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LASER INDUCED BREAKDOWN SPECTROSCOPY (LIBS):

CHARACTERIZATION OF GUNSHOT RESIDUE AND BULLET WIPE

by

KRISTINA TRUITT

ELIZABETH GARDNER, COMMITTEE CHAIR

JASON LINVILLE

MITCH RECTOR

A THESIS

Submitted to the graduate faculty of the University of Alabama at Birmingham,

in partial fulfillment of the requirements for the degree of

Master of Science

BIRMINGHAM, ALABAMA

2011

ii

LASER INDUCED BREAKDOWN SPECTROSCOPY (LIBS):

CHARACTERIZATION OF GUNSHOT RESIDUE AND BULLET WIPE

Kristina Truitt

Master of Science in Forensic Science

Laser Induced Breakdown Spectroscopy (LIBS) can be used to quickly determine

the elemental composition of gas, liquid, and solid samples with minimal sample

preparation. A LIBS instrument commonly incorporates a ND:Yag Laser and a CCD or

Eschelle detector. The laser pulse ablates material from a sample, producing a high

temperature plasma. The plasma emits light at wavelengths that are characteristic of the

elements ablated from the sample. The emission of the plasma is collected and analyzed

by a detector within the LIBS system.

The advantages of LIBS are that the method is relatively non-destructive, very

little sample preparation is required, and the spectra can be obtained within just a few

minutes. The disadvantages are that the limit of detection is presently only 4-10 ppm and

that the percent composition of trace elements cannot be determined to the level of

accuracy required for forensic analyses. However, as shown by this project, LIBS can be

used as a quick test when the presence of specific elements are used to identify a sample,

such as lead for a bullet fragment or lead (Pb) and barium (Ba) for a suspected bullet

hole.

In cases that involve the use of firearms, traces of lead, barium, and antimony can

be detected on the victim, criminal, and other objects that have come in contact with the

firearm and/-or fired projectiles. However, when the point of impact is more than six feet

from the source of the gunshot, the only residue at the site of impact may be bullet wipe,

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a dark ring of lead or barrel residue wiped from a bullet as it passes through the material.

This study focused on the effects that distance on the detection of gunshot residue and

bullet wipe on clothing, cement block, wood, and drywall shot at distance from 1” to 12’.

Peaks at 280.16, 368.49, and 405nm are characteristic of lead and at 455.4, 493.4, and

553.5 nm for barium.

The techniques developed in this project have the potential to establish an area of

bullet impact detection in the presence and absence of gunshot residue.

Keywords: Spectroscopy, Gunshot Residue, Bullet

iv

DEDICATION

In Loving Memory of William Benson

v

ACKNOWLEGDEMENTS

Dr. Elizabeth Gardner, Research Advisor

Dr. Jason Linville, UAB MSFS Director

Janey Deimling, UAB Graduate

Mitch Rector, Birmingham Police Department Forensic Scientist

Perry Gordon, Birmingham Police Department Firearms Examiner

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TABLE OF CONTENTS

Page

ABSTRACT ............................................................................................................ ii

DEDICATION ....................................................................................................... iv

ACKNOWLDEGEMENTS .....................................................................................v

LIST OF TABLES ............................................................................................... viii

LIST OF FIGURES ............................................................................................... ix

INTRODUCTION ...................................................................................................1

MATERIALS AND METHODS ...........................................................................10

Preliminary Research Samples .........................................................................10

Preliminary Samples LIBS Procedure ..............................................................12

Experimental Samples ......................................................................................13

Experimental Controls ......................................................................................15

Experimental LIBS Procedure ..........................................................................16

RESULTS AND DISCUSSION ............................................................................18

Preliminary Research .......................................................................................18

Experimental Samples .....................................................................................24

T-shirt Samples ................................................................................................24

Cement Samples...............................................................................................29

Wood Samples .................................................................................................33

Drywall Samples ..............................................................................................36

CONCLUSION ......................................................................................................41

LIST OF REFERENCES .......................................................................................44

viii

LIST OF TABLES

Table Page

1. Comparison of Wavelength and Intensity of the

Lead Standard and Shotshell 1 .....................................................................................19

2. Lead Spectral Lines of T-shirt Samples

and Controls .................................................................................................................25

3. Barium Spectral Lines of T-shirt Samples

and Controls .................................................................................................................26

