A forensic study: Lead determination in gunshot residues

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A forensic study: Lead determination in gunshot residues

Maiara O. Salles

a

, Juliana Naozuka

b

, Mauro Bertotti

a,

⁎

a_{Instituto de Química, Universidade de São Paulo, São Paulo, SP 05508-900, Brazil}

b_{Departamento de Ciências Exatas e da Terra, Universidade Federal de São Paulo, Diadema, SP 09972-270, Brazil}

a b s t r a c t

a r t i c l e

i n f o

Article history: Received 11 August 2011

Received in revised form 14 October 2011 Accepted 14 October 2011

Available online 20 October 2011

Keywords: Gunshot residue Microelectrode Stripping analysis Lead

Electrochemical lead analyses of gunshot residues (GSRs) were performed using an acidic solution with a bare gold microelectrode in the presence of chloride ions. GSRs from four different guns (0.38 in. revolver, 12 caliber pump-action shotgun, 0.38 repeating rifle, and a 0.22 caliber semi-automatic rifle) and six different types of ammunition (CleanRange®, normal, semi-jacketed, especial 24g®, 3T®, CBC®, and Eley®) were analyzed. Results obtained with the proposed methodology were compared with those from an atomic absorption spectrometry analysis, and a paired Student'st-test indicated that there was no significant differ-ence between them at the 95% confidence level. With this methodology, a detection limit of 1.7 nmol L−1

(3σ/slope), a linear range between 10 and 100 nmol L−1_{, and a relative standard deviation of 2.5% from 10}

measurements were obtained.

1. Introduction

Forensic analysis has improved significantly over the past few years, mainly owing to the continued increase in the requirement for more reliable results in order to diminish erroneous convictions. Lead determination in the forensicfield can be performed on different kinds of samples, such as drugs[1,2], disease cases[3,4], teeth[5,6], and particularly gunshot residues (GSRs)[7–18]. GSRs are residues fromfirearm discharge, and they consist of vapors and particulate materials that are deposited onto the hands (mainly the indexfingers and thumbs), face, and clothes of the shooters. If these particles are collected and reliably analyzed, the suspect can be successfully iden-tified. GSRs have three main inorganic elements: lead, barium, and antimony. The analysis of lead from GSRs can be used as an indicator of the presence of the residues.

Electrochemistry methods, particularly stripping analysis, are often used for the determination of metals such as lead[19–24]. In the past few years, efforts have been made to replace the commonly used mercury electrode[25–27]with less toxic ones. Nowadays, the most common alternative is the bismuthﬁlm electrode[28–32], but carbon[33–35]and gold electrodes[36,37]have also been used.

As for other metallic substrates, the deposition of another metal (e.g., lead) onto gold electrodes occurs through bulk and underpoten-tial deposition (upd)[38]. The upd is a consequence of the stronger bonding force between the metal being deposited and the substrate as compared to the bonding force between similar atoms[38,39].

This difference in bonding energy results in the formation of a mono-layer of the metal, which is followed by the formation of successive layers owing to the bulk deposition[38].

The use of a microelectrode to perform stripping analysis has some advantages, such as the possibility of carrying out the experiment in low-volume samples and the miniaturization of the apparatus[36]. In addition, since one of the dimensions of a microelectrode is smaller than the thickness of the Nernst diffusion layer, an efﬁcient mass transport to the electrode surface is achieved, resulting in a steady-state response in a very short time. The efﬁciency of mass transport eliminates the need to stir the solution in the pre-concentration step (as required when a conventional sized electrode is used in stripping analysis), considerably reducing an important source of errors.

In this study, the amount of lead in GSR samples was quantiﬁed using a gold microelectrode and stripping analysis. All of the parameters involved in the lead determination, including the amount of chloride used, were optimized. The results obtained by the electrochemistry technique were compared with those of an atomic absorption analysis.

