https://doi.org/10.1007/s41664-020-00152-x REVIEW
Trends in Gunshot Residue Detection by Electrochemical Methods for Forensic Purpose
Abhimanyu Harshey1 · Ankit Srivastava1 · Tanurup Das1 · Kriti Nigam1 · Raj Shrivastava2 · Vijay K. Yadav1
Received: 4 October 2020 / Accepted: 19 November 2020 / Published online: 31 January 2021
© The Nonferrous Metals Society of China 2021
Abstract
Gunshot Residue (GSR) has been a subject of interest for the forensic fraternity. Numerous analytical contributions towards the GSR analysis have been reported. Sensitivity, portability, cost-effectiveness, speed, etc. are such factors of electrochemi- cal methods that have attracted the researchers across the globe to test the applicability of these as a potential analytical tool for forensic evaluation of GSR. With the development of scientific technology, efforts have been made towards the hand- held device for the on-field analysis of GSR. Recently, chemometric treatment of data generated from the electrochemical analysis of GSR has offered more effective approach. It makes the analysis more conclusive and minimizes the chances of false-positive detection. It will be very fruitful to anticipate the analytical potential of electrochemical tools for GSR analysis.
This article reviews the research progress towards the development of electrochemical sensor for GSR detection reported during 2013–2020 along with challenges and future perspectives.
Keywords GSR(gunshot residue) · Forensics · Electrochemical sensing · Voltammetry · Amperometry
1 Introduction
Gunshot Residue (GSR) analysis is one of the most frequent and highly significant practices of forensic ballistics exami- nations. GSR originates in the form of a gaseous cloud (also known as plume), encompassing the combustion products of primer (vaporized Lead (Pb), Barium (Ba), and Antimony (Sb)) and propellant (Nitro-aromatic explosives present in the propellant, such as Ethyl Centralite (EC), 2, 4- Dinitro- toluene (2, 4-DNT), Diphenylamine (DPA), Nitro Glycerin (NG), and some of its nitrated derivatives), which are gener- ally referred to as Inorganic Gunshot Residue (IGSR) and Organic Gunshot Residue (OGSR), respectively, and pre- sent distinguished analytical perspective [1, 2]. Since, GSR originates as a consequence of firing, it is a surest sign of the firearm discharge. GSR analysis requires expertise and utmost care while interpreting the results. GSR escapes from the available openings in the firearm and deposits on the
objects present in the close vicinity of the firing range [3].
Morphology and chemical content of the GSR particle inter- estingly depend upon the configuration of the weapon as well as on the location from where the samples have been collected. Here, it is germane to note that several studies emphasise that presence of the GSR on individuals may not be alone suffice to identify him as a shooter, and the pres- ence of GSR may be subjected to the transfer mechanism.
Therefore, it is more appropriate to refer that the person was present in the close proximity of firing range [4].
Numerous analytical approaches have been reported for the GSR analysis. Color tests, such as Walker test, Heris- son—Gillroy test, Dermal Nitrate test, Griess test, offer the on-spot field presumptive and rapid detection of IGSR and OGSR [5–7]. Over the period, many advanced instrumental techniques have been introduced to increase the precision, sensitivity of the analysis. Scanning Electron Microscope Energy-Dispersive X-rays (SEM–EDX) [8], Neutron Acti- vation Analysis (NAA), Atomic Absorption Spectroscopy (AAS) [9], X-ray Fluorescence (XRF)[10], Ion Beam analy- sis (IBA) [11], Inductively Coupled Plasma Optical Emis- sion Spectroscopy (ICP-OES) [12], etc. target IGSR (metal- lic content) and confirm the presence of GSR. Among all the techniques, SEM–EDX is supposed to be a very specific and gold standard technique of the detection and analysis of
* Ankit Srivastava
ankit_forensic81@rediffmail.com
1 Dr. A.P.J. Abdul Kalam Institute of Forensic Science and Criminology, Bundelkhand University, Jhansi 284128, India
2 State Forensic Science Laboratory, Sagar 470002, India
GSR [13]. The particle analysis by SEM–EDX, for the con- firmation of GSR, includes the morphological study of the particle. While the techniques, such as High-Performance Liquid Chromatography (HPLC) [14, 15], Gas Chromatog- raphy–Mass Spectroscopy (GC–MS) [16–18], Attenuated Total Reflection Fourier-Transform Infrared (ATR-FTIR) spectroscopy [19], etc. facilitate the OGSR detection [20].
In addition, Raman Spectroscopy can target both IGSR and OGSR [21]. Simultaneous detection of IGSR and OGSR has also been attempted [22–25].