4. Lead Spectral Lines of Cement Samples

and Controls .................................................................................................................30

5. Barium Spectral Lines of Cement Samples

and Controls .................................................................................................................31

6. Lead Spectral Lines of Wood Samples

and Controls .................................................................................................................34

7. Barium Spectral Lines of Wood Samples

and Controls .................................................................................................................35

8. Lead Spectral Lines of Drywall Samples

and Controls .................................................................................................................38

9. Barium Spectral Lines of Drywall Samples and Controls ...........................................39

ix

LIST OF FIGURES

Figure Page

1. Image of pellet arrangement for Shotshell #3 (AR23L22) ..........................................11

2. Image of Cement Block shot at varied distances;

Label on left Sample #3, 6’; Label on right: Sample #4, 9’ .........................................14

3. Pb composition comparison of a PbNO3 Standard (#121)

and Shotshell 1 .............................................................................................................18

4. Shotshell Comparison: Pb composition comparison of four

different shotshells using spectral analysis overlay .....................................................20

5. Ba composition comparison of clean spot and

GSR spot on T-shirt .....................................................................................................21

6. Ba composition comparison of all preliminary samples

and Pb Standard ...........................................................................................................23

7. Pb composition comparison of all preliminary samples and

Pb Standard ..................................................................................................................22

8. Ba Composition of T-shirt Samples 1-5 (1” to 1’) ......................................................27

9. Pb and Ba Composition of T-shirt Samples 6-10 (1’to12’) .........................................28

10. Elemental Composition Comparison of Cement Sample and Control

Prominent Pb and Ba Peaks .........................................................................................32

11. Elemental Composition Comparison of Wood Sample and Control

x


12. Elemental Composition Comparison of Drywall Sample and Control


1

INTRODUCTION

Laser Induced Breakdown Spectroscopy (LIBS) uses a short pulse laser to

produce a plasma that can be analyzed for the elemental composition of gas, liquid, and

solid samples. The LIBS instrument uses a laser, commonly a ND: Yag Laser to produce

a plasma that evolves with time from the point of impact of the incident laser pulse. The

emission of the plasma is collected and analyzed by either a CCD or Eschelle detector

within the LIBS system. The intensities of the spectral emission lines vary with the

following conditions: the type of sample, the distance from the center of the plasma, and

the wavelength of the incident laser light.1 In most recent years, LIBS has been used to

detect trace elements in various types of samples in varied fields of study. Given LIBS’

ability to detect elements in a sample, LIBS could be used as a presumptive test for

gunshot residue in evidence samples. For example, if a fragment of metal, suspected to be

a fired bullet, was retrieved from a crime scene, LIBS could be used to detect elements

associated with gunshot residue on the metal fragment, further suggesting the fragment is

a bullet.

The advantages of applying LIBS as a presumptive test for gunshot residue is its

ability to identify the elemental composition of a sample in less than four minutes and

operate without the use of reagents and sample preparation. One limitation of LIBS as a

presumptive test is that if further analysis required the quantitation of the concentration of

the trace elements present in the sample, accomplished second analytical method would

2

be required. In a case study done at Iowa State University, determination of trace

element concentrations lead to the class characterization of a bullet fragment as a

Cascade, Federal, Remington, or Winchester by identifying only three elements.2

Characterization of the bullet fragment involved detecting the trace elements bismuth,

arsenic, and copper, and calculating the concentration of these trace elements.

Unfortunately, LIBS lacks precision and the current technology does not allow for

quantitation. Most trace elements are present in low concentrations, making it difficult to

assign elements to individual peaks in the spectra and distinguish these trace elements

from background noise. The peak intensity for trace elements can be increased with an

increase in laser power, however if the main constituents are too concentrated the

detector becomes over saturated, making the wavelength measurements inaccurate.

Overall, LIBS precision in measuring the concentration of trace elements can range from

1-40% accuracy.

Researchers at the Fraunhofer- Institute for Laser Technology have taken

measures to gain insight into relevant parameters that have a significant effect on the

spectral data retrieved from ablating the surface of a sample.3 Wester and Noll developed

a heuristic model under the assumption that the plasma content and state of the plasma is

known in order to model or represent the emission spectra of the plasma created by LIBS.

The plasma in terms of the model is described as a spherical shell. In the model, the LIBS

plasma created by LIBS was recreated as two shells, the inner layer, the plasma core, and

the outer layer, ambient gas, both assumed to only consist of atoms, ions, and electrons.

