Orthogonal Identification of Gunshot Residue with Complementary

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Orthogonal Identification of Gunshot Residue with Complementary Detection Principles of Voltammetry, Scanning Electron Microscopy, and Energy-Dispersive X‑ray Spectroscopy: Sample, Screen, and Confirm Aoife M. O’Mahony, Izabela A. Samek, Sirilak Sattayasamitsathit, and Joseph Wang* Department of Nanoengineering, University of California, San Diego, La Jolla, California 92093, United States S Supporting Information *

ABSTRACT: Field-deployable voltammetric screening coupled with complementary laboratory-based analysis to confirm the presence of gunshot residue (GSR) from the hands of a subject who has handled, loaded, or discharged a firearm is described. This protocol implements the orthogonal identification of the presence of GSR utilizing square-wave stripping voltammetry (SWSV) as a rapid screening tool along with scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) to confirm the presence of the characteristic morphology and metal composition of GSR particles. This is achieved through the judicious modification of the working electrode of a carbon screen-printed electrode (CSPE) with carbon tape (used in SEM analysis) to fix and retain a sample. A comparison between a subject who has handled and loaded a firearm and a subject who has had no contact with GSR shows the significant variations in voltammetric signals and the presence or absence of GSRconsistent particles and constituent metals. This initial electrochemical screening has no effect on the integrity of the metallic particles, and SEM/EDX analysis conducted prior to and postvoltammetry show no differences in analytical output. The carbon tape is instrumental in retaining the GSR sample after electrochemical analysis, supported by comparison with orthogonal detection at a bare CSPE. This protocol shows great promise as a two-tier detection system for the presence of GSR from the hands of a subject, whereby initial screening can be conducted rapidly onsite by minimally trained operators; confirmation can follow at the same substrate to substantiate the voltammetric results.

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Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) allow the identification of a single GSR particle, affording high selectivity currently unparalleled by any other bulk analysis method.2 In forensic casework, SEM/ EDX continues to be the method of choice for GSR identification.4 In a landmark paper by Wolton et al., the uniqueness of the elemental composition and morphology of GSR was studied in depth.6 GSR particles originating from the discharged firearm have been studied using SEM and are mainly spherical in shape with a cracked shell morphology. Analysis of these particles with EDX can then show the presence of Ba, Sb, and Pb, which is characteristic of GSR.3 The combination of this spherical morphology and elemental composition is unique

unshot residue (GSR) detection methods are based on the analysis of chemical residues produced by discharge of a firearm whereby trace amounts of metallic and organic species can be deposited on the hands, face, hair, and clothing of the shooter.1−5 Inorganic GSR is generally analyzed using neutron activation analysis (NAA), atomic absorption spectroscopy (AAS), inductively coupled plasma (ICP), scanning electron microscopy in conjunction with energy dispersion Xray (SEM/EDX), and organic GSR is regularly separated and identified using gas chromatography (GC) and high-performance liquid chromatography (HPLC) with mass spectrometry and capillary electrophoresis (CE).1 GSR particles originating from the primer are unique to the discharge of firearms due to the characteristic composition of lead, antimony, and barium.2 Particle analysis identifies individual GSR particles consisting particularly of these metals, whereby both morphological and elemental characteristics are examined.3,4 © 2014 American Chemical Society

