Elsevier

Science & Justice

Volume 60, Issue 1, January 2020, Pages 63-71
Science & Justice

Comparison of three collection methods for the sodium rhodizonate detection of gunshot residues on hands

https://doi.org/10.1016/j.scijus.2019.09.004Get rights and content

Highlights

  • The Filter method had a high-quality detection with both dry and humidified hands.

  • The adhesive foil with photo paper performed better on dry than humidified hands.

  • The adhesive with a PVAL method did not perform as well as the other methods.

  • Both Filter and PVAL methods provided satisfactory SEM/EDX analysis.

  • SEM/EDX analysis preparation was more tedious for the AF Photo and PVAL methods.

Abstract

The aim of this study was to compare three gunshot residue (GSR) collection methods used in conjunction with chemographic detection applied by different regional Swiss police services. The specimens were collected from the hands of a shooter with either filter paper (Filter method) or adhesive foil. The adhesive foil was then either applied against photographic paper during visualisation (AF Photo method) or coated with a layer of polyvinyl alcohol (AF PVAL method). The experiments involved two conditions of the examined hands, i.e. dry and humidified. The residues were revealed using the sodium rhodizonate test (SRT). Preliminary tests assessing the possibility of conducting a confirmatory Scanning Electron Microscopy coupled to Energy Dispersive X-ray spectroscopy (SEM/EDX) analysis after the chemographic test were performed on a number of specimens by cutting positive spots and mounting them on stubs. Obtained results were compared in terms of effectiveness - number of positive spots, time requirements, quality of subsequent SEM-EDX analysis, ease of use and cost.

The Filter method generally yielded a high-quality detection with both dry and humidified hands, as well as a simple, quick and efficient confirmation by SEM/EDX. The AF Photo performed well on dry hands, but not on humidified hands. The AF PVAL method performance was lower compared to the other methods in both examined conditions of the hands. The SEM/EDX analysis showed that the Filter and AF PVAL method provided satisfactory results when a sufficient carbon coating thickness was applied to the cuttings. It was also observed that the thinner the PVAL layer, the better the quality of the spectra and obtained images in SEM/EDX. Furthermore, the surface of the photographic paper did not seem to be conductive, even after the application of a thick layer of carbon.

In conclusion, the Filter method gave the best overall results, but its application required slightly more time and expertise than the two other methods.

Introduction

Gunshot residue (GSR), also known as firearm discharge residue, is produced during the discharge of a firearm [1]. The residue exits the muzzle and all other firearm openings, such as the ejector port or the barrel-drum gap [2]. While a part of GSR travels with the bullet and is thus generally detected on the target, the remainder loses its kinetic energy and then settles down on surfaces around the firearm, including on the shooter [3,4]. GSRs are a mixture formed by primer, smokeless powder and lubricant residues as well as metals arising from the projectile, cartridge case and gun barrel [5]. In casework, the forensic examiner's tasks include identifying bullet holes, estimating the firing distance, and evaluating whether an individual has discharged a firearm [5]. For these purposes, various methods can be applied, namely optical, chemographic and instrumental methods [6]. Depending on the context and investigation requirements, these methods can be employed individually or in sequence. As a rule, the examination begins with simple and non-destructive approaches, generally optical methods such as observation under various lighting conditions or using a macroscope/microscope. In a second stage, more efficient and to some extent destructive methods may be needed. Depending on the context, the sequence of applied methods has to be optimised to obtain the greatest amount of relevant information [6,7].

Chemographic methods target GSR using chemical reagents that are specific to an element, thus enabling the visualisation of GSR distribution pattern [7]. They are used to reveal the presence of latent GSR as well as for shooting distance estimation. Such tests are often used as a screening tool and thus require confirmatory analysis respectively by Scanning Electron Microscopy coupled to Energy Dispersive X-ray spectroscopy (SEM/EDX) or liquid chromatography coupled to mass spectrometry (LC-MS) for inorganic and organic GSR. These methods are specific to one or more chemical elements contained in GSR, but not uniquely present in GSR. Thus, false positive reactions from environmental contaminants may occur. However, the main advantage of these methods compared with instrumental methods is their low cost, easy implementation (partly applicable directly on the crime scene) and short analysis times, leading to frequent use in police services for investigative purposes.