With the formation of the two shell plasma model, the presence of an ambient gas, such

as Argon, was determined to have a positive influence the spectral data by lessening the

3

variation in spectral conditions. They were able to determine that factors such as the

interaction of the laser with sample, the generation of the plasma, and the interaction of

the laser with the plasma all affect the conditions of the spectral data. In this model and as

assumed in many other studies, the spherical plasma created by LIBS is assumed to be in

local thermodynamic equilibrium (LTE). The LTE effect is used to depict the effects of

laser pulse interaction with the surface of the sample and how this correlates to plasma

emission.

Another method that incorporates the LTE effect and addresses the issue of

quantification of elements in a sample is the Calibration Free LIBS (CF-LIBS). This

particular method focuses on analyzing LIBS spectra generated from samples consisting

of multiple elemental components by measuring the relative intensity of peaks generated

and the property of the plasma generated from ablating the sample.4 Quantitative analysis

can be done based on assumptions regarding the Boltzmann population of levels of

electrons at their excited states. In order to ensure the LTE effect, this method employs

the McWhirter principle; which requires that at all electronic levels, the depopulation rate

of electron collision must be at least ten times greater than the depopulation rate of

electron radiation.

Based on a series of integration and log calculations of the transition between

particular electronic levels, the CF-LIBS method is able to accurately quantify the

concentration of elemental components of a sample. Their results showed that the CF-

LIBS method would be suitable for the detection of elements in metal alloys. The

disadvantage of CF-LIBS is that the calculation of concentrations in organic and mineral

4

samples cannot be calculated with sufficient accuracy. This is due to organic and mineral

samples containing multiple elements at very small concentrations.

Abdel-Kareem et.al carried out a study on the analysis of historical metal threads

collected from different museums.5 Metal threads have been used since 3rd century BC

and are commonly used today in couture garments. The almost instant deterioration of

these threads causes scratches, significant changes in the surface morphology, and loss of

surface definition. The analysis of these samples using LIBS would help validate its

potential use for the detection of trace elements in degraded samples and its ability to

produce comparable, reproducible results. In order to test the efficiency of the spectral

data produced from LIBS analysis, the historical metal threads were also analyzed by

Scanning Electron Microscope (SEM) with energy-dispersive x-ray analyzer (EDX).

LIBS was compared to this particular technique since it had been proven to be the best

technique for the elemental analysis of samples such as the historical metal threads. From

analysis of the metal samples, it was determined that LIBS was a useful technique for the

detection of relatively small quantities, or trace amounts of copper (Cu), silver (Ag), and

gold (Au) in the historical metal elements.

The same elements were detected by LIBS even though the samples were in a

much degraded state. Their experimental procedures also indicated that the number of

times the samples were hit with the laser was based on the conditions of the sample being

analyzed. Spectral data could vary based on the amount of degradation of the samples

and the sensitivity of the surface area undergoing ablation. These results indicate that

LIBS could have the potential to be applied to other areas of interest such as the detection

of gunshot residue and bullet wipe.

5

LIBS has also been used in research studies involving the analysis of different

types of cement. In an article published by Gondal et al. LIBS was used to detect the

elemental concentration of chloride in three different cement samples.6 The objective was

to develop a more cost effective and time efficient analytical technique to characterize the

durability and safety of concrete structures. The LIBS spectral data were compared to

spectral data results from Inductively Coupled Plasma Emission Spectroscopy (ICP-ES).

LIBS spectral data indicated that the primary elemental components of the cement

samples were calcium, aluminum, silicon, iron, chromium, magnesium, sodium, chlorine,

sulfur, phosphorous, and manganese. Calibration curves were calculated in order to

accurately measure the concentration of trace amounts of metal in each cement sample.

The concentration of each of the metal components was verified by comparing the results

to the data generated by the calibrated ICP spectrometer. The spectral data was compared

the standards from the NIST database and the data produced from ICP-ES analysis, and

were found to be within +/- 2%. From this comparison, it was determined that LIBS was

an efficient technique in the detection of elemental components. The reproducibility of

LIBS was calculated by determining the relative standard deviation (RSD), resulting in a

precision of 2-4%.