2. Experimental section

2.1. Reagents

Solutions of lead nitrate, potassium chloride, potassium ferricya-nide, and EDTA were prepared by dissolving the reagents in deionized water processed through a water puriﬁcation system (18.0 MΩcm−1_,

Nanopure Inﬁnity, Barnstead, Iowa, USA). The nitric acid solution was prepared by diluting the stock solution as necessary. All solid reagents (Merck, Darmstadt, Germany) were of analytical grade, and were used without further puriﬁcation.

–

⁎Corresponding author. Tel.: + 55 11 30912694. E-mail address:[email protected](M. Bertotti).

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2.2. GSR collection and analysis

The GSR was collected during a shooting lesson of the “Dr.

Coriolano Nogueira Cobra” police academy (ACADEPOL) of São

Paulo, and at a shooting range (Centaurus®) in São Paulo, Brazil. Four different guns were used: one handgun (0.38 in. revolver) and three long-barreled guns (12 caliber pump-action shotgun, 0.38 re-peating riﬂe, and a 0.22 caliber semi-automatic riﬂe). The ammuni-tion types used were CleanRange®, normal, semi-jacketed, especial 24g®, 3T®, CBC®, and Eley®.

GSRs were collected from the right hand of the shooters immedi-ately after the shoot, using a cotton swab soaked in a 2% (m/v) EDTA solution, as recommended by Reis et al.[40]. The entire hand was scrubbed in order to collect the gunshot residue. The cotton swabs were then placed in sterile vials, and 10 mL of a 10% (m/v) HNO3 so-lution was added. The vials were subjected to ultrasound for 2 h at 60 °C, before the extracted solutions were analyzed by square wave anodic voltammetry (SWV) and atomic absorption spectrometry (AAS).

2.2.1. Square wave voltammetry

An Autolab PGSTAT30 bipotentiostat (Eco Chemie, Utrecht, Netherlands) connected to a microcomputer was used for the voltam-metric measurements. The gold microelectrode was constructed by connecting a gold microﬁber (diameter=25μm) to a nickel/chromium wire with a silver ink conductive paint (Joint Metal Comércio LTDA, São Paulo, Brazil), which was then inserted in a glass capillary and ﬂame-sealed. The surface of the microelectrode was polished with alumina powder (1μm, Alfa Aesar, Ward Hill, MA, USA) on a microcloth polishing pad. The radius of the microelectrode was determined by measuring the steady-state current in a potassium ferricyanide solution of known concentration.

Prior to each lead analysis, the microelectrode surface was polished with the alumina powder and washed with deionized water. The lead analysis was then conducted on the extraction solution plus a known amount of chloride using SWV. And if necessary, the sample was diluted. The analysis parameters were optimized to the following values: Ecleaning= 0.7 V, tcleaning= 120 s, Edeposition=−0.7 V, Eﬁnal= 0.1 V, tdeposition=480 s, tequilibrium= 15 s, frequency= 500 Hz, Estep= 5 mV, and Eamplitude= 25 mV.

2.2.2. Atomic absorption spectrometry (AAS)

The lead content of the samples was also determined using a Zeenit 600 graphite furnace atomic absorption spectrometer (Analytik Jena AG, Jena, Germany). This apparatus has transverse heated graphite tubes with integrated pyrolytically coated platforms, a Zeeman-effect background correction system, and a hollow lead cathode lamp (Analytik Jena AG, Jena, Germany). The instrument settings for the spectrometer were 4 mA of lamp current, a band-pass of 0.8 nm, and a wavelength of 283.3 nm. The heating program consisted of five steps (given here as temperature (°C), ramp (s), hold (s)): 1: (130, 10, 10); 2: (200, 5, 20); 3: (800, 5, 20); 4: (2100, 0, 5); and 5: (2400, 1, 2). Aliquots of 10μL of sample or analytical solution were introduced into the graphite furnace along with 10μL of chemical modifier (5μg Pd +3μg Mg). The chemical modifier was prepared using Suprapur® solutions of 10 g L−1_{Pd in 15% (v/v) HNO3}_{and 10 g L}−1_{Mg, prepared}

from Pd(NO3)2and Mg(NO3)2 salts, respectively (Merck, Darmstadt, Germany).

