Laser-Induced Breakdown Spectroscopy for the Rapid Characterization of Lead-Free Gunshot Residues

Abstract

This study investigated the use of laser-induced breakdown spectroscopy (LIBS) and scanning electron microscopy energy dispersive X-ray spectroscopy (SEM-EDX) as means of characterizing gunshot residue (GSR) originating from commercially available lead-free rounds. Data from two experiments are presented in this work. One experiment focused on identifying prominent analytical markers present in lead-free GSR by LIBS while the other applied SEM-EDX to determine the degree of evidence preservation after LIBS analysis. Samples of GSR were collected via tape-lift method from the hands of volunteer shooters and instrumental analyses were conducted in triplicate. As a result, the lead-free ammunition analyzed in this work generated GSRs comprising primarily Ba, Al, Si, and/or K. Trace amounts of Ti, Fe, and S were also apparent in some compositions. Through SEM-EDX analysis, a spheroidal geometry consistent with traditional lead-containing GSR was observed. Additionally, it was determined that evidence is preserved after LIBS analysis which supports the implementation of LIBS as a rapid preliminary screening method followed by confirmatory testing via SEM-EDX on the preserved evidence.

Keywords

Gunshot residue laser-induced breakdown spectroscopy LIBS evidence preservation scanning electron microscopy energy dispersive X-ray spectroscopy SEM-EDX

Introduction

Modern inorganic gunshot residue (GSR) particle analysis primarily focuses on identifying Ba, Sb, and Pb present in a spheroidal metal alloy.^1–3 Scanning electron microscopy coupled with an energy dispersive X-ray spectrometer (SEM-EDX) is the most widely accepted technique in these studies.⁴ However, the manufacture and distribution of lead-free alternatives to small caliber firearms ammunition has resulted in a reformulation of the priming mixture, which varies widely between manufacturers. With this emerging market, the forensic community aims to rapidly detect and classify the composition of these residues as evidence collected from the hands of suspected shooters.

When a firearm is discharged, the weapon cycles through several processes in a relatively short time period. Pulling the trigger is merely the start of this chain of events. Once triggered, the firing pin strikes the primer and detonates the primary shock explosive (e.g., lead styphnate). A chemical oxidant (e.g., barium nitrate) provides oxygen to the flame creating a rise in temperature until the fuel (e.g., antimony sulfide) is ignited. Finally, the energy produced from burning the fuel in the presence of the oxidant ignites the propellant (gun powder or black powder) and provides sufficient kinetic energy to force the bullet out the barrel of the gun.⁵ The GSRs resulting from this process are significant to forensic scientists investigating criminal cases involving a firearm.^6–9

Gunshot residue scatters in all directions and can be found on the clothing and skin⁵ of both the shooter and victim. The heterogeneous mixture of burned and partially burned chemical residues can be classified as either organic gunshot residue (o-GSR) or inorganic gunshot residue (i-GSR).⁴ Organic gunshot residue particles largely originate from the ammunition’s propellant, but additional sources include the primer mixture, lubricants, and any organic debris already present within the firearm.¹⁰ This is because propellants and lubricants are mainly comprised of hydrocarbons and nitrocarbons.⁵ The chemical structure of the propellant allows for it to be easily combusted which produces chemical energy that is converted to kinetic energy necessary to propagate a mechanical shockwave. This forces the projectile out of the barrel of the gun to complete the firearm discharge. Recent works have investigated methods that apply statistical thresholds for presumptive testing for o-GSR.¹¹

To the contrary, i-GSR originates mainly from the priming mixture, bullet, or cartridge case which typically contain halides, nitrates, nitrites, and metallic particles.¹² Many modern analytical techniques aimed at analyzing i-GSR concentrate on these metallic elements: barium, antimony, and lead.⁵ However, environmentally friendly ammunition, often advertised as lead-free, is gaining popularity. This poses potential problems for the analysis of i-GSR because as the name would suggest, lead-free primers do not incorporate lead-containing compounds into the mixture. Furthermore, recent studies indicate laser-induced breakdown spectroscopy (LIBS) has proven successful in characterizing particulate matter, pyrophoric materials, and analogs of GSR.^13–16 Thus, the objective of this study is to conduct forensic analysis on lead-free GSRs produced from commercially available ammunition by LIBS and SEM-EDX.