LIBS has also been used in forensic science laboratories for the analysis of trace

evidence. One research project in particular focused on the discrimination of glass

fragments for forensic applications.7 Rodriguez et al. made a comparison of the spectra of

glass fragments collected from various car windows, using linear and rank correlation

methods. The basis of this research relied on the sole fact that glass has a unique spectral

fingerprint that can be characterized. The potential of LIBS was evaluated based on its

6

ability to discriminate glass samples of similar refractive index (RI) values. This was

accomplished by comparing the spectra generated from LIBS within the same day.

Analysis of the glass fragments involved comparing the spectra of unknown glass

fragments to a spectral database, direct comparison of the glass spectra to one another,

and validation of the instrument’s reproducibility. From this study, it was determined

that LIBS could provide an effective identification and discrimination of glass fragments

at a 95% confidence level. These results indicated that LIBS could become a useful

technique in the analysis of forensic glass samples.

Due to the ease in identifying trace elements, it may be possible to use LIBS to

quickly screen evidence samples for the presence of gunshot residue (GSR). Gunshot

residues are primarily composed of burnt and un-burnt particles from combustion and

residual components of the bullet, firearm, and primer.8 Currently, color tests are often

used to detect GSR in evidence samples. The main elements of detection in gunshot

residue are primer residue (lead, barium, and antimony), nitrites, and nitrates.9 Romolo

and Margot evaluated several color testing methods to determine the best inorganic

chemical method to use for GSR analysis. A portion of their surveys focused on the

comparison of various color tests used for presumptive testing. They compared the

paraffin test by Teodoro Gonsalez of the Mexico City Police to the chemical test of

Harrison and Gilroy produced in 1959.10

Both the paraffin test by Gonasalez and chemical test by Harrison and Gilroy

were able to detect GSR. The Paraffin Test consisted of using hot paraffin to make the

cast of a suspected shooter’s hand. Once the wax cooled, it was peeled off and the cast

was sprayed with N, N,-diphenylbenzidine in concentrated sulphuric acid. This chemical

7

reaction results in a blue color change in the presence of nitrates. The color change to a

deep blue indicated the presence of partially burnt and un-burnt particles. The color tested

examined by Harrison and Gilroy, the sodium rhodizonate test, detected lead styphnate,

barium nitrate, and antimony sulphide from primer residue. Their GSR samples were

collected from the hands of the shooters using swabs moistened with hydrochloric acid.

The swabs were dried and then treated with two reagents: triphenylmethylarsonium

iodide and sodium rhodizonate. In their color testing, the first reagent produced an orange

color if antimony was present and the second reagent produced a red color if lead or

barium was present. If the swab turned red, dilute hydrochloric acid was added and the

sample turned a purple color if lead was present. The Harrison and Gilroy chemical test

was found to be more reliable in that it produced the least number of false positive

results.

Currently, the modified Griess test is used for the detection of nitrite residues.

Nitrite residues can be detected in unspent gunpowder residue that has not fully burned

when a bullet is projected out of the muzzle of a firearm. The residue occurs on surfaces

pierced by a bullet when nitrocellulose is not completely burned during flame ignition

after the firing pin is hit. The presence of GSR, in the form of nitrites, is indicated by an

orange color change. The modified Griess test consists of 1% sulfanilamide, 0.1% N-1-

naphthyl-ethylenediamine dihydrochloride, and 5% phosphoric acid. This chemical test is

most effective for the detection of nitrite gunshot residue on samples or materials shot

within 3’ or less. The level of GSR present is also dependant on the type of ammunition

and the type of firearm used.

8

Although chemical testing is more efficient for detecting the presence of GSR

over a large area, it is limited to shots fired from less than 6’ from the target. The range

from which a firearm is shot determines the level of element detection. At close ranges,

the gaseous cloud of GSR particles is directly dispersed onto the surface of the target. In

farther ranges, the deposit of gunshot residue decreases and disperses before it reaches

the target. GSR can also be transferred to the target by having actual contact with the

bullet itself, this is known as bullet wipe. It has been determined that transfers of GSR, at

close range, from far range, or by bullet wipe, can be characterized and distinguished

from one another.11 If a dark blackish-grey area of GSR is visible on the sample, this is

an indication that the firearm was fired in close range. The presence of GSR in this dark

colored area, or in the bullet wipe area, could be characterized by the presence of the lead

peaks using LIBS.