In the current scenario, available statistics indicate an alarming increase in the involvement of firearms in criminal activities across the globe [26–30]. In India, as per the sta- tistics provided by the National Crime Record Bureau, Min- istry of Home affairs, Government of India (NCRB, MHA, Govt. of India), huge numbers of cases are being registered under arms act [31–34]. Table 1 compiles the data from the reports of NCRB during 2014–2018. The increasing rate of firearm-related offences poses the need for rapid detection and analytical tools. Real-time or on-location analysis may contribute significantly to enhance the speed of the investiga- tion. Therefore, the development of electrochemical sensors, paper-based devices (PADs), IR, and Raman Spectroscopy- based portable devices for the forensic purpose was initiated and has increased also during the last decades [35]. In this regard, the advents of electrochemistry have significantly contributed to the forensic analysis. The analysis of the sam- ples of forensic interest by the means of electrochemical tools presents a good agreement towards the sensitive, rapid detection, and screening [36]. In recent years, the potential application of electrochemistry for the detection and forensic analysis of illicit drugs [37–39], intoxication [40–43], explo- sives [44], etc. have been reported. Electro-analytical tools offer a promising approach for the GSR analysis. It offers portable, rapid, sensitive, and low-cost analysis of the GSR.
Recently, de Oliveira et al. [35] presented a detailed review demonstrating the contribution of portable and poten- tially portable devices (not only limited to the electrochemi- cal sensors but also including field tests and intelligence devices) reported during 2013–2017, that can be applied for
the forensic analysis illicit drugs, beverages, biological flu- ids, agrochemicals, explosives, and GSR. However, another review article authored by de Araujo et al. [45] has reviewed a wide range of portable devices, e.g. portable Raman and NIR device, paper-based devices, microchip electrophoresis, ambient ionization on the portable MS and electrochemical sensors that could be potentially applicable for the in-loco analysis of the latent evidence, screening controlled sub- stances and hazardous materials. Both of the articles have discussed the electrochemical detection of GSR in brief along with all other possible portable methodologies for GSR analysis.
No detailed review solely dedicated to the electrochemi- cal detection of GSR is reported with the exception of the article authored by O’ Mahony and Wang [46] which was published 7 years ago in 2013. The presented article reviews electrochemical studies and advances dedicated to forensic detection and the analysis of GSR reported during the past 7 years, i.e. 2013 to 2020. In addition, this article discusses studies on the grounds of existing challenges in GSR analy- sis and underlines the future perspective of electrochemical sensors for the GSR.
2 Electrochemistry and Electrochemical Sensors
Electrochemistry deals with the charge transfer between an electrode and electrolyte and thus defines the chemi- cal perspectives of electricity [47]. Superficially, all the electro-analytical methods can be classified into two major categories, namely interfacial methods and bulk methods, as shown in Fig. 1 [48]. Chemical sensing provides the infor- mation (qualitative and quantitative) regarding any chemi- cal substance by interpreting the amplified electrical signals originated from the interaction between the sensor and the chemical species (Fig. 2) [49, 50]. Among various chemical- sensing methods, the electrochemical ones are the oldest and seem to be covering a wide arena [51, 52]. Electrochemi- cal sensors, based on different types of electro-analytical
Table 1 Number of seized firearms, ammunitions and cases registered under Arms Act, 1959
Year Number of cases regis- tered under Arms Act, 1959
Number of firearm seized Number of
ammunition seized Licensed/
Factory made
Country-made/ Impro- vised/ Unlicensed/
Crude
Total
2014 55,255 1,198 32,319 33,517 1,09,110
2015 53,300 1,214 32,564 33,778 3,42,478
2016 55,660 1,052 36,064 37,116 1,06,900
2017 58,053 3,525 59,694 63,219 92,107
2018 66,305 3,742 71,135 74,877 1,08,444
methods, possess specific configuration of the electrochemi- cal cell accordingly [51, 53].
Voltammetric methods have been commonly procured to develop electrochemical sensors as these have found to be offering more advantages in terms of increased sensitivity and speed of the assay as compared to their potentiometric counterparts [51]. Voltammetric assessment involves the use of a three-electrode configuration (working, reference, and counter electrode) with a potentiostat for the determination of the electrochemical behavior by applying known variable potentials between working and reference electrodes [54].
The electric field generated at the surface of the working electrode stimulates the exchange of electron (between the sample and the working electrode surface) which is meas- ured in terms of the current and is referred to as Faradaic
current [47]. Voltammetry lays on the foundation of the fact that the current is proportional to the analyte concentration.