Received: May 2, 2014 Accepted: August 1, 2014 Published: August 1, 2014 8031

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to GSR when compared to a variety of other sources.7 SEM/ EDX analyses have a number of disadvantages, however: analysis is time-consuming, restricted to centralized laboratories, and requires highly trained personnel. The implementation of voltammetric methods allows for rapid detection of metal constituents of GSR and has recently been reviewed.1 Several authors have described the simultaneous detection of two or more GSR-consistent metals utilizing a variety of voltammetric methods.8−12 Advances in such analyses have been reported through the use of chemometric treatment of data obtained from voltammetric detection of GSR samples.13,14 Some drawbacks to the voltammetric detection of GSR have been identified. The detection of antimony in GSR is complicated by the fact that the Sb and Cu signals can overlap.8,15,16 Barium has previously been detected electrochemically in GSR9,17 but presents the most difficulties of any of the GSR constituents due to the fact that it strips at a very negative potential and causes hydrolysis of an aqueous solvent.18,19 The combination of different powerful and independent detection principles for identification of a broad range of trace GSR particles has been reported. In 1987, Dahl et al. were also utilizing orthogonal detection techniques for the discrimination of organic and inorganic components in GSR with the use of high-performance liquid chromatography coupled with oxidative electrochemical detection and graphite furnace atomic absorption spectrophotometry, respectively.20 More recently, Bueno et al. reported spectroscopic data from two complementary techniques, Raman and FT-IR, which are combined into a single data set to improve discrimination of nonequivalent GSR particles.21 The integration of these techniques is, however, confined to a centralized laboratory. In this work, we implement the orthogonal detection of GSR using voltammetry, microscopy, and spectroscopy. We have developed a disposable screen-printed carbon electrode which utilizes carbon tape as both a working electrode for voltammetric detection and a platform for SEM and EDX. This permits a two-tier system of detection, whereby a fielddeployable voltammetric screening step can be conducted onsite by minimally trained operators, and the follow-up analysis acts to confirm the voltammetric results. Such complementary use of voltammetry and microscopy has not been previously reported. The integration of the three orthogonal detection modes of SWV, SEM, and EDX offers substantial information on a variety of properties of GSR, as well as a built-in redundancy on a single hand-held SWV device, and is unique among existing GSR detection systems. A “swipe-and-scan” technique involving mechanical transfer of GSR directly onto the surface of screenprinted sensor strips followed by electrochemical analysis renders sampling to be field deployable with minimal intermediate treatment.15 The simplicity of the sampling method coupled with the speed of the voltammetric sensing allows for a field-deployable screening step utilizing equipment that any law enforcement office or forensic technician can carry and operate. With the sample fully intact, the same substrate is then ready to be used for SEM/EDX analysis on return to a centralized forensic laboratory, for confirmation utilizing preferred existing identification methods. The goal is to combine triple independent analytical platforms based on complementary detection principles, including a recently developed highly sensitive voltammetric device, and trusted, existing techniques that offer maximum specificity. The three

sensing platforms have strengths that are complementary to each other and are extremely compatible with automation.

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EXPERIMENTAL SECTION Chemicals and Materials. Acetate buffer (pH 4.6) was purchased from Fluka (St. Louis, MO). Conductive carbon tape was obtained from Ted Pella (Redding, CA). Remington UMC Target 40 S&W ammunition and Federal Range 9 mm automatic ammunition were acquired from P2K shooting range (P2K, El Cajon, CA). Instruments and Procedure. Electrochemical measurements were performed with the use of an Autolab PGSTAT 12 (Eco Chemie, The Netherlands). Carbon screen-printed electrodes (CSPEs) were utilized for all measurements. The carbon ink used for the working and counter electrodes was Acheson Electrodag 440B (Henkel Electronic Materials LLC). The fabrication of these SPCEs has been detailed in the literature.22 In short, a semiautomatic screen-printer (model TF100, MPM, Franklin, MA) was employed to print the electrodes through a patterned stencil on 10 × 10 cm ceramic plates containing 30 strips (3.3 cm × 1.0 cm each). A silver/ silver chloride ink (E2414, Ercon, Wareham, MA) was used to define the conductive under layer as well as the reference electrode. A carbon ink (Acheson, Henkel Corp., Madison Heights, MI) was then overlaid on the conductor to define the working and counter electrodes. Insulating ink (E6165, Ercon, Wareham, MA) was subsequently printed to define the electroactive area of the electrodes. All printed layers (Ag/ AgCl, carbon, and insulating ink) were cured at 125 °C for 20 min. The CSPE was modified using a conductive carbon double-sided tape (Ted Pella, Catalogue number 16073), cut to fit exactly over the working electrode and acted as thus. Squarewave voltammetry (SWV) was employed to characterize GSR electrochemical signatures. A potential of −1.3 V was applied for 120 s, and a scan to a final potential of 0.1 V versus Ag/ AgCl was completed. An accumulation time was implemented for deposition of metals in GSR. All scans were performed with the use of an acetate buffer (pH 4.6) at a frequency of 25 Hz, an amplitude of 25 mV, and a potential step of 4 mV. Scanning electron microscopy (SEM) images were obtained with an XL30 SEM instrument (FEI Co., Hillsboro, OR) using an acceleration potential of 30 kV. Energy dispersive X-ray analysis (EDX) was performed with an EDAX acquisition system (Ametek Inc., Mahwah, NJ) installed on the XL30 SEM instrument. Abrasive Sampling from Various Surfaces. Integrated sampling was performed at a local shooting range (P2K, El Cajon, CA). Samples were collected from the skin of two different subjects under different control scenarios: at the laboratory, prior to any contact with GSR (N = no contact); having handled ammunition and loaded the firearm (L = loading); and after discharging several rounds from the weapon (F = firing).