Materials and Methods

Materials

Materials used were 9-mm Blank Training Rounds (Force on Force, Speer Le, Lewiston, ID, USA), Zero Pollution Primers (Fiocchi Munizioni, Ozark, MO, USA), Train and Defend (Winchester, East Alton, IL, USA), Unprimed Brass (Remington Arms Company, Madison, NC, USA), Georgia Bureau of Investigation Gunshot Primer Residue Collection Kit Stubs (TriTech Forensics, Southport, NC, USA), Walther CCP 9-mm Luger (Walther Arms Inc., Germany), and Rock Chuck Bullet Swage Reloading Kit (RCBS, Orville, CA, USA).

Formation and Collection of Lead-Free Gunshot Residue

Nine-millimeter Blank Training Rounds (Speer Le, Lewiston, ID, USA), Zero Pollution Primers (Fiocchi Munizioni, Ozark, MO, USA), and Winchester Train and Defend (Winchester, East Alton, IL, USA) ammunition were selected for use in this study due to the absence of lead in the primer composition.^17–19 These rounds were fired under ambient conditions and all guidelines regarding the safe discharge of a firearm were adhered to during sample formation and collection. Samples were collected using Georgia Bureau of Investigation (GBI) primer residue collection kits (TriTech Forensics, Southport, NC, USA) covered with a double-sided adhesive carbon tape. The collection stubs were applied to the area from the first knuckle of the index finger, through the webbing between the index finger and thumb, and around to the first knuckle of the thumb.^20,21

With approval from the Institutional Review Board (Study 15-431), volunteer shooters discharged one or five shots of blank ammunition from a Walther CCP 9-mm Luger handgun (Walther Arms Inc., Germany) in triplicate. Samples were collected immediately after the completion of firing each set of shots and analyzed by LIBS. Samples were collected from the area of interest on the volunteers’ hands after the completion of firing either one or five shots. To prevent cross-contamination, the samples were collected intermittently at one-week intervals between trials.

Lastly, an experiment was conducted to determine the degree of evidence preservation relative to this method for GSR analysis. In this study, GSR collected immediately from the hands of a volunteer shooter after firing five shots were initially analyzed via SEM-EDX to determine chemical morphology and composition. Following the preliminary characterization, the same sample was analyzed via LIBS and subsequently re-analyzed by SEM-EDX as proof of concept for a rapid screening followed by confirmatory testing on preserved evidence.

Preparing the Cartridge Casings

Upon the initial LIBS analysis of the samples collected on the stubs from the GBI primer residue collection kit, the distribution of GSR across the tape-lift failed to produce a quantifiable emission signal for the test firing experiment with the 9 mm blank training rounds. One apparent reason for this could be that the blank training rounds examined in this study possess a unique design. The aluminum casing is rounded as if it were to contain a projectile. The inside of the casing, though hollow, is mostly enclosed and there is a pinhole partway down the enclosure. This distinct design suppresses the noise associated with firing the round, helps generate enough pressure within the barrel for the spent cartridge to properly eject, and is suitable for close combat training with a one-foot safety radius. Consequently, most of the primer residue remains trapped behind the enclosure.

For this reason, a drill press was used to bore out the dividing wall inside of the casing. After firing the drill press modified version of the training rounds, the spent cartridges had to be manually ejected. However, there was a substantial increase in both noise at the time of firing and visible formation of GSR. In preparing the Fiocchi ammunition, the Zero Pollution primers were loaded by hand into unprimed brass casings (Remington Arms Company, Madison, NC, USA) using Rock Chuck Bullet Swage reloading equipment (RCBS, Orville, CA, USA). The Winchester Train and Defend ammunition was purchased as a complete round containing bullet, propellant, and primer. To prepare these rounds for firing, the bullet was removed and the propellant was emptied leaving only the priming cup and mixture in the cartridge case.

Detection of Gunshot Residue

Samples were loaded directly into an OOI LIBS 2000+ Spectrometer (Ocean Optics, Inc. Dunedin, FL, USA) coupled to a Big Sky Ultra 50 mJ neodymium-doped yttrium aluminum garnet (Nd:YAG) laser (Quantel USA, formerly Big Sky Laser Technologies, Bozeman, MT, USA). This LIBS system uses a Q-switch to pulse the laser which operated at a wavelength of 1064 nm. A time delay of 2 microseconds was applied to discriminate against the early occurring continuum emission resulting from the generated microplasma. Shorter time delays were shown to have increased background noise from the continuum emission and decreased the signal-to-noise (S/N) ratio.^20,22–25 Using a 3 × 3 raster pattern across the tape surface, a total of nine spectra were collected per sample. Due to the heterogeneous nature of GSR, it is beneficial to collect an emission spectrum at multiple points across the sample surface to increase the probability of striking analyte rather than the tape matrix.^21,22,24,25 Accordingly, only the maximum emission signals per sample were considered for analysis, rather than the average signal over all spectra collected. Spectra were displayed by OOILIBS Operating Software (Ocean Optics, Inc., Dunedin, FL, USA).