There exists a linear relationship between both of them. This current may be plotted as a function of either voltage or time and plots are often referred to as voltammogram. An analyst determines the analyte concentration by interpreting the current–potential plot [51, 52]. However, amperometry is a subclass of the voltammetry in which the concentration of the sample is supervised at fixed potentials (for different time durations, electrodes are placed at constant potential).
The strides towards the development of electrochemical sen- sors for the forensic purpose have been started for a very long period and are still going on. Trends in electrochemical sensing of GSR have been discussed in the next section.
Fig. 1 Classification of electro- analytical methods
Fig. 2 Schematic representation of the different sensing processes
3 Endeavors to Detect the GSR by Electrochemical Means
Detection of GSR by electrochemical methods have been well attempted for the last 3–4 decades, over the period, strides have been made to improve the selectivity, sensi- tivity and the specificity of the method and analysis. The transition of GSR analytical methodology, from the use of bulky electrodes (conventional electrochemical cells) to the field-deployable electrochemical sensor, has taken a period of more than 4 decades.
3.1 Historical Background: A Survey (1977–2012) Electrochemical detection of GSR is found to be initi- ated with the detection of metallic traces by voltammetric methods. Initially, in 1977, Anodic Stripping Voltamme- try (ASV) was employed for the detection of the Pb and Sb using Mercury-filmed graphite as a working electrode.
This work laid down the basis for the further detection of GSR by Voltammetric methods [55]. Further, different voltammetric methods, such as ASV, differential pulse vol- tammetry, and differential pulse anodic stripping voltam- metry (DPASV), were applied for the detection of different metallic components, such as Zn, Cu, Cd, Pb, Sb [56–61], and nitrites [62]. Till the end of the twentieth century, Ba could not be detected in GSR by electro-analytical meth- ods since it stripps at electronegative potential followed by the hydrolysis of the solvent. With the beginning of the twenty-first century, Woolever and Dewald reported the detection of the Ba and Pb in GSR by DPASV using Hg film glassy carbon electrode (GCE). Nonetheless, being non-destructive, cost-effective, and simple, this technique encountered challenges, such as the evolution of hydro- gen at electrodes, a wide standard deviation for Pb and Ba, and concentration of Pb below the threshold values [63]. Along with notable attempts of IGSR detection in the twentieth century, electrochemical detection of nitro-aro- matic, nitro-amine and nitrate ester explosive compounds is commonly present in the propellant powder, such as NG, 2,4-DNT, TNT, EC, and DPA was performed by different methodologies, namely reductive electrochemistry using glassy carbon and amalgamated gold electrodes (with Liq- uid Chromatography also) [64], oxidative electrochemistry (with HPLC) [65], screen-printed voltammetric sensor for on-site screening [66]. Towards the simultaneous detec- tion of the metallic components and organic residues, Vuki et al. reported the detection of primer and propellant resi- dues in a single run using cyclic voltammetry (CV) and Cyclic Square Wave Stripping Voltammetry (CSWV). Ba was also detected with voltammetric measurements at Hg film GCE [67].
The batch injection analysis (with ASV) and multi- commuted flow system (with DPASV) were also applied for the detection of metallic components of GSR [68, 69].
Different voltammetry techniques, e.g. differential pulse cathodic adsorptive stripping voltammetry (DPCAdSV), square wave cathodic adsorptive stripping (SWCAdSV) voltammetry, CV and, SWV, have been also employed for the detection of metals (Pb, Sb) [70] and trace nitro- aromatic, nitro-amine, and nitrate-ester explosives [71].
In the year 2012, several significant and landmark stud- ies directing novel methodologies and attempts towards the development of portable devices for the on-site detection of the GSR were reported. Different electrochemical meth- ods were used with pattern recognition techniques (Che- mometrics), namely Principle Component Analysis (PCA) and Hierarchical Cluster Analysis (HCA). It enabled the descriptive GSR analysis; discrimination GSR originated from different ammunitions and guns using gold micro- electrodes [72, 73], rapid identification of shooter by the SWV analysis of GSR sample collected abrasively [74].
The abrasive sampling of GSR (a novel sampling method for GSR collection, swiping of the electrode at the sur- face bearing the sample) followed by the ASV analysis at screen-printed electrodes were presented by O’Mahony and co-authors to counter the problems, such as errors at sam- ple collection and transportation stage. This protocol was termed as abrasive stripping voltammetry (AbrSV) [75].
3.2 2013 Onwards
Over the period, different voltammetric techniques have been applied for the GSR sensing. This section discusses the application of different voltammetric operations for GSR detection.