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RESULTS AND DISCUSSION Orthogonal Detection and Analysis of GSR Using Voltammetry, SEM, and EDX. The protocol described herein allows for rapid, field-deployable sampling and screening of a subject suspected of involvement in a gun crime, followed by confirmation analysis from the sample substrate utilizing the present benchmark analytical system for identification of GSR particles. The sequence of integrated sampling and analysis for 8032

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Figure 1. Gunshot residue detection sequence involving the stubbing of a subject’s hand, screening with voltammetric analysis, and confirmation with SEM/EDX analysis of the working electrode’s surface.

Figure 2. Comparison of (i) voltammetric, (ii) SEM, and (iii) EDX responses for samples from (A) a subject who has loaded a firearm and (B) a subject who has had no contact with a GSR at a SEM tape-modified electrode. Voltammetric currents are in the same range.

detection. The sample can then be loaded into the SEM, without any further modifications, and the GSR sample remains intact on the working electrode surface. SEM equipped with EDX provides close examination of particle morphology and high sensitivity analytical information on individual particles. The GSR particles display a distinctive morphology of a spherical, “cracked shell” appearance. Subsequent EDX analysis can identify the presence of Ba, Sb, and Pb, all three together being unique to GSR. The minimum emission voltage values of each metal are 4.465 (Lα), 3.604(Lα), and 2.342 (M) keV for Ba, Sb, and Pb, respectively, resulting in no overlap peaks or error of EDX analysis. Different ammunitions were tested herein, but a distinction could not be made among them as varying amounts of Pb, Sb, and Ba were present in each particle.

screening and orthogonal detection methods is outlined in Figure 1. Sampling is carried out using a variation of the “swipe and scan” method outlined in previous literature.15 The working electrode surface of the modified screen-printed carbon electrode was deployed over the hands of a suspect using a stubbing mode of sample collection, allowing the SEM tape electrode contingent to stick to the thumb and back of the subjects’ hand. The sample is then immediately ready for electrochemical analysis. SWSV is employed to identify the electrochemical targets of Pb, Sb, and Cu. A distinct increase in signals for the presence (red) of GSR compared to the absence (green) is observed in the voltammetry of Figure 1. Due to the presence of the carbon tape, the sample remains fixed to the working electrode surface subsequent to electrochemical 8033

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Figure 3. Effect of voltammetric scanning on EDX analysis for samples from a subject who has discharged a firearm. EDX analysis for GSR sample prior to A(i) voltammetry and B(ii) postvoltammetry. Voltammetric analysis for samples from a subject with no contact with GSR (green-dashed) and a subject who has discharged a firearm (red) A(ii) subsequent and B(i) prior to EDX analysis.

the voltammetry resulting from the scan of the “no contact” sample in the same potential window and the same current range as the scan taken in Figure 2A (i). This scan displays a featureless baseline for the voltammetric screening, and thus the subject would be labeled as not involved. No particles fitting the description of the cracked-shell spherical morphology were observed during SEM analysis. Some spherical particles were observed, although these were smaller than the particles identified in Figure 2A (ii). Upon inspection with EDX analysis, confirmation of the absence of GSR particles was made, as outlined in Figure 2B (iii). It is clear, when compared with the EDX scan from the subject in Figure 2A (iii), a featureless scan to corroborate the SEM analysis and GSR screening is acquired. Thus, no false positive identifications of a shooter were encountered. Examination of the Effect of Voltammetry upon EDX Analysis of GSR. The effect of performing voltammetry on the GSR sample immobilized on the carbon tape prior to SEM analysis was examined. Two different samples taken from the hands of a subject who has discharged a firearm were utilized for this study, first performing voltammetry and subsequent EDX analysis and second performing EDX analysis, with subsequent voltammetric screening analysis. Figure 3A displays the (i) initial EDX and (ii) subsequent voltammetric analysis from a sample immobilized from the hand of a subject who has discharged a firearm. The EDX scan in Figure 3A (i) clearly displays the Pb, Sb, and Ba signals at values 2.342, 3.604, and 4.465 keV, from a particle which is unique to GSR. This is confirmation of the presence of GSR particles. Subsequently, a voltammetric scan was taken of the sample and the results are shown in Figure 3A (ii). The signal shown in green is that taken from the hand of the subject prior to exposure to GSR, and the signal in red is that taken from a subject post discharge of the firearm. A clear and characteristic electrochemical fingerprint is observed for the GSR sample from the shooter. The signals for Pb and Sb/Cu are observed at potentials −0.7 V and −0.15 V, respectively. There is a significant and quantifiable difference between the scan for the subject prior to contact to GSR,