The calibration curve (2–40μg L−1_{) was constructed using}

ana-lytical grade Tritisol® solutions of 1000 mg L−1_{of Pb (Pb(NO3)3)}

di-luted in 0.1% v/v HNO3. The samples were analyzed without prior treatment. Samples with high concentrations of lead were diluted in high-purity water, and the analytical signals of each sample were recorded in triplicate.

3. Results and discussion

3.1. Optimization of parameters of SW stripping voltammetry

As described inSection 1, the use of microelectrodes in stripping analysis can be highly advantageous, as the efﬁciency of the mass trans-port eliminates the need to stir the solution in the pre-concentration step, considerably reducing an important source of errors in stripping analysis.Fig. 1 shows ten successive stripping voltammograms, re-corded without stirring the solution using a gold microelectrode in a 0.1% (m/v) HNO3+ 0.01% (m/v) EDTA+ 0.1μmol L−1_{Pb(II) solution.}

In all voltammograms a peak at−0.6 V can be observed, which can be related to the electrochemical oxidation of lead. The peak current varied between 49 and 53 nA with a mean current peak of 51 nA and a relative standard deviation of 2.5%. These results conﬁrm the good repeatability obtained using a microelectrode in stripping analysis.

Experiments to optimize the SWV stripping parameters for the lead

analysis were performed using a 0.05% (m/v) HNO3+0.01% (m/v)

EDTA+0.1μmol L−1 _{Pb(II) solution. All parameters involved in the}

SWV were evaluated and were chosen according to the highest current obtained for each parameter.

Fig. 1.Square wave voltammograms obtained with a gold microelectrode (r = 12.5μm) in a solution containing 0.1% (m/v) HNO3, 0.01% (m/v) EDTA, and 0.1μmol L−1Pb(II). Parameters: Ecleaning: 0.7 V, tcleaning: 120 s, Edep:−0.7 V, tdep: 480 s, Eﬁnal: 0.1 V, fre-quency: 500 Hz, Estep: 5 mV, Eamplitude: 25 mV, tequilibrium: 10 s.

Fig. 2.Inﬂuence of frequency on peak current. Depositing solution: 0.1% (m/v) HNO3, 0.01% (m/v) EDTA, 50 mmol L−1_Cl−_{, and 0.1}_μ_{mol L}−1_{Pb(II). Studied frequencies:} 20, 50, 90, 120, 150, 200, 250, 300, 400, 500, and 1000 Hz. Parameters: Ecleaning: 0.7 V, tcleaning: 120 s, Edep:−0.6 V, tdep: 480 s, Eﬁnal: 0.1 V, frequency: 500 Hz, Estep: 5 mV, Eamplitude: 25 mV, tequilibrium: 10 s.

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The effect of the deposition time on the peak current was evaluated at seven different values (30, 60, 120, 240, 480, 600, and 720 s). A sig-niﬁcant increase in the current value was observed as the deposition time was increased to 480 s; beyond this value, the enhancement was negligible, and so the deposition time was chosen to be 480 s.

The parameter most affected by the current value was the fre-quency. A signiﬁcant increase in the peak current was observed as the frequency changed to higher values (Fig. 2). As reported by Bard and Faulkner[22], frequencies between 1 and 500 Hz are usually used in SWV, and therefore, we chose to work at a frequency of 500 Hz.