According to Martiny, the characterization of gunshot residue is essential when

investigating firearm-related incidents. Martiny detected gunshot residue by using a

combination of chemical testing and Scanning Electron Microscopy/ Energy Dispersive

X-ray Spectroscopy (SEM/EDS) analysis.12 This study analyzed the elemental

composition of heavy-metal free environmental ammunition primers from Brazil. They

chose this ammunition type based on the fact that more lead free ammunition is being

produced in response to increased exposure to heavy metal-rich airborne particles. Their

analysis compared the morphology and characterization of samples produced by

Companhia Brasileira de Cartuchos (CBC) and Clean Range ammunition. They used

SEM/EDS to examine GSR from the shooter’s hands upon firing a firearm. Differences

were observed in the shape and composition for the Clean Range samples. The Clean

9

Range samples consisted of first and second generation lead-free ammunition including 9

mm Luger, .380 AUTO, .38 Special, .40 Smith and Wesson, and .45 Automatic Colt

Pistol calibers. Their results indicated the detection of strontium with spherical shaped

particles in the first generation ammunition and the detection of potassium, aluminum,

silicon, and calcium with irregular shaped particles in the second generation ammunition.

From this study, it was concluded that the identification of gunshot residue from the CBC

ammunition was impossible without a distinct metallic marker within the composition of

the primer.

This project has been constructed to determine the validity and essential use of

LIBS in crime scene investigations that involve the use of firearms. The purpose of this

study is to determine whether LIBS has the potential to be used as a presumptive test for

the detection of gunshot residue and bullet wipe from suspected impact areas through the

analysis of bullet fragments, T-shirt, cement, wood, and drywall samples.

10

MATERIALS AND METHODS

Preliminary Research Samples

The LIBS System (Crossfire Model, Photon Machines, Inc.) was used to analyze

the elemental composition of six types of samples, including a cotton T-shirt shot at close

range, three sets of known lead, unfired shotgun pellets, one unfired lead bullet, a

2”x4”wooden block and a cement block, each shot two years ago. The cement block and

2”x4” wooden block were shot from a distance of 10’. The known lead shot samples

analyzed were made by Remington and included:

Shotshell#1: Slugger 20 gauge Hollow Point rifled Slug, Lot # BP26B527

Shotshell#2: Game Load 20 gauge- 8 shot, Lot# BG16F520

Shotshell#3: Express Plastic 16 gauge-7 ½ birdshot, Lot # AR23L22

Shotshell #4: Premier Target Load 20 gauge- 9 shot, Lot # BF08U521

These samples were collected from a previous research project carried out by

University of Alabama at Birmingham Graduate, Janey Deimling between 2007 and

2008. The objective of her project was to use a microcrystalline test to detect the presence

of lead and prove it to be a much more suitable test than the currently used chemical test,

sodium rhodizonate. Research results showed that the microcrystalline test was

successful on samples that had at least 300µg/mL of lead. From the conclusions drawn

from this project, it was determined that collection of a sample’s elemental composition

would be easier with less sample preparation and faster data generation by using LIBS.

11

Since the samples were already proven to contain a detectable amount of lead composite,

the application of LIBS presented a possible avenue to the detection of trace amounts of

elements such as lead.

A T-shirt that had been shot at close range, with visible gunshot on a wide area

residue surrounding the hole, was used to develop a gunshot residue detection method.

The T-shirt had been shot 2 years previous to the research and was analyzed primarily to

develop the experimental method. Approximately 1.2 cm samples were cut from four

spots on the T-shirt, at different distances (3/4” to 10”) from the bullet hole. The control

spectrum of the T-shirt was collected from an apparently clean section on the back, lower

portion of the T-shirt. The clean area was cut out using scissors and was mounted onto a

business card using double-sided tape. An approximate diameter of 1.2cm of the gunshot

residue sample was cut out near the hole using scissors as well.

The pellets from unfired cartridges were removed and separately mounted onto a

business card using double-sided tape. The pellets were placed in a close, compact

arrangement as shown in Figure 1.

Figure 1. Image of pellet arrangement for Shotshell #3 (AR23L22)

12

The cement block and wood slab samples were gathered by chipping away pieces

of each material where discoloration from the bullet wipe was obvious. The pieces of

each material were mounted on a card using double-sided tape as well.