3.2.1 Square Wave Voltammetry (SWV)
Towards the development of the wearable sensor, Bandodkar et al. (2013) [76] synthesized a novel Forensic Finger to be used for the detection of GSR. It was the first attempt of its kind. This sensor possessed electrodes and electrolytes that were fabricated on the flexible nitrile fingertip. For the ease of use, a solid-state ionogel was employed as an electrolyte due to low cost, ionic conductivity and mechanical flexibil- ity. Electrode screen-printed fingertip and ionogel containing fingertips were employed on the index finger and thumb, respectively. After the abrasive rubbing of the electrode on the GSR-containing surface, the index finger was placed in contact with the thumb thereby completing the circuit (con- tact of electrodes and the electrolyte). The electrochemical signature of GSR was characterized by the means of square wave stripping voltammetry (SWSV) by applying the poten- tial of − 0.95 V for 120 s with scan potential of 0 V vs Ag/
AgCl. Peaks at − 0.6, − 0.4 and − 0.2 V were observed cor- responding to the Pb, Sb, and Cu, respectively. This method enables a promising on-site detection of GSR. Neverthe- less, this Forensic Finger was also employed for the detec- tion of explosive powder. 2,4-DNT was identified by the peaks observed at − 0.9 and − 1.2 V during the SWSV. This wearable sensor was found to be cost-effective, resistant to mechanical stress as well durable. Bandodkar et al. found it efficient up to 7 days while efficiency and the durability were not assessed beyond 1 week. This study provides a promis- ing on-site qualitative detection of GSR and 2,4-DNT rather than quantitative assessment [76]. To impart the robustness in the analysis, some other explosive substances, that are usually present in the ammunition, such as NG, NC, DPA, EC, etc., may also be tested. Further, attempts towards the simultaneous detection of more explosives (constituents of OGSR) may also be efficacious.
An approach demonstrating the orthogonal identifica- tion of GSR utilizing Voltammetry and SEM–EDX was presented by O’Mahony et al. (2014) [77]. The carbon screen-printed electrode (CSPE) was modified by carbon tape. Further, − 1.3 V potential was applied for 120 s and scanned at the final potential of 0.1 V vs Ag/AgCl dur- ing SWSV to electrochemically assess the characterization of GSR collected by swiping the working electrode over the hands of the shooter. Pb, Sb, and Cu were character- ized. Voltammetric signals of Pb and Sb, with potential contributions of Cu, were obtained at − 0.7 and − 0.15 V, respectively. The splitting of the peak near the − 0.8 V was observed possibly due to the coexistence of the nitro compounds and other electro-active species with Pb or may be due to antimony coated carbon stripping. Subsequent SEM–EDX analysis revealed the morphology of the GSR particle as cracked shell spherical and simultaneous detec- tion of Pb, Sb, and Ba that is unique to the GSR. Further, on the voltammetric analysis of the negative sample (swiped from the hand of a person who has not fired the firearm), a baseline with no characteristic peaks (featureless) indicated the absence of GSR and it was confirmed by the SEM–EDX analysis. This work presented the non-destructive nature of electrochemical analysis and its non-influence over the subsequent SEM-EDX analysis. Bare CSPE offers dimin- ished signals as compared to that of Carbon tape-modified CSPE. Carbon tape-modified CSPE offers an enhanced col- lection of GSR particles and thereby the enhanced signal as well as it also provides the means for the sample retention for further sample. Thus, it may be referred that Carbon tape-modified CSPE offers several advantages over the bare CSPE [77]. Pb, Ba, Cu may be a presumptive indication for the presence of GSR. Here, electrochemical sensing offers on-spot preliminary field sensing. Orthogonal analysis pro- vided two-tier examination and increased reliability and robustness of the examination.
The potential of fused deposition modeling (FDM) 3-D printing of polylactic acid, containing graphene (G-PLA), that provides a conductive, flexible, biodegradable platform, was tested for the detection of TNT using square wave vol- tammetry measurements. Further, a polished 3-D printed device was applied for the detection of Pb and Cu by square wave anodic stripping voltammetry (SWASV). The appli- cation of − 0.3 V voltage was followed by the deposition of the metals. On the basis of the findings of this work, Cardoso et al. found the 3-D screen-printed device is rapid, cost-effective as well as may offer on-site investigation of the forensic samples. The feasibility of this 3-D printed device might be a promising approach for the detection of GSR [78].
Recently, 3-D printed G-PLA platform coupled with voltammetry has been reported as potential and effective method for the simultaneous detection of Pb2+ and Sb3+. Direct swiping of 3-D printed electrode on the hands of the shooter was followed by the immersion in the supporting electrolyte for SWASV scan. The method does not require sample preparation and provides identification and semi- quantification of metallic components. As far as the stabil- ity of IGSR is concerned, it was found that IGSR remained stable on 3-D printed electrode surface even after 8 months.