Historically, the first chemical test documented for GSR was the paraffin test, also called the dermal nitrate test, introduced by T. Gonzalez at the Mexico City Police Laboratory [5,8,9]. It consisted of applying molten paraffin to the hand and, after removal, spraying it with diphenylamine in concentrated sulfuric acid to produce a blue reaction product. The paraffin test targeted nitrates and nitrites. However, it was abandoned due to its unreliability [5] as it was found to react with both nitrates and chlorates leading to an unacceptably high rate of false positive results [9]. While nitrates are ubiquitous in the environment, nitrites are less common. This ion, formed by the burning of propellant, is the target of the modified Griess test that results in an orange colour [10]. Both the paraffin and modified Griess tests detect residues from the propellant. However, assays for metallic elements are also available and frequently used in casework. A list of the most common methods is provided in the Best Practice Manual published by ENFSI [7]. One of the most frequently used by police services is the sodium rhodizonate test (SRT), which detects lead and barium. Sodium rhodizonate reacts with divalent metals by forming coloured complexes [11]. Depending on the pH, the resulting colour varies from blue-violet (neutral pH) to scarlet (pH 2.8) for lead, whereas the red-brown colour obtained with barium is independent of the pH [11]. Other bivalent metals such as strontium and copper also react at neutral pH, but produce no coloration at acidic pH. The SRT can be applied directly on the target surface, such as the hands of a presumptive shooter or the clothing of a victim. However, visualisation on dark-coloured fabric can be difficult, leading to the introduction of a transfer step by Bashinksi et al. [12]. A filter paper was soaked in 10% acetic acid and pressed onto the residue pattern, leading to the partial transfer of the residue. The filter paper was then sprayed with sodium rhodizonate, with lead becoming bright pink and barium orange. This procedure is nowadays named Bashinski transfer. A similar indirect procedure was also described by Suchenwirth, who used 1% tartaric acid instead of acetic acid [13]. Nowadays, the indirect procedure is generally applied. However, as highlighted in the ENFSI Best Practice Manual [7], there are at least four variants of this chemical test, differing from each other in transfer and diffusion media. Regarding the transfer medium, various protocols use adhesive foils, filter paper, polyethylene photo paper or cellophane™. The diffusion medium is in all cases acidic, using either acetic or tartaric acid, but an additional layer of polyvinyl alcohol (PVAL) is used in conjunction with the adhesive foil transfer protocol. Yet, there are other variants that are not mentioned in the Best Practice Manual, leading to the aim of the present work.

In Switzerland, some regional police services use pattern visualising methods such as the SRT in addition to SEM/EDX analysis as a rapid screening test for the presence of GSR. In some cases, the distribution of GSR particles on the hands of a suspect or a victim is used to infer the type of activity leading to that specific distribution. At least three variants of the SRT have been reported in Switzerland. However, at present no data allows a direct comparison of their performance. Thus, the aim of this work was to compare the effectiveness of three protocols: one using filter paper, one using adhesive foil (AF) with further GSR transfer onto a photographic paper and the last one using an adhesive foil with the addition of a thin layer of polyvinyl alcohol (PVAL). Experiments were carried out on dry and humidified hands to simulate conditions that can be encountered routinely, such as the presence of blood or perspiration. After visualisation with sodium rhodizonate, the treated samples were scanned and positive reactions optically verified. Then, some positive spots were excised and mounted on a carbon stub for further confirmation by SEM/EDX. Finally, the protocols were compared in terms of the number of positive spots, time requirements, quality of subsequent SEM-EDX analysis, ease of use and cost.

Section snippets

Shooting experiments

Various factors affecting the formation and the deposition of GSR were standardised to minimise contamination throughout the experiments. In this case, a single person carried out all shooting sessions in an indoor shooting range with the ventilation turned off. The same 9 mm Luger semi-automatic pistol – a Sig Sauer P220 – with Geco Sinoxid® ammunition (124 g, FMJ) from a single batch was used for all experiments. Before every study the firearm was completely dismantled, cleaned and

Dry hands

The aim of the first experiment was to compare the three methods in ideal conditions in terms of transferred particles (time t ≈ 0 after shooting, dry hands). For each method, the procedure was repeated 15 times in order to take into account the high variability in GSR production and transfer. Before and between each experiment, the shooter washed his hands and a blank was collected from each hand using the sampling technique applied in the given set of experiments to check for potential

Discussion

The comparison of the medians showed that the Filter and AF Photo methods performed better than AF PVAL on dry hands in terms of the number of positive reactions (Table 1). When applying the methods on humidified hands, a lower amount of residue was detected (between 33 and 68% fewer particles than on dry hands). While the Filter method showed a higher variability between replicate experiments, it did yield a significantly higher median than the two other methods on humidified hands (i.e.

Conclusions

This research project evaluated the performance of three GSR collection methods to be used in conjunction with the SRT. These methods, which are commonly used by Swiss police services for the detection of GSR and its distribution, were compared in terms of the number of positive reactions on dry and humidified hands, time requirements, ease of use and possibilities of subsequent SEM-EDX analysis and cost. All in all, the Filter method showed the best results with both dry and humidified hands

Declaration of Competing Interest

None.

Acknowledgements

The authors would like to acknowledge Mr. Urs Nachbur, from the police services of Canton of Basel-Landschaft in Switzerland, and Mr. Stefan Meichtry from the police services of Canton of Bern in Switzerland, for providing their protocols and training to reproduce them and Dr. Amanda Frick from Ecole des Sciences Criminelles of the University of Lausanne for revising early drafts of the article.

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