For analysis by SEM-EDX, samples were loaded into the chamber of a JSM-5800LV (Jeol USA, Peabody, MA, USA) scanning electron microscope and subsequently subjected to X-ray analysis (Bruker AXS Microanalysis, Germany). A heated tungsten filament served as the electron source and the SEM operated under high vacuum conditions at an accelerating voltage of 20 kilovolts (kV) and a working distance of 10 mm. Only images resulting from the detection of secondary electrons are included in this work, but backscattered electrons can also be detected. Additionally, an X-ray detector was used to determine the elemental profile of each sample. The combination of scanning electron microscope with X-ray detector produced an elemental map that provided a spatial representation for elements comprising each analyzed GSR particle.

Statistical Analysis

A blank library of 20 volunteers who self-indicated not having recently discharged a firearm was used to establish threshold values for a positive GSR. The threshold is referred to as the signal detection limit (y_dl). This value is calculated using Eq. 1, where ${\bar{x}}_{bl}$ and s_bl represent the average and standard deviation of the emission signal for a particular analytical marker, respectively.

y_{dl} = {\bar{x}}_{bl} + {3 s}_{bl}

(1)

Ten laser pulses were taken from the area of carbon adhesive of each GSR collection stub to give a total of 200 spectra in the blank population of non-shooters. Leave-one-out cross-validation was performed over the entire blank library to assess how the results of the statistical analysis generalized to the independent data set.²⁶ Although the signal detection limit is independent of the ammunition type, this value must be calculated for any potential analytical markers to determine associated rates of error. Table 1 presents the calculated signal detection limit for select analytical markers relevant to this discussion. The resulting rates of error (false positive and true negative) were defined as when the emission signal was greater or less than the signal detection limit for the corresponding wavelengths, respectively.

Table 1.

Statistical analysis on selected analytical markers present in the Force on Force Blank Training Rounds. Displays the selected analytical markers, calculated signal detection limit (y_dl), and rates of error from the leave-one-out cross-validation (false positive/true negative; where n = 200) given in percentage for the Force on Force blank training rounds.

Element/Ion	Wavelength (nm)	Mean	Standard deviation	Y_dl	False positive (%)	True negative (%)
Ba⁺	455.403	13.890	12.036	49.998	1.000	99.000
Ba⁺	493.408	9.926	8.271	34.739	1.000	99.000
Ba⁰	553.548	11.550	8.514	37.092	1.000	99.000

Results and Discussion

Laser-Induced Breakdown Spectroscopy Analysis

A representative emission spectrum for Force on Force (shown in Figure 1) shows that the GSR samples from blank training rounds emit at various barium wavelengths. Three prominent barium emission lines, shown in Table 1, were selected as analytical markers for statistical analysis. The leave-one-out cross-validation indicated that the blank library served as an adequate representation of non-shooters. Approximately 1% of the blank population produced false positives when compared to the calculated threshold value for each marker. This finding indicates that the majority of the samples which comprise the blank population of non-shooters fall below the calculated threshold value, representing a true negative result.

Additionally, Calcium: Ca(I) 422.673 nm, Sodium: Na(I) 588.995 nm, and Na(I) 589.592 nm emission lines are readily apparent in the blank spectrum and analyte spectrum (Figure 1). Alkali metal (sodium) and alkaline earth metal (calcium) possess an ubiquitous nature which results in elevated signal detection limits for these and other prevalent elements.²¹ This would suggest that some emission lines are more suitable as representative analytical markers than others. Previous research, implementing LIBS for the detection of GSR, used a polytetrafluoroethylene (PTFE) extruded film tape which was specifically selected for its low emission background.^21,22,24,25 These experiments revealed that the presence of calcium in the blank spectra comes from the volunteers and not the adhesive.

The results from the test firing experiments suggest that the likelihood for distinguishing a shooter from a non-shooter increases as consecutive shots are fired (Table 2). Each test firing experiment of the Force on Force blank training rounds was conducted in triplicate and there are three analytical markers of interest, this yields nine wavelengths where it is possible for the emission signal of the sample to exceed the detection limit. When one shot was fired, 3/9 wavelengths exceeded the detection limit. However, this value increased to 100% where 9/9 wavelengths exceeded the detection limit when five shots were fired. With the current data, a correlation between the number of emission lines detected and the time elapsed since firing the shot has not been established. However, for a single marker it has been determined that the intensity of the emission signal decreases with time from the initial discharge of the round.²²

Table 2.