However, OGSR components substantially degrade after 4 days. It showed better analytical performance in HCl media than that of HNO3. One more thing that makes it more advantageous is that it was found to be reusable for three times without any loss electrochemical performance.
It showed a good agreement for GSR analysis by providing a rapid, sensitive, portable, specific analytical approach [79].
3.2.2 Cyclic Voltammetry (CV)
CV had also offered an effective detection of the Cu (II) in the GSR sample. CSPE coated with Au was directly swapped over the shooter’s hand to collect the GSR sam- ple for voltammetric analysis. While for ICP-OES analysis, GSR was collected through the swabbing of the shooter’s hands by cotton soaked in 0.5 mol·L−1 HNO3. CV analysis was performed at the scan rate of 100 mV·s−1 with − 100 to 100 mV scan potential. The rise in the concentration of Cu (II), because of the increased diffusion of Cu (II) at the electrode, resulted in a linear increase in the peak current.
However, the high concentration of electro-active substances caused a gradual increase in the peak current. Modification of the electrode by nano-particles caused effective catalysis, increased effective surface area, and therefore followed by the enhanced electrochemical response. Results obtained from the cyclic voltammetry analysis were validated by the results of ICP-OES at 94% level of confidence. ICP-OES was found to be less sensitive as compared to CV for Cu detection (Cu detection limits of 1–7 ppm and 1–50 ppm
for ICP-OES and CV, respectively). Additionally, cyclic vol- tammetry offers a relatively convenient and reliable method [80]. Gold-coated CSPE offered enhanced reproducibility.
Here, it is worthy to note that Cu alone cannot be considered as specific for GSR. This study may be referred as proof of concept. However, the diagnosis of other metals (Pb, Ba, Sb), that are considered as specific for GSR, may be tested with this methodology.
Trejos et al. [25] reported a combined screen protocol using LIBS and the electrochemical tools. In this method, sample was collected in compliance with the guidelines for that of SEM–EDX. Laser-induced breakdown spectros- copy (LIBS) and electrochemical method, namely CSWV using gold and CSPE, provided detection of both IGSR and OGSR. LIBS spectrum having multiple emission lines con- firmed the presence of metallic components in GSR (Pb, Ba, and Sb). The spectral lines at 368.3 nm and 405.8 nm were indicative for Pb. While, Sb was determined by the lines at 252.8 nm and 259.8 nm. While spectral lines at 455.4 nm, 493.3 nm, 553.4 nm, 614.1 nm, and 705.9 nm confirmed the presence of Ba. On the course of electrochemical analysis, an optimized reduction potential of −0.95 V was applied and acceptable measurements were found with the accumulation time of 120 S. Ba could not be detected by the electrochemi- cal analysis; however, the electro-analytical approach was found to be offering lower detection limits of Pb than that of LIBS. This orthogonal approach presented a good agreement for the rapid and sensitive on-site detection with high accu- racy (98%) of the GSR [25]. Reporting of field-deployable LIBS makes this much promising approach as compared to others. This method seems to be more robust than that of reported previously. LIBS was found to be possessing the capabilities of detecting most of the elements that might be beneficial in the context of modern ammunition (free with heavy metals). However, this simultaneous detection of IGSR and OGSR provides a significant contribution to the GSR analysis as eliminating the probabilities of misin- terpretation of results.
3.2.3 Amperometry
Previously, Yan et al. reviewed the technological advance- ments dedicated to the electrochemical sensor development for the detection of nitrite ions [81]. Promsuwan et al. [82]
presented a novel sensor based on amperometry for the detection of Nitrite in the GSR. For nitrite detection, modi- fied electrodes are used rather than conventional electrodes.
Due to enormous electro-catalytic features (conductivity and catalytic activity), chemical stability, and nontoxic nature, synthesis of Palladium (Pd) nano-particles on carbon micro- spheres offers significant advantages over the other metal particles, such as platinum, gold, silver, etc. Other carbon materials, e.g. Carbon nano-tubes, graphene may be used.
As being cost-effective, easy to modify, chemically stable and due to improved electrochemical properties as compared to carbon nanotubes and graphene, carbon microspheres were preferred. This was the first approach demonstrat- ing the detection of GSR using a glassy carbon electrode modified by Pd nano-particles synthesized on carbon micro- spheres (Pd-GCMs/GCE) and flow injection amperometry.