Seven different values of the deposition potential were studied (−0.4,−0.5,−0.6,−0.7,−0.8,−0.9, and−1.0 V). An applied po-tential of−0.7 V was chosen, as this presented the highest current value not to be significantly influenced by hydrogen evolution. Fi-nally, the step potential and amplitude potential were optimized, from experimental ranges of 1–15 mV and 5–35 mV, respectively. No significant change in these parameters with respect to the current was observed, and thus, values of 5 mV for the step potential and 25 mV for the amplitude potential were chosen. All optimized param-eters and the chosen values are summarized inTable 1.

As already reported [37], the addition of chloride ions to the depositing solution plays an important role in lead deposition. There-fore, the amount of chloride added to the solution was studied.Fig. 3 shows the effect on the current peak, relative to the electrochemical oxidation of lead, with variation in the chloride concentration. An in-crease in the current of a factor of three, until the chloride concentra-tion reaches 50 mmol L−1_{, can be clearly observed. In an effort to}

understand this effect, experiments with an electrochemical quartz crystal microbalance (EQCM) and a scanning electronic microscopy (SEM) were conducted, and the results will be reported elsewhere. It was found that the presence of chloride ions in the depositing solu-tion caused an increase in theﬁlm roughness and facilitated the lead ﬁlm dissolution, leading to the observed improvement in the lead electrochemical oxidation stripping current.

Subsequently, theﬁgures of merit (listed inTable 2) were deter-mined using the previously optimized parameters and conditions.

3.2. GSR analysis

As detailed in theExperimental section, samples were obtained from a 0.38 in. revolver, 12 caliber pump-action shotgun, 0.38 repeating rifle, and a 0.22 caliber semi-automatic rifle. The ammunition types used were CleanRange®, normal, semi-jacketed, especial 24g®, 3T®, CBC®, and Eley®. With the CleanRange® ammunition, each shooter fired 18 times in a row, while only one shot was discharged with the other types of ammunition.

After the extraction process, the lead content of the GSRs was determined using both the proposed method and a reference method (AAS). It is important to note that the lead determination by SWV stripping required the use of the standard addition method (see Fig. 4).

The Student'st-test was applied to the results obtained by SWV stripping and those found using AAS, and a comparison is shown in Table 3. The paired Student'st-test indicated that there was no signif-icant difference between the results obtained from both methods at the 95% conﬁdence level. Hence, it can be concluded that the pro-posed strategy is reliable and can be successfully applied to lead determination in GSR analysis.

Two interesting aspects can be highlighted from analysis of the results. Firstly, it can be seen that the amount of lead present in the GSR from the revolver is smaller than that from long-barreled guns. Secondly, the CleanRange® ammunition is not supposed to have lead as a constituent[41]. However, as shown here, lead was detected in the residues originating from the discharge of a gun containing this type of ammunition. Sarkis et al. have already reported the presence of lead in this ammunition[42]. Our result also corroborates reports from police ofﬁcers, who declared that they felt a slight sweet taste in their mouth following shooting lessons with the CleanRange® am-munition, indicating the presence of lead in the air.

Table 1

Square wave optimized parameters and the chosen values.

Parameter Optimum value

Deposition time 480 s

Deposition potential −0.7 V Ag/AgCl

Frequency 500 Hz

Step potential 5 mV

Amplitude potential 25 mV

Fig. 3.Effect of Cl−_{concentration on IP}_{of the stripping of lead. Parameters: Ecleaning: 0.7 V,} tcleaning: 120 s, Edep:−0.7 V, tdep: 480 s, Eﬁnal: 0.1 V, frequency: 500 Hz, Estep: 5 mV, Eamplitude: 25 mV, tequilibrium: 10 s. Deposit solution: 0.1% (m/v) HNO3, 0.01% (m/v) EDTA, 50 mmol L−1_Cl−_{, and 0.1}_μ_{mol L}−1_Pb(II).

Table 2

Figures of merit obtained after optimization of the proposed method.

