Impact-disrupted gunshot residue:A sub-micron analysis using a novel collection protocol

V.Spathis

School of Physical Sciences,University of Kent,Canterbury,Kent CT2 7NZ,United Kingdom

Impact-disrupted gunshot residue:A sub-micron analysis using a novel collection protocol

V.Spathis

School of Physical Sciences,University of Kent,Canterbury,Kent CT2 7NZ,United Kingdom

A R T I C L E I N F O

Article history:

Gunshot residue

Impact-disrupted gunshot residue

Sub-micron

Scanning electron microscopy

Cold field emission SEM

FEG-SEM

X-flash

Energy dispersive X-ray spectrometry

The analysis of gunshot residue(GSR)has played an integral role within the legal system in relation to shooting cases.With a characteristic elemental composition of lead,antimony,barium,and a typically discriminative spheroidal morphology,the presence and distribution of GSR can aid in firearm investigations.In this experiment,three shots of low velocity rim-fire ammunition were fired over polished silicon collection substrates placed at six intervals over a 100 cm range.The samples were analysed using a Field Emission Gun Scanning Electron Microscope(FEG-SEM)in conjunction with an X-flash Energy Dispersive X-ray(EDX)detector,allowing for GSR particle analyses of composition and structure at the sub-micron level.The results of this experiment indicate that although classic spheroidal particles are present consistently throughout the entire range of samples their sizes vary signi ficantly,and at certain distances from the firearm particles with an irregular morphology were discerned,forming“impactdisrupted”GSR particles,henceforth colloquially referred to as“splats”.Upon further analysis,trends with regards to the formation of these splat particles were distinguished.An increase in splat frequency was observed starting at 10 cm from the firearm,with 147 mm-2splat density,reaching a maximal flux at 40 cm(451 mm-2),followed by a gradual decrease to the maximum range sampled.Moreover,the structural morphology of the splats changes throughout the sampling range.At the distances closest to the firearm,molten-looking particles were formed,demonstrating the metallic residues were in a liquid state when their flight path was disrupted.However,at increased distances-primarily where the discharge plume was at maximum dispersion and moving away from the firearm,the residues have had time to cool in-fight resulting in semi-congealed and solid particles that subsequently disrupted upon impact,forming more structured as well as disaggregated splats.The relative compositions of the characteristic elements that are present in GSR also change in the different splat morphologies sampled, which may contribute to the particles'physical structures.Two distinct populations of splats were also observed:circular and elongated,which suggest the residues hit the substrate at different angles.The difference in the splat impact angle can be ascribed to the position of the residues within the firearm discharge plume;particles get caught up in the vortex that is created by the discharge gases behind the projectile as it leaves the barrel,thereby affecting their directionality and flight time.This reasoning could also justify the existence of both spheroidal and splat particles at certain distances.The novel sampling and analytical techniques used in this experiment have provided previously unknown information in relation to GSR structure and formation which could have greater implications to its current analysis amongst laboratories and law enforcement agencies worldwide.

?2017 The Author.Published by Elsevier Ltd.This is an open access article under the CC BY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/).

1.Introduction

When a cartridge-based weapon is fired,both gaseous and solid residues consisting of organic and inorganic particles from the ammunition are produced and expelled from the barrel[1-3].Once these residues are in flight they cool very rapidly producing GSR, which is identi fied by its characteristic inorganic elemental composition of lead,antimony and barium[4].The analysis of GSR provides important evidence in firearms incidents,making the quality of its analysis crucial.Therefore,it was established that SEM-EDX analysis was the idealtechnique to do so due to its nondestructive ability to analyse samples and being able to providemorphological and elementalcomposition data in a short amount of time[5,6].Although GSR has a characteristic elementalcomposition,the origin of those particles cannot be concluded based on that alone.It has been acknowledged that to establish a particle is in fact GSR,the compositional data must be coupled with a characteristic spheroidal morphology to distinguish it from environmentalaggregates such as residue from fireworks or vehicle brake pads[7-11].In this experiment,the morphology and elemental composition ofrim fire ammunition GSR is examined.Although the concept of coalescence of molten droplets to form GSR is not unknown[12]the novel sampling and analytical techniques used in this experiment demonstrate that particles may not always have suf ficient time to coolinto spheroidalresidues,in particular at low velocities.