For the collection of GSR, hands and clothes of the shooter were swiped with the cotton swab. Phosphate buffer was added to swabs and it was ultrasonicated followed by the filtration of the extract. This extract was analyzed by the Griess spectrophotometry and Flow injection amperom- etry system. In the Griess spectrophotometry, a mixture of Griess reagent and the extract was incubated followed by the absorption analysis using UV–Vis spectrophotometer at 548 nm. Amperometric analysis with flow injection system was conducted under the optimum conditions (i.e. pH of the buffer: 5.0; potential: 0.90 V; sample volume: 300 μL;
flow rate: 2.50 mL· min−1) providing high current response and short analysis time. The nitrite detection through this amperometric analysis was found to be uninfluenced by the other IGSR and OGSR. The statistical analysis of the results obtained from the Griess test and the electrochemi- cal analysis revealed that there is no significant difference and Pd-GCMs offered a cogent approach for the detection of nitrite in the screening of the GSR [82]. Here, it is sig- nificant to note that detection of GSR by the tests for nitrite is considered as presumptive. For the robust interpretation, alone nitrate analysis cannot be considered as sufficient.
To evaluate the robustness and specificity of the technique, some more experiments may be performed to diagnose the rate of false-positive results.
Previously discussed analytical approaches, that have been applied for the GSR detection onwards 2013, are sum- marized in Table 2. Further, Table 3 summarizes the time- line of the development over the decade. Electrochemical sensors offer an effective approach for the field detection of GSR by targeting the IGSR and OGSR components with enhanced sensitivity as compared to that of available spot test methods. As reported by Koons [83], ICP-MS presents superior detection limits for Pb, Ba, and Sb (0.14, 0.020, 0.052 ng·mL−1, respectively) than that of AAS and ICP- AES. However, SEM–EDX enables the morphological and chemical content detection of a GSR particle which requires several hours for sample analysis. Electrochemical sensor gives comparatively lower sensitivity in terms of detection limits as that of previously mentioned methods, but the major advantage provided by this sensor is that it offers a rapid on-site analysis as well as reduces the sample analysis cost. That readily eliminates the sample preparation steps.
However, the challenges faced by electrochemical sensing method, emphasise the need for further developments, have been discussed in next section.
The exhaustive review of the existing literature reveals that along with the advancements in the analytical tools, sample collecting methods have been evolved also. Initially, the GSR analysis was performed using conventional bulky electrodes and samples were collected by rinsing of the hands by acids. This methodology had a major issue. Actu- ally, the analysis of morphological and chemical constituent differences of GSR collected from different locations has been well attempted in forensic and it may contribute in shooting scene reconstruction [4, 12, 20]. But the sample collected by acid washing cannot be distinguished as that coming from the dorsal surface or the palm of the hand.
However, it is worthy to note that the determination of the
shooter on the basis of the presence of GSR is a cumber- some task. In the light of transfer phenomenon (primary, secondary, tertiary, and quaternary transfer), the presence of GSR on one’s hand does not indicate that the individual has fired a gun although, it is more appropriate to refer that the person was in the close vicinity of firing range [4]. Subse- quently, electro-analysis of the GSR collected by the swab- bing counters the previously discussed challenge. Adhesive tape was applied for sample collection and it was found bet- ter than swabbing method, however, acid wash presented efficiency over the tape lifting method. Since tape lifting is a dry method, therefore, it is more suitable for on-site sample collection. The advents of screen-printed electrode abrasive
Table 2 GSR screening strides (2013–2020)
Sr. No. Working electrode Sampling Technique Analyte Analytical performance Reference
1. Screen-printed electrode Abrasive SWSV and SWV Pb, Sb , DNT N/A [76]
2. CSPE Abrasive SWSV Pb, Sb, Cu N/A [77]
3. CSPE Swabbing CV Cu (II) Detection limit: 0.3 ppm
Linear ranges: 1–50 ppm [80]
4. CSPE, Au-SPE Carbon
conduc- tive tape
SWASV Pb, Sb,
2,4-DNT, NG Detection limit: .1–1 ng·mL−1 Linear ranges: Pb: 0–15, Sb: 0–5, 2,4 DNT: 0–60,
NG: 0–50 (in ng·mL−1)
[25]
5. 3-D printed G-PLA Abrasive SW, SWASV TNT, Pb, Cu Detection limit: 0.40 μmol·L−1
Linear range: 1.00–870 μmol· L–1 [78]
6. 3-D printed G-PLA Swabbing SWASV Pb2+, Sb3+ Limit of detection: 0.5 μg·L−1 and 1.8 μg·L–1 for Pb2+, Sb3+ respectively
Linear ranges: 50 μg·L−1 and 1500 μg·L−1
[79]