Preliminary Samples LIBS Procedure

The LIBS instrument is coupled with a Nd: Yag Laser with a CCD detector. LIBS

is used to identify the elemental composition of samples analyzed by laser pulsing the

samples, ablating the surface of the sample at 1500˚C, and using the detector to collect

the spectra of electrons after atoms relax from their excited state. Lead nitrate was used

for the lead standard (#121). The powder was pressed into pellets and analyzed by the

OOILIBS software (December 2008, Q-Switch delay -1.7us).

All samples were pulsed with the laser and the atomic spectral lines were

collected. The instrument was set at an integration of -0.42, a Q-switch delay time of -

0.4us, and the power level of 200mJ. The LIBS software, OOILIBS, was used to analyze

each sample and identify the elements of lead, barium, and antimony. The spectral data

collected from each sample was collected within the range of 200 to 600nm. Analysis of

the samples varied depending upon the intensities of the peaks detected; therefore the

OOILIBS settings of peak height (threshold) and search width were adjusted in order to

detect trace elements of lead in the samples. The threshold was adjusted been 300 and

800 counts based on the intensity of the peaks, and the search width varied based on the

location of distinct, sharp peaks in the spectra.

13

Experimental Samples

In light of the preliminary work done to determine whether or not LIBS could

detect GSR, further research was done to determine if LIBS could be used to detect bullet

wipe without the presence of GSR patterns and tattooing. Samples analyzed included ten

cotton T-shirt samples, a cement block, dry wall samples, and a 2”x4”wooden block. The

T-shirt samples were shot at a distance of 1 inch to 12 feet in order to find a trend in

decreasing element detection as the distance increased.

The shooting range utilized to soot the samples analyzed was an open, training

field utilized by the Birmingham Police Department. The training field contained a

wooden fixture (setup) used for training; this particular wooden structure was used to

prop up the samples for shooting preparations. The shooting distance was measured by

using measuring tape to construct and assure the correct distance between the mounted

samples and the firearm. The cement block, wood sample, and dry wall were propped up

on a wooden stand before shooting. The cement block, wood sample, and dry wall

samples were divided into sections based on shooting distance by labeling the perspective

distance on each section of the sample, as shown in Figure 2. The samples collected for

experimental analysis included a total of 10 T-shirt samples (shot from 1 inch to 12 feet),

4 cement samples (shot from 3 feet to 12 feet), 5 wood samples (shot from 1 foot to 12

feet), and 5 dry wall samples (shot from 1 foot to 12 feet). Each sample was shot with a

lead semi-wad cutter bullet (Pb-SWC) using a Smith and Wesson, 357 Magnum Caliber

Revolver (Model 13).

14

Figure 2. Image of Cement Block shot at varied distances; Label on left: Sample #3, 6’;

Label on right: Sample #4, 9’

The 10 T-shirt samples were separately mounted onto pieces of cardboard with

staples and properly labeled based on sample number and distance. The mounted T-shirt

samples were individually placed on a wooden stand and shot. The T-shirt samples were

shot from a distance of 1”, 3”, 6”, 9”, 12” (2), 3’, 6’, 9’ and 12’ at a 90˚ angle with

respect to the position of the firearm. Each bullet hole was analyzed by cutting a piece of

the fabric near the bullet hole entry using scissors. Each piece of fabric was mounted onto

a separate business card for LIBS analysis.

As shown in Figure 2, the cement block was divided into sections using a

permanent marker. Each section of the cement block was labeled with the respective

sample number and the distance. The block was shot once in each section from a distance

of 3’, 6’, 9’, and 12’ at an angle of 45˚ with respect to the position of the firearm. The

cement samples were then collected by chipping away pieces in the area of bullet impact

using a flat head screwdriver and a hammer. The pieces of cement from each section of

the cement block were mounted on separate, labeled business cards using double sided

tape.

15

The wooden block was divided into sections like the cement sample in Figure 2.

The wood samples were also propped up on the wooden bench and shot from a distance

of 3’, 6’, 9’, and 12’ at a 90˚ with respect to the position of the firearm. The wood

samples were collected by cutting pieces from the area of bullet impact using a box-

cutter. Like the T-shirt and cement samples, the pieces from each section of the wood

block were mounted onto labeled business cards.

