The effect of zinc chloride, humidity and the substrate on the reaction of 1,2-indanedione–zinc with amino acids in latent fingermark secretions
Introduction
1,2-Indanedione (Ind) reacts with α-amino acids present in latent fingermark secretions to form a pink Ruhemann's purple analogue referred to as Joullié’s pink (JP) [1]. JP exhibits an absorption maximum at 550 nm and produces a luminescence emission at 555 nm when excited by a green light source (typically around 500–530 nm). The 1,2-indanedione reagent was initially trialled as a latent fingermark reagent after it was isolated as an intermediate in the production of 5-methylthioninhydrin [2]. Following the publication of this initial study, several research groups conducted further trials to determine the suitability of 1,2-indanedione as a luminescent alternative to ninhydrin for fingermark detection on paper substrates [3], [4], [5].
Previous studies into the optimisation of the indanedione reagent indicated that the reaction between 1,2-indanedione and amino acids is highly sensitive to the chemistry of the paper substrate, ambient humidity, the solvent system and the application of heat. Many formulations required the addition of a small volume of glacial acetic acid to neutralise alkaline paper stocks and improve the development of latent fingermarks, although research performed by researchers at the Casali Institute of Applied Science in Israel indicated that the addition of acetic acid to the working solution resulted in the development of diffused fingermarks on local paper stocks [6].
Research into the indanedione reagent by the University of Technology, Sydney (UTS) in conjunction with the Australian Federal Police (AFP) indicated that the performance of the reagent reduced dramatically in low relative humidity conditions, leading to the inference that the reaction requires the presence of water in a similar manner to that of ninhydrin [7]. Furthermore, the addition of a 1:25 molar ratio of ethanolic zinc chloride solution to 1,2-indanedione in the working solution was shown to significantly improve the performance of the indanedione reagent in low humidity environments [8]. The dependence of the indanedione reagent on humidity has since been reported anecdotally by practitioners and demonstrated experimentally by Bicknell and Ramotowski on common USA paper stocks [9].
Although the reaction mechanism of 1,2-indanedione with α-amino acids in solution has previously been characterised as involving a Strecker degradation analogous to the ninhydrin and 1,8-diazafluoren-9-one (DFO) reactions [10], [11], the role of zinc(II) ions and the cellulose substrate in the reaction has not been extensively characterised. Zinc(II) ions were initially considered to form a complex with JP in the same manner as for the ninhydrin reaction product when applied as a post treatment, with research by Ramotowski et al. and Hauze et al. demonstrating a darkening of indanedione-developed fingermarks upon the application of ethanolic zinc chloride solution [2], [12]. However, similar research by other groups only noted slight changes in visible colour, luminescence intensity and the visible absorbance wavelength maximum [3], [8], [11].
The minor red shift in the absorption spectrum reported by Stoilovic et al. [8], combined with an observed decrease in luminescence and colour intensity observed with high concentrations of zinc chloride, suggested that the primary role of zinc(II) ions in the indanedione-amino acid reaction is that of a Lewis acid catalyst. However, this hypothesis, formed primarily through anecdotal observations of the behaviour of the indanedione-zinc reagent in forensic laboratories, has not been confirmed in the literature. The emergence of several “green catalysis” methods for the production of amines via Strecker degradation using transition metal salts as Lewis acid catalysts [13], [14], [15] further supports the hypothesis that a low concentration of zinc chloride added to the indanedione working solution catalyses the reaction with amino acid residues by accelerating an otherwise rate-limiting step.
The experiments undertaken in this study aimed to investigate whether the reaction of 1,2-indanedione with amino acid residues on cellulose proceeds via the same mechanism identified by Petrovskaia et al. and to identify any potential surface interactions between the supposedly inert paper substrate and reaction intermediates [10]. The amino acids used for this particular study were chosen as representative models for two classes of amino acid: aromatic (phenylalanine) and aliphatic (leucine). Alanine was used as a universal model for all common amino acids as it exhibits a greater reaction rate and fewer side reactions than glycine [16]. Furthermore, these experiments aimed to explain the previously reported observations regarding the correlation between a high yield of JP and the rehumidification of latent fingermarks prior to treatment with 1,2-indanedione or the addition of zinc chloride to the working solution [16].
Section snippets
Preparation of the cellulose substrate
l-Alanine (Fluka, 98% w/w; 0.1782 g, 2.00 mmol) was dissolved in MilliQ water (18.2 mΩ, 20 mL) and the solution mixed thoroughly with chromatography-grade cellulose (SigmaCell type 101; 5.00 g). The slurry was then spread in a thin layer and allowed to air dry overnight. Once dried, the alanine-impregnated cellulose was ground into a fine powder. The procedure was repeated with l-phenylalanine (0.3304 g, 2.00 mmol) and l-leucine (0.2623 g, 2.00 mmol) (both reagent grade; Sigma–Aldrich). Further
Characterisation of the indanedione–zinc reagent reaction mechanism on paper substrates
The initial analysis of the reaction of Ind and Ind–Zn with amino acids on the cellulose substrate using ESI–MS indicated that the reaction proceeded through a similar Strecker degradation mechanism to that of ninhydrin. Due to the soft ionisation conditions, the carrier solvents used for analysis and the increased stability of the enolate conformation of the intermediate and product structures, the most comprehensive data set was obtained using the negative ionisation mode.
Of the three amino
Conclusions
The various experimental approaches employed in the course of this study confirmed that Ind-Zn reacts with amino acids in latent fingermark deposits via the same Strecker degradation pathway as for ninhydrin. A secondary pathway to JP formation through the deamination of the 2-amino-1-indanone reaction intermediate was also identified to occur in warm and high relative humidity laboratory environments. The addition of a low concentration of zinc chloride to the 1,2-indanedione working solution
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Effect of carrier solvent in 1,2-indanedione formulation on the development of fingermarks on porous substrates
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2020, Forensic Science InternationalCitation Excerpt :However, a ninhydrin oven was not used in this research in order to simulate scene conditions. In addition, the poor performance of ninhydrin on matt paint could be due to the lack of cellulose within the paint, which is known to affect the development of latent marks [15,22,23]. Fingermarks that are absorbed into cellulose materials are considered more stable, and therefore marks can be developed more effectively with chemical reagents such as ninhydrin [24–26].
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2018, Forensic Science InternationalCitation Excerpt :A recent study found the effectiveness of DFO and 1,2-indandione formulations to be close but proposed the use of an 1,2-indandione formulation over DFO [23] because of the additional cost of DFO and two other factors, toxicity and flammability (although these assertions are inconsistent with the relevant Safety Data Sheet [24]). Other studies have investigated the use of sequences of processes from examining the amounts of amino acids reacted by each of the reagents and have concluded that, in addition to the known advantages of using ninhydrin after DFO, there is also a need to follow 1,2-indandione/zinc with ninhydrin to maximise fingermark recovery [25–27]. Based on the results obtained from brown paper and cardboard [20], work has continued in the UK to refine the 1,2-indandione/zinc formulation [28] and to identify the most appropriate processing conditions for operational implementation [29].
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