7. Pd-GCMs/GCE Swabbing Amperometry Nitrite Linear range: 0.10 μmol· L−1 to 4.0 mmol· L−1 Detection limit: 0.030–0.11 μmol· L−1 Qunatification limit: 0.11 μmol· L−1
[82]
Table 3 Timeline of developments of
electrochemical strategies for the GSR analysis
Timeline Major arena Strides and milestones
1976–1977 Inception Initial attempts for the electrochemical detection of GSR 1977–2001 Development and explo-
ration Development of methodology
Simultaneous detection of metals
Explosives substances and OGSR detection
2001–2012 Augmentation Stripping voltammetry
Batch injection analysis Multicommutated system
Replacement of HMDE by Bi electrodes 2012–2020 Sophistication Towards the portable device
Use of screen-printed electrodes
Introduction of abrasive sampling for GSR collection Chemometric treatment of the data
Introduction of wearable sensors Two tier examination
2020 onwards Standardization Development of hand-held electrochemical sensor Towards more robust analysis
Focusing NTA
Elimination of false-positive detection Quantitative analysis
Standardization of the protocol
sampling were stepped forward towards the field-deployable device. As a rapid, sensitive, and accurate method, a two-tier detection system for the GSR (combined analysis by electro- analytical tool and other instruments, such as SEM–EDX and LIBS) has been reported. It appears that attempts after 2013 are going towards the developments of more robust, sensitive and portable electrochemical analysis enhancing the objectivity.
4 Challenges and Future Perspective
Notwithstanding, electrochemical detection of the GSR presents several advantages, such as cost-effective, non- destructive in nature, field-deployable, rapid, etc., it poses some challenges to the GSR detection that have been dis- cussed as below.
4.1 Selectivity
Traditionally, the screening of GSR is based on the detection of Pb, Sb, and Ba. While analyzing the real GSR sample, the electrochemical detection of Sb and Ba puts some com- plexities. The stripping peak of Cu may overlap with the Sb peak. In addition, the electro-analysis of Ba results in the hydrolysis of the aqueous solutions because of its stripping at a negative potential. Therefore, Ba puts forth challenges to its electro-analysis and the problem still persists even after the advent of the portable screen-printed electrodes. This is the major limitation of this approach [46].
4.2 Non‑Toxic Ammunition
Advents in ammunition manufacturing, with the aim to elim- inate the heavy metals because of their hazardous effects, towards the development of heavy metal-free (HMF) ammu- nition also known as non-toxic ammunition (NTA) have posed restrictions to the conventional analytical methodol- ogy. The primer of these NTA contains elements, e.g. Ti, Zn, Ca K, Sr, etc. The absence of the lead and heavy metals in primer results as the absence of the characteristic unique particles and thus in this situation SEM–EDX analysis is not useful. However, the OGSR analysis and the application of lanthanide-based metal–organic frameworks (Ln-MOFs) as luminescent GSR markers provide significant approaches towards the analysis of GSR originated from the NTA [84].
Since the elements present in the modern ammunitions are also abundantly present in an open environment, therefore, the utmost care is needed while interpreting the analytical findings to obviate the misinterpretation. In this regard, the electrochemical analysis, of GSR originated from the NTA poses several concerns. None of the aforementioned studies
include NTA. Therefore, electrochemical analysis of the GSR may be very beneficial to strengthen the analytical and investigative context.
4.3 Environmental Sources of Contamination
The abundance of elements, present in the GSR, in the open environment, poses some serious concerns while analyzing the GSR and requires utmost care while interpreting the resuts. However, Salles et al. [73] distinguished among the ammunitions on the behalf of the lead concentration in the GSR by means of SWV. It was found that the CleanRange
® ammunition unexpectedly possesses small amounts of Pb.
While dealing with real GSR samples in casework, it may not suffice enough to interpret on basis of the single metal only as there are a number of source in the environment that might lead to the Pb contamination also. Interestingly, nowadays, a variety of air-operated weapons are being used and these propel lead pellets or steel balls (in BB guns) using air pressure rather than including any chemical reaction (i.e burning of ammunition) [85, 86]. In case of shooting at inanimate objects, such as doer, glass pane, fiber sheet, etc.
by air-operated weapons [87–89], GSR might be absent but microtraces of Pb are likely to be present at target and thus the results from the interpretation of single metal detection may be false-positive for the GSR. Existing challenges pre- sent the need for further technological development. The future perspective is presented below.
