the background than ninhydrin [12]. However no derivative provided a higher sensitivity when compared to their parent compound. For this reason, ninhydrin has not been replaced in the United Kingdom with any of these new compounds. More recently, the development of 1,2 indandione [13] has offered an alternative to DFO and there have been several studies comparing the relative effectiveness of these two reagents [14], [15], [16]. 1,2 Indandione also produces a fluorescent reaction product with amino acids, and for some marks a visible bright pink reaction product can be observed. Another investigated approach has been the development of formulations which produce marks that are simultaneously coloured and fluorescent. Such ‘dual action’ reagents may allow replacement of the two-step DFO–ninhydrin sequence. Nevertheless, in order to obtain such prints, a secondary treatment of the items with a metal salt solution appeared to be necessary [6], [8], [11] and consequently these previously reported formulations could not be considered true dual action finger mark reagents. True dual action reagents have recently been prepared by different study groups. Genipin, a natural compound extracted from the fruit Gardenia jasminoides, was also shown to be a dual action reagent, developing dark blue marks that fluoresced under illumination with green/yellow light [17], [18]. Incorporation of ZnCl₂ into 1,2 indandione (1,2 IND) formulations, as opposed to its sequential use in post-treatment, has also been shown to be capable of producing formulations with dual action [14], [15], [16]. Similarly, pre-mixed ninhydrin/ZnCl₂ formulations have shown the potential for dual action, but require the use of liquid nitrogen to produce both fluorescent and coloured marks simultaneously[19], making them unfeasibly cumbersome to perform operationally. However, ninhydrin analogues, such as 5-methylthioninhydrin (5MTN), have been shown to develop both coloured and fluorescent marks when used in conjunction with ZnCl₂ without the need for the post-development liquid nitrogen cooling step. In a recent comparative study conducted in Israel by Almog and co-workers, a pre-mixed 5MTN/ZnCl₂ solution indicated a higher efficiency than ninhydrin in the colour mode and a stronger fluorescence when compared with DFO [20]. However, it is important to note that climatic variation between countries can introduce an added dimension of performance variability for fingerprint development reagents. This variability has been attributed to a direct correlation between the moisture content of the paper and the % relative humidity (RH) in the air [17], although it should be recognised that paper chemistries (acidity/alkalinity, trace metals, types of filler) may also vary between countries and may contribute to the results observed.

The overall aim of this study was to conduct the first investigation of a 5MTN/ZnCl₂ reagent under United Kingdom climatic conditions to assess whether it could replace currently employed approaches for finger mark development on representative paper types. The objectives were therefore to optimise a formulation of 5MTN/ZnCl₂; to obtain the excitation and emission spectra of the 5MTN/ZnCl₂ reaction with amino acids; to carry out comparative studies with existing reagents/procedures and with the 1,2 indandione/ZnCl₂ process on a range of paper types; to perform a sequencing experiment in order to assess at what stage of the sequence 5MTN/ZnCl₂ could be incorporated to develop the highest cumulative number of marks; and finally to assess 5MTN/ZnCl₂ on pseudo-operational articles and compare it to ninhydrin.