Equation I/nA = 3 · 10−10_{± 1 · 10}−10_{+ 0.28 ±} 2 · 10−3_⁎[Pb2+_{]/nmol L}−1_R2_{= 0.99} Linear range 1 · 10−8_{–1 · 10}−7_{mol L}−1 Detection limit (3σ/slope) 1.8 nmol L−1

Quantiﬁcation limit (10σ/slope) 5.6 nmol L−1

Fig. 4.Square wave voltammograms obtained with a gold microelectrode (r = 12.5μm) in the presence of a solution containing GSR (full line) and successive additions of a Pb(II) solution of known concentration (dashed lines). Parameters: Ecleaning: 0.7 V, tcleaning: 120 s, Edep:−0.7 V, tdep: 480 s, Eﬁnal: 0.1 V, frequency: 500 Hz, Estep: 5 mV, Eamplitude: 25 mV, tequilibrium: 10 s. Inset: calibration curve obtained from the voltammograms.

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4. Conclusions

The possibility of using a gold microelectrode in acidic media in the presence of chloride ions to analyze lead originating from GSRs has been demonstrated. The proposed method was compared with results from AAS, and a paired Student'st-test indicated that there was no signiﬁcant difference between the methods at the 95% conﬁ -dence level.

The results showed that the lead content in GSRs obtained from the same ammunition (semi jacketed, caliber 0.38 in.), but different guns (revolver (0.38 in.)) and long-barreled gun (0.38 repeating riﬂe), was not the same. The amount of lead found in the GSR from the revolver was lower, and this is likely to be due to a more signiﬁcant dispersion of the residues from the revolver in comparison with the long-barreled gun. The major difference between the gunshot residues from two different ammunitions (Especial 24g® and 3T®) was noticed using the 12-caliber pump-action shotgun. Finally, it was found that the CleanRange® ammunition contained a small amount of lead, which is unexpected since this kind of ammunition is not supposed to have this metal as a constituent in order to avoid exposure of policemen.

Acknowledgments

The authors are grateful to FAPESP (Fundação de Amparo à Pesquisa do Estado de São Paulo), grant number: 2006/60078-0, CAPES, and CNPq (Conselho Nacional de Desenvolvimento Cientíﬁco e Tecnológico) forﬁnancial support. The authors also thank the volunteer shooters and the Centaurus® shooting range for allowing us to collect the GSR samples.

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Table 3

Results obtained from two different methods (stripping and AAS) for analysis of GSR samples.

Gun Ammunition Number of shots “Stripping”/mg Pb2+ _{AAS/mg Pb}2+ _tcalculated

0.38 revolver CleanRange® 18 0.014 ± 0.001 0.0147 ± 0.0001 1.00a

0.38 revolver Normal 1 0.019 ± 0.008 0.026 ± 0.001 1.47b

0.38 revolver Semi jacketed 1 0.019 ± 0.008 0.0260 ± 0.0004 1.44b

12-caliber pump-action shotgun 3T® 1 0.021 ± 0.008 0.019 ± 0.002 0.40b

12-caliber pump-action shotgun 3T® 1 0.021 ± 0.008 0.0217 ± 0.0004 0.07c

0.38 repeating riﬂe Semi jacketed 1 0.022 ± 0.008 0.022 ± 0.002 0.20b

Semi automatic 0.22 caliber riﬂe Eley® 1 0.039 ± 0.006 0.046 ± 0.001 1.9b

Semi automatic 0.22 caliber riﬂe CBC® 1 0.06 ± 0.01 0.0670 ± 0.0008 1.03b

Semi automatic 0.22 caliber riﬂe Eley® 1 0.07 ± 0.02 0.078 ± 0.002 1.28b

12-caliber pump-action shotgun Especial 24g® 1 0.12 ± 0.02 0.1441 ± 0.0004 2.22b

a_ttheoretical_{95% = 2.132 (n = 4; n = degrees of freedom).} b _ttheoretical_{95% = 2.353 (n = 3; n = degrees of freedom).} c _ttheoretical_{95% = 6.314 (n = 1; n = degrees of freedom).}

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