This has previously been observed,3 reflecting the nature of the formation of the particles. Examination of GSR from a Subjects Hand before and After Discharging a Firearm. We demonstrate the orthogonal detection of GSR using the screening tool of SWSV and the confirmation tools of SEM and EDX with samples taken from subjects in two different control scenarios. Figure 2 outlines the analysis of a sample taken at a carbon tape-modified electrode from the hand of a subject who has (A) loaded and handled a firearm and (B) had no contact with GSR. For each of these control scenarios, (i) SWSV was first performed as the screening step, followed by analysis utilizing (ii) SEM and (iii) EDX as confirmation steps. Figure 2A shows the analysis for a subject who has loaded and handled a firearm at the carbon tape modified CSPE. The voltammetry in (i) is similar to previous reports and characteristic of the GSR voltammetric fingerprint, encompassing signals for Pb and Sb (with potential contributions from Cu) at potentials −0.7 and −0.15 V, respectively. This GSR voltammetric fingerprint was confirmed by EDX analysis; the elemental analysis showed that Pb and Sb were indeed present in this sample. EDX analysis also confirmed the presence of Ba. Orthogonal analysis validated the simultaneous presence of three elements, Ba, Pb, and Sb, which is indicative of GSR. Upon completion of voltammetric screening, the electrode was placed into the SEM and images were taken of GSR particles. Figure 2A (ii) shows a cracked-shell spherical morphology indicative of GSR, confirming the presence of GSR on the hand of the subject. Finally, EDX analysis is implemented on the working electrode surface. Figure 2A (iii) shows the results of the dispersive analysis on the area over the particle shown in (ii). Clear signals are observed at values 2.342, 3.604, and 4.465 keV, corresponding to Pb, Sb, and Ba, respectively, which are noted to be unique to GSR, thus confirming the presence of GSR particles on the hands of a subject. Similar analysis was conducted on a sample taken from the hands of a subject who has had no prior contact with GSR. The results are outlined in Figure 2B (i−iii). Figure 2B (i) shows 8034

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Figure 4. Comparison of (i) voltammetric, (ii) SEM, and (iii) EDX responses for samples taken from a subject who has discharged a firearm at a (A) SEM tape-modified electrode and (B) bare carbon screen printed electrode. Voltammetric currents are in the same range.

compared with the subject having fired a weapon. To confirm that this voltammetric screening step has no effect on the subsequent EDX analysis, a second GSR sample was taken from the hands of a subject upon discharge of a firearm, and a voltammetric scan was taken, outlined in Figure 3B (i). As before, the scan in green is that from a subject who has had no prior contact with GSR and the scan in red is taken upon discharge. The characteristic signals for Pb and Sb/Cu are again observed at −0.7 and −0.15 V, respectively, for the sample from the subject upon firing, while the sample prior to firing displays a featureless baseline. Subsequently, the electrode underwent EDX analysis, and the results are shown in Figure 3B (ii). Clear and distinct signals for Pb, Sb, and Ba are observed at values 2.342, 3.604, and 4.465 keV, denoting not only the validity of the voltammetric screening step but also the nondestructive nature of this electrochemical technique. This study illustrates the complementary nature of voltammetric screening in conjunction with EDX analysis confirmation. Examination of Use of Carbon Tape-Modified CSPE Compared with Bare CSPE. Additionally, we examined the potency of the carbon tape-modified electrode compared with a bare carbon electrode, utilizing this analytical collaboration of voltammetry, microscopy, and spectroscopy. Figure 4 outlines (i) voltammetry, (ii) SEM analysis, and (iii) EDX analysis at (A) a carbon tape modified CSPE and (B) a bare CSPE. Both samples were taken from the hand of a subject who had discharged a firearm. Figure 4A (i) displays the characteristic voltammetry for such a subject, as previously observed in Figure 3. Signals for Pb and Sb/Cu are observed at −0.7 and −0.15 V, indicating a significant presence of GSR-related species. Figure