2.Materials and method

2.1.Experimental setup

In this experiment,six silicon collection substrates sized approximately 15 mm×20 mm were set up perpendicular to the firearm and 7 cm below the firing line atbench level.The collection substrates were kept upright and in place with the use of plastic spine bars that were fixed onto the bench,allowing the substrates to be at a consistent height and perpendicular to the firearm for maximum residue collection.Prior to the experiment,these silicon substrates were cleaned for 5min in an ultrasonic bath using HPLCgrade acetone and left to dry,placed in their individual containers and were only taken out for the test firing in a closed room using gloves.This ensured that they remained as uncontaminated as possible,and residues found on them during the analyticalprocess were a product of the firing and not prior contaminants.These substrates were arranged in a straight line down the centre of the firing line at distances of 10 cm,20 cm,40 cm,60 cm,80 cm and 100 cm.Three consecutive shots were fired using 0.22′376 m s-1CCIMinimag full metal jacketed(FMJ)rim fire ammunition with a Browning Buckmark long ri fle pistol.

2.2.Analyticalmethod

The silicon samples were collected immediately after the test firing and mounted on double sided carbon adhesive tape on aluminium stubs,allowing for SEM analysis.They were then analysed using a HitachiS-4700 cold field emission SEMequipped with a Bruker X-flash 5060f flat quad energy dispersive X-ray detector [13],which allowed for the examination of particle structural morphology(Fig.1)and elementalcomposition(Fig.2)at the submicron level.The morphology of the particles was examined using an accelerating voltage of 1.5 kV,which provided a more elaborate insight into the surface topography of a particle as the electron beam does not penetrate deep into the sample[14].However, during the elemental composition acquisition process,the voltage was changed to 20 kV.This gave a poorer image of the particle(as the electron beam is more penetrating and thus surface features are rendered invisible)but the higher accelerating voltage is required to stimulate X-ray emission from metallic elements comprising the particle.

3.Results

Upon FEG-SEM analysis of the samples,it appeared that the frequency at which particles were present on each silicon substrate varied signi ficantly.It was determined that the majority of the residues collected had an irregular morphology rather than their frequently encountered spheroidalshape.This was done by setting up a‘Particle Mapping Job’on the FEG-SEMusing the Bruker Esprit software which allowed for image and X-ray data to be acquired overa customisable area.Approximately halfthe surface area ofthe sample was analysed using this automated method at 20 kV.The results were then reviewed and the particles were then manually examined,as well as the rest of the sample,using a random selection process.Within these particles,there were two populations distinguished,normal incidence and angled splats(Fig.3).Consequentto further analysis,this selection ofparticles could be divided further into severalsubcategories(Fig.4 and Fig.5).

A class of particles shown in Fig.6 was also encountered throughoutallofthe samples.However,as they primarily consisted of organic material with only small amounts of the inorganic‘characteristic’GSR elements,they were excluded from the classification process.The silicon substrates were analysed and a random selection of particles was chosen from each.These particles were then classi fied using the splatclassi fication diagram and archetypes mentioned in Figs.4 and 5.The results are displayed in Table 1.

A selection of splats from each class was analysed using EDX (Fig.7).The elementaldata was collated and is displayed in Table 2. The relative proportions ofthe‘characteristic’GSR elements in each class was also plotted to show elemental composition change per classi fication type(Fig.8).

Table 1 Table showing the number of particles per silicon substrate and their corresponding classi fications.

Although copper does not come under the ASTM guide for characteristic GSR elements,it was present in the vast majority of particles analysed(in conjunction with Pb,Sb and Ba)and was therefore included to ensure more accurate relative particle compositions.