4.4 Anticipation of Novel Approaches
Development of the method providing quantitative details, simultaneous sensing of other chemical species that are sup- posed to be characteristic OGSR, such as DPA, EC, NG, 2,4- DNT, simultaneous detection of IGSR and OGSR, studies dedicated to the sensing of GSR originated from the NTA and tagged ammunitions is an important future perspective.
Recently, Raman spectroscopy has been reported as a poten- tial, advantageous approach for the analysis of GSR origi- nated from NTA. Raman spectroscopy offers the detection of both IGSR and OGSR. The field-deployable Raman spec- troscope presents on-site screening and detection of GSR and chemometric data treatment eliminates the chances of misinterpretation. Raman spectroscopy seems to be more efficient as compared to electrochemical methodologies [21]. However, some other methodological advancements and anticipation of other approaches with electrochemistry may present a good agreement for GSR analysis. Orthogonal of GSR analysis by electrochemistry and other instruments, such as SEM–EDX, LIBIS, etc., has been discussed in the previous section. In this consonance, Raman spectroscopy
may also be used with electrochemical methods orthogo- nally that may increase the robustness and specificity of the analysis.
In recent years, ring sensors, glove-based sensors have been developed for the detection of drugs, alcohol and explosives [39, 42]. Furthermore, integrated sensors for the GSR detection in the form of a ring, wrist band, etc. should be anticipated. Towards the rapid investigation, wireless communications should be encouraged. Paper-based micro- fluidics (µ-PADs), developed in 2007, has a profound impact on clinical diagnosis and chemical analysis. Distance-based µ-PADs have offered advantages over ladder- and time-based µ-PADs [90, 91]. Distance-based µ-PADs were introduced for the GSR analysis by Buking et al. [92]. The coupling of the µ-PADs with detectors (such as electrochemical, lumi- nescence, colorimetric) offers sensitive, easy, and objec- tive analysis. PADs have offered an efficacious analytical approach for the explosives [93]. In the future, PADs cou- pled with the electrochemical sensor may be anticipated for the GSR analysis.
4.5 Chemometrics
Chemometric treatment of the data yields the extraction of the information. Therefore, along with the development of the sensor for the simultaneous IGSR and OGSR detection, the application of the chemometric tools may reduce the chances of reporting false-positive, false-negative results and misinterpretation of the analytical findings. Quantita- tive analysis of the GSR by electrochemical means may be performed in consonance with the secondary and tertiary transfer of the GSR. Yañez et al. quantified the metals (such as Pb, Ba, Sb, Zn, Ca) by AAS and ICP-OES followed by the chemometric data treatment and, thus, differentiated two ammunitions. It was reported that chemometric tools, such as PCA and Regularized Discriminant Analysis, effi- caciously contributed in analysis with no risk of misclassi- fication [94]. Further, research may be pursued with the aim to standardize the analytical procedure that will ensure the optimum and effective utilization of the electro-analytical tools for the purpose of forensic investigation of GSR.
5 Conclusion
GSR is the evidence of the utmost importance in forensic ballistics investigation. Due to the complex nature of GSR, on the course of analysis, distinguished care is needed. How- ever, timely developments have significantly contributed to enhancing the sensitivity, accuracy, and objectivity of the investigation. Notwithstanding centralized and sophisticated analytical tools, the development of the field-deployable
devices is the need of the hour. In this regard, electrochemi- cal sensors have a good promise with the on-site detection of the GSR. Its sensitive, rapid and non-destructive nature has attracted the researcher across the globe. Over the period, several contributions have been made towards the techno- logical and methodological developments from the conven- tional bulky electrochemical cells to the field-deployable and wearable sensors. The present scenario presents the requirement of a much wider array of possible developments for the electrochemical analysis of the GSR. To enhance the selectivity, specificity, accuracy, and robustness of the analysis, much more developments, such as combining the electrochemical methods with other established tools, e.g.
Raman spectroscopy, development of other novel method- ologies, chemometric models, are needed. As well as, stud- ies on the GSR originated from NTA, simultaneous IGSR and OGSR detection is a relevant future perspective. It may be anticipated that the proper exploration, validation and standardization of the electro-analytical tools may provide robust on-site detection and offer an efficient alternative to present time-consuming methods.
Acknowledgements Not applicable
Author contributions AH, AS and TD have equally contributed to all aspects of this work. All authors read and approved the final manuscript.
Funding The authors are sincerely thankful to the University Grants Commission (UGC), Ministry of Education, Government of India, for providing financial assistance (UGC-JRF vide Letter No.
190521040928) to the first author (AH).
Compliance with ethical standards
Conflict of interests The authors declare that they have no known com- peting financial interests or personal relationships that could have ap- peared to influence the work reported in this paper.
Ethics approval and consent to participate Not applicable.
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