Five drywall samples were placed onto a wooden bench and shot from a distance

of 1’, 3’, 6’, 9’, and 12’ at a 90˚ with respect to the position of the firearm. The dry wall

samples were also collected by cutting away a piece from the area of bullet impact using

a box cutter. However, the dry wall sample had three layers, the chalky, white gypsum

material sandwiched between two layers of paper, so the top coating was sliced off for

easier, cleaner mounting and detection purposes.

Experimental Controls

The control samples consisted of the Pb-SWC bullet head and jacket (positive

controls), scissors, flat head screwdriver, box-cutter, and cardboard. Samples from the T-

shirt, cement, wood, and drywall controls before being shot were also collected as

negative controls in order to distinguish between bullet wipe and the elemental

composition of the samples themselves. The spectral data of the controls were collected

before they were used in the experimental procedures.

16

Experimental LIBS Procedure

Like the preliminary samples, the experimental samples were analyzed using

OOILIBS software for the detection of lead and barium. AddLIBS software was also

used to detect trace amounts of elements in the sample that may not have been detected

using the OOILBS software. Collection of spectral data was collected in the 200 to

600nm range and the analysis of each sample varied based on the intensity of the peaks,

varying the threshold adjustment, but not accounting for elements detected below a

threshold of 300. All the samples were compared to each other to identify the similarities

or differences in their spectral data.. The elemental lines from each sample were

compared to The National Institute of Standards and Technology (NIST) Database of

common element wave numbers. The NIST Database was also used to validate the results

of the LIBS instrument.

All the samples were mounted onto a business card using double sided tape,

placed on the laser platform, and then ablated. For the analysis of the T-shirt, cement,

wood, and drywall samples, the instrument parameters were set at an integration of -

0.42., a Q-switch delay time of 0.4us, and the power level of 200mj. OOILBIS was used

to analyze each spectrum within the threshold range of 300 to 800 counts and 200 to 600

nm, with a +/- 3 search width of the spectrum. All samples were compared to each other,

to the spectra of the Pb-SWC bullet, and to the spectra of their perspective control

samples. T-shirt samples were also compared to T-shirt, cardboard, and scissor controls.

The spectra of the 4 cement samples were also compared to the flat head screwdriver

controls. The spectra of the wood sampled and drywall samples were also compared to

box-cutter controls.

17

The controls were analyzed individually by mounting them on a business card,

using double-sided tape. The Pb-SWC bullet head was analyzed by mounting the bullet

onto the card and shooting it with a laser on various areas of the top portion of the bullet.

The spectral data of each of the controls were analyzed using the OOILIBS software at

the same settings. The Pb-SWC bullet jacket was also analyzed by mounting the bullet

onto a business card and shooting various areas of the lower portion of the bullet. The

metal portion of the flat head screwdriver and box-cutter were detachable and were also

placed on a business card and shot at various spots. The business card was simply placed

on the sample mount and shot at various spots as well. The T-shirt, cement, wood, and

drywall controls were collected from areas of the sample that had not come in contact

with the bullet itself. This was ensured by collecting the control samples before the

samples were shot at varied distances.

To prevent cross contamination between the collection of samples, the tools used

were cleaned with ethanol each time applied. The tools were washed with ethanol

between the collection of the same type of samples, however in some cases the same tool

was used for the collection of different sample types. A box-cutter was used for the

collection of both wood and drywall samples so to ensure that cross contamination did

not occur the blade was changed between collection of the wood samples and dry wall

samples.

18

RESULTS AND DISCUSSION

Preliminary Research

The spectral data results from the LIBS instrument were consolidated into scatter

plots, as shown in Figures 3-4. The scatter plots generated for each sample were used to

compare the spectra by graph overlay and to distinguish each sample from one another

through element identification. Each sample was characterized by distinguishing sharp,

concise peaks of elements distinct from peaks generated by background noise.

Figure 3. Pb composition comparison of a PbNO3 Standard (#121) and Shotshell 1

In analyzing each sample, it was determined that there were significant

similarities between the Pb Standard and each shotshell sample. In a comparison of the

19

Pb Standard and Shotshell 1, the major lead lines were detected at 280.19nm, 283.30nm,

363.95nm, 368.34nm, and 405.78 nm, as shown in Figure 3. There is a direct overlay of

the lead peaks detected in both the lead standard and Shotshell 1. These results validated

the operating system of LIBS and verified the prominent lead peaks that should be

detected in the bullet fragments.