4.Discussion

4.1.Morphological analysis

The results from the morphologicalexamination of the impact disrupted GSR indicate that there is a clear pattern in class type as a function of distance.As it can be seen from Table 1,at the distances closest to the firearm a high number of class 2 and class 5 particles are observed,with minor contributions to the other classes.However,around the centre of the sampling range,at approximately 40 cm from the firearm,the residues analysed primarily fallwithin classes 1,3 and 4.Here,the firearm discharge plume is at its maximum dispersion for this speci fic firearm and ammunition combination.This was established by having previously carried out a test firing with the same firearm and ammunition combination where a 25 cm×25 cm piece of cotton fabric was setup at 10 cm intervals from the firearm up to a maximum of 100 cm.A single shot was fired through each piece of fabric and the Modi fied Griess and Sodium Rhodizonate tests were carried out to revealthe nitrite and lead residue dispersion pattern on the fabric[15].These data suggestthatalthough these residues appear further away from the firearm,they have nothad suf ficienttime to cool in-flight,resulting in their liquid and molten forms.This is contradicted by the fact that residues closer to the firearm inhabit a more solid structure despite being only 10 cm or 20 cm away. Therefore,this exhibits thatthere are two differentdirectionalities to the firearm discharge plume,each ofwhich may be subject to a different temperature or velocity.As is demonstrated in Fig.9, when the projectile leaves the barrel,it creates an expanding vortex behind it.The vortex keeps expanding until it reaches maximum dispersion,after which it starts to diminish.However, although the particles get trapped in that vortex,the majority of them stilldo not have suf ficient time to cooldespite the increased flighttime.This suggests thatthe residues found atthese distances are either experiencing a higher temperature effect or are travelling at a higher velocity than the particles found closer to the firearm.The travelling of particles in the firearm discharge plume is also responsible for the different splat angles seen in Fig.3 in section 3.Although some particles maintain a fairly straight flight path resulting in normalincidence splats,others gettrapped in the vortex that is created,resulting in a change in their directionality, and therefore impacting the substrate at an angle.Moreover,the sphericalparticles thatwere found atthese distances were smaller than 3μm.This information supports the aforementioned hypothesis as small,spheroidalparticles would have had the chance to cool because of their small volume.To first order,the relative particle cooling time is a function of the volume of a particle,and thus is 8 times faster for a particle with half the diameter.On the other hand,the larger particles would not have had the chance to coolbecause oftheir added size,resulting in their disruption upon impact.In consequence,this means that the residues found closer to the firearm are present as a result of the lower velocity stream of particles that leaves the muzzle,labelled‘A’.This is reinforced by the factthatatthe distances farthestfrom the firearm,there is a gradual increase in more semi-congealed and solidi fied residues, as well as the characteristic spherical GSR.

4.2.Elemental composition analysis

Upon examination ofthe elementalcomposition ofthe different particle classes,it can be seen that the more liquid the appearance of the particle,the higher the lead content of said particle appears to be.Conversely,as it can be seen in Fig.8 in section 4.2,the barium content seems to increase as the particles become more solidi fied.Antimony and copper both show a slight decrease as the residues become more structured and solid.However,as multiple particles throughoutthe sampling range were analysed,itappeared thatthese particles are fairly antimony-poor and upon EDX analysis of the cartridge case,projectile and primer,it was determined the ammunition itself contained no antimony.In fact,the presence of antimony in the samples was due to the projectile itself.Some anomalous particles have in fluenced these results,leading to larger than average error.Nevertheless,a change in the compositional ratios in these particles could in fluence their structural morphology,therefore,suggesting that the ratio of the elemental composition may also in fluence a particle's structure,inconjunction with in-flight cooling time.

Table 2 Table showing the relative proportions of‘characteristic’elements present in GSR±1 standard deviation.

5.Conclusions

The study that was carried out has given new insights into the formation of GSR and its analysis.The use of a FEG-SEMwith an X-flash flat quad detector has allowed for in depth particle analysis of a higher spatial resolution at sub-micron magni fications.This,in turn,has led to a deeper understanding of particle morphology, showcasing new impact disrupted GSR structures that were previously unknown.Similarly,the ability to determine the elemental composition of speci fic features within particles measuring less than 1μm in diameter,creates new possibilities into the standard of GSR analyses.This information may have a greatimplication on the automated analysis of GSR in laboratories and law agencies.Particles collected that may have previously been discarded due to theirlack of all three’characteristic’GSR elements may in fact be a segment ofa greater splat structure,leading to the re-evaluation of sample collection protocols and analysis world wide.

Acknowledgements

The author would like to thank Dr Mark Price for his vitalsupport and encouragement during this process as well as Mr Mark Johnson for his time and resource contribution to this experiment.

This research did not receive any speci fic grant from funding agencies in the public,commercial,or not-for-pro fit sectors.

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30 January 2017

E-mail address:v.spathi@kent.ac.uk.

Peer review under responsibility of China Ordnance Society.

http://dx.doi.org/10.1016/j.dt.2017.03.007

2214-9147/?2017 The Author.Published by Elsevier Ltd.This is an open access article under the CC BY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/).

Received in revised form 19 March 2017

Accepted 29 March 2017

Available online 4 May 2017