Results of the test firing experiment. Displays the number of analytical markers that exceed the signal detection limit per replicate sample.

Experiment	Replicate	Analytical markers exceeding the detection limit (n)
1 shot fired	1	1
	2	1
	3	1
5 shots fired	1	3
	2	3
	3	3

Representative emission spectra for Fiocchi Zero Pollution and Winchester Train and Defend rounds are displayed in Figure 2a and b. The LIBS multi-elemental nature is evident in each emission spectrum as several emission lines are observed for each sample. Scanning electron microscopy energy dispersive X-ray spectroscopy data, presented in Figure 3, suggests that it may be possible to see Al, Si, and K in the LIBS emission spectrum. For the Fiocchi samples, it would be beneficial to examine potential Al, Si, and K emission lines as chemical markers for further statistical analysis. Similarly, for the Winchester Train and Defend rounds, SEM-EDX images suggests that it may be possible to see K LIBS emission lines since K is present in the GSR particle composition. Another advantage of LIBS analysis is that that there are multiple wavelengths at which atoms and ions emit. Even though, the SEM-EDX spectrum indicates only one element comprising the GSR particle, with LIBS it is possible to perform a statistical analysis at each of the detectable emission lines for a single atom type. Spectra from the Fiocchi and Winchester samples are included in this work solely to further solidify proof of concept. More information regarding these samples is pending as discussed in the conclusion.

Chemical Morphology and Evidence Preservation

Chemical morphology plays a crucial role in GSR analysis by SEM-EDX. Recently, Hogg et al. determined that it is possible to differentiate various brands of non-toxic ammunition using principal component analysis to classify particles by surface characteristics.^2,27 The Force on Force training rounds analyzed in this work produced GSR particles with a smooth spheroidal morphology, comprising mostly Ba, Al, and Si (Figure 5). The spherical shape is mostly attributed to the presence of Al.²⁸ Similarly, the Fiocchi primers primarily contained Al, Si, and K in a smooth round metal alloy. Although, the priming mixture for the Winchester Train and Defend round was predominantly organic constituents, SEM-EDX revealed that particles of GSRs maintained a spheroidal geometry solely composed of K, likely originating from potassium nitrate. These particles exhibited rough edges and were less smooth than those containing aluminum. Figures 3 –5 display secondary electron images, X-ray spectra, and elemental maps for each type of ammunition. Trace amounts of Ti, Fe, and S were present in several of the analyzed particles. Based on these results, variations of the Ba–Al–Si and Al–Si–K combinations could be added to a laboratory’s list of elements to be considered in the protocol for some lead-free GSR identifications.

Given that LIBS is a minimally destructive technique,^29,30 it is a logical assumption that some evidence remains after laser ablation. Forensic investigators are often analyzing trace evidence in minute quantities and in some cases and/or jurisdictions confirmatory testing is required.^1,31 Therefore, it was necessary to investigate LIBS in regards to evidence preservation. Despite the fact that a small volume of the sample surface is being ablated during generation of the microplasma, particles of GSR maintain a chemical morphology and composition comparable to data collected prior to LIBS analysis. Secondary electron images and X-ray emission spectra acquired before and after laser ablation are presented in Figures 3 and 6. This information further supports proof of concept for the use of LIBS as a potential field portable prescreening method for GSR when multiple suspect shooters are encountered at a crime scene.

Conclusion

Gunshot residue produced by commercially available lead-free ammunition and/or primers was successfully characterized using both LIBS and SEM-EDX. The data acquired in this work suggest that LIBS may be a suitable method for the analysis of lead-free GSR. Laser-induced breakdown spectroscopy exhibits significantly higher throughput compared to SEM-EDX and offers decreased analysis times to forensic investigators. Additionally, this study serves as a proof of concept that evidence is preserved which may allow for subsequent confirmation by SEM-EDX. Implementing LIBS as a prescreening method for a subset of analytical markers representative of lead-free GSR may help to alleviate backlogs in SEM-EDX workload. The analytical markers presented for the ammunition characterized in this work are only applicable to primers with a similar composition. Several other markers have been analyzed in the thesis work from the Dockery research group.³² Future research should include efforts to characterize various brands and calibers of both classical and lead-free ammunition (e.g., Winchester WinClean and Speer LawmanCleanFire) in order to create a library of chemical profiles associated with different manufacturers, taking advantage of the inherent multi-elemental nature of LIBS.

Footnotes

Acknowledgments

Portions of this work were previously presented at Pittcon.

Conflict of Interest

The authors declare there are no conflicts of interest.

Funding

This work was supported in part by the Department of Chemistry and Biochemistry at Kennesaw State University.

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