4A (ii) displays a prime example of the “cracked-shell” spherical morphology of the GSR particle, which substantiates the screening voltammetry. Finally, EDX analysis was implemented, and the resulting scan is displayed in Figure 4A (iii) showing distinct signals for Ba, Sb, and Pb at values previously observed in the first three sections of the Results and Discussion, confirming finally the presence of these GSR-unique metals. We compared the analysis of the sample taken from the shooter in Figure 4A at the carbon tape-modified CSPE with that of a sample taken from a bare CSPE, outlined in Figure 4B. Initial examination of the voltammetry in Figure 4B (i) shows significantly diminished signals for Pb and Sb/Cu in the experimental window. While signals are observed for these species at the bare CSPE, they are significantly lower than those observed for the modified electrode in Figure 4A (i). Figure 4B (ii) shows a SEM image of the electrode surface. No particles matching the GSR characteristic description were observed. In general, no spherical particles were observed. Any GSR that was detected during the screening step was removed along with the electrolyte. EDX analysis was finally applied and similar to the SEM analysis, no metals consistent with GSR particles were observed. Therefore, we can conclude that the use of carbon tape to modify the working electrode of a CSPE is instrumental in not only gathering increased amounts of GSR from a subject’s hand but also retention of the sample, which is crucial for further analysis. Further work has been conducted, outlining the selectivity of this system toward GSR. In short, different surfaces, including aluminum foil, paper, plastic, and glass, were examined voltammetrically as well as using SEM and EDX and compared 8035

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to a sample taken from the surface of a used bullet casing. The results are outlined in Figure S1 of the Supporting Information. The results indicate that no voltammetric signals, indicative of the presence of Pb, Sb and Cu, are obtained from surfaces other than the bullet casing (Figure S1, panels A, B, D, and E vs C). The peak potential of Cu/Sb (in C(i)) is shifted to −0.25 V as compared to the peak potential characteristic of Sb in Figure 2A (i). EDX and SEM analyses confirm the absence of target metals on these surfaces. These studies validate the reliable method for screening and confirm gunshot samples. The peak near −0.8 V is split, possibly due to other coexisting electroactive components (e.g., nitro species) along with the Pb signal22 or due to stripping from the carbon and its antimony “coating”.

Notes

CONCLUSIONS We have demonstrated, for the first time, the orthogonal detection of GSR, coupling complementary voltammetric techniques for screening and SEM/EDX analysis for confirmation. This was achieved through modification of the working electrode on a CSPE with standard carbon SEM tape, which acts to immobilize and retain a GSR sample to the working electrode surface, as well as providing a conductive background for SEM/EDX analysis. Effective screening of different control scenarios from subjects exposed to different levels of GSR have been observed voltammetrically, and the resulting observations have been substantiated with SEM and EDX analysis. Subjects who have loaded and discharged a firearm delivered characteristic voltammetric fingerprints showing the presence of Pb and Sb/Cu, as observed in previous literature. Subsequent examination with SEM displayed typical cracked-shell spherical GSR particles, which contained Ba, Sb, and Pb according to EDX analysis. Examination of similar circumstances at a bare CSPE for a subject who had discharged a firearm displayed characteristic voltammetry of Pb and Sb/Cu signals, although these signals were approximately five times lower in current output than the equivalent test at the carbon tape-modified CSPE. The carbon tape is thus responsible for increased sampling of GSR from the hands of a shooter due to its “sticky” nature. Finally, SEM/EDX analysis of the bare CSPE showed no GSR-consistent particles in SEM analysis and no GSR-consistent metals in EDX analysis. This further endorses the use of the carbon tape-modified working electrode, which is responsible for retention of particles even after voltammetry was conducted. Tests also confirmed that voltammetry prior to SEM/EDX analysis has no detrimental effects of the output of the microscopy or spectroscopy. The rapidity and portability of the initial voltammetric screening approach coupled with the specificity of the subsequent SEM/EDX analysis indicates promise for use of a field-deployable, hand-held device for investigating firearmrelated crimes, the substrate of which can later be further analyzed in a centralized laboratory.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the Washington Headquarters Services-Acquisition Directorate [Assistant Secretary of Defense for Research and Engineering (Defense Biometrics and Forensics Office)]. This material is based upon work supported under Contract HQ0034-11-C-0034. We would like to thank the staff at the P2K gun range (El Cajon, CA) for their cooperation.

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REFERENCES

ASSOCIATED CONTENT

S Supporting Information *

Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +1 (858) 246 0128. Fax: +1 (858) 534 9553. 8036

dx.doi.org/10.1021/ac5016112 | Anal. Chem. 2014, 86, 8031−8036