There were also significant similarities between each shotshell sample. The

spectral lines in Table 1 indicate that the major lead lines in each shotshell sample were

again 280.19nm, 283.30nm, 363.95nm, 368.34nm, and 405.78nm. As shown in Figure 4,

the spectra from each of the four shotshells were very similar to each other. The primary

elemental compositions of the shotshells are shown.

Table 1

Comparison of Wavelength and Intensity of the Lead Standard and Shotshell 1

PbNO3 Standard #121 Shotshell 1 280.16 nm/ 1833cts. 280.19 nm/ 4095cts. 283.42 nm/1084cts. 283.30 nm/ 2381cts. 364.01 nm/ 1016cts. 363.95 nm/ 4095cts. 368.48 nm/ 2003cts. 368.34 nm/ 4095cts. 405.89 nm/ 2060cts. 405.78 nm/ 4095cts.

*Cts: The counts represent the relative intensity of a particular peak relative to other peaks detected in the spectra.

20

Figure 4. Shotshell Comparison: Pb composition comparison of four different shotshells

using spectral analysis overlay

The composition shown in Figure 4 can be used as a control for the detection of

lead in bullet wipe. Lead was the major element detected in each bullet sample. The

spectra of the clean and GSR samples of the T-shirt are shown in Figure 5. There were

significant differences between the clean and gunshot residue spots, indicating that an

area that has come in contact with a bullet fired from a firearm can be distinguished from

an area that did not have contact with the fired bullet.

Intensity (counts) vs. Wavelength (nm)

21

Figure 5. Ba composition comparison of clean spot and GSR spot on T-shirt

Along with lead, barium was the other prominent element detected in the GSR

spot, suggesting that these elements can characterize an area that has come in contact

with a fired bullet. The prominent barium lines were detected at 455.40nm, 493.41nm

and 553.54nm. Barium peaks were detected in the clean T-shirt sample; however, it was

determined that it was as a result of background noise.


0

500

1000

1500

2000

2500

3000

0 100 200 300 400 500 600 700

Soiled frag

Clean frag

553.55 nm493.41

455..40 nm

22

Figure 6. Ba composition comparison of all preliminary samples and Pb Standard

As shown in Figure 6, there were slight similarities between the cement block

sample, gunshot residue spot from the t-shirt, lead standard, and wood slab sample. In

Figure 6, the prominent barium lines were detected at 350.11nm and 553.55nm in the

cement block sample, gunshot residue spot, and wood slab sample. In Figure 7, the

prominent lead lines were detected at 280.2nm, 283.31nm, 363.96nm, and 368.35nm, and

405.78nm in the cement block sample, gunshot residue spot, and wood slab sample in

comparison to the lead standard. From the elemental composition analysis of each

sample, it was determined that LIBS could detect the prominent lead lines. These


*This is a zoomed –in view of the spectral data, lead standard is not shown in this image.

23

prominent lead and barium lines detected in the gunshot residue spot, wood slab, and

cement block indicated that bullet wipe had resulted from the bullet making contact with

each of the materials.

Figure 7. Pb composition comparison of all preliminary samples and Pb Standard

The T-shirt shot at close range was sampled at varied distances from the bullet

hole. Spectra results showed that the detection of elements becomes increasingly difficult

as the distance from the bullet hole increases. Thus far, the element detection was the

most effective nearest the hole and as far as 1 inch from the hole. The analysis of these

samples indicated that LIBS would be efficient for its use as a presumptive test for the

detection of gunshot residue and bullet wipe from suspected impact areas. In comparison

to chemical testing, LIBS would be more useful in the detection of bullet wipe if gunshot

residue is not visible. This reasoning is supported by the fact that there is an immediate

detection of gunshot residue in chemical testing a visible gunshot residue.

24

Experimental Samples

From the preliminary results, it was determined that further work could be done

by determining limits of detection for bullet wipe in T-shirts and other samples shot at

varied distances and that the development of controls would be important for comparative

analysis of the materials analyzed and the tools that could be used to make holes in

construction materials.

T-shirt Samples

As previously mentioned, 10 T-shirt samples were analyzed. The first set, T-shirts

labeled 1 through 5, were shot from a distance of 1, 3, 6, 9, and 12 inches. Analysis of

these samples only showed the presence of prominent barium peaks at 455.40, 493.41,

and 553.55 nm, as shown in Figure 8 and Table 2 and 3. Of the first set of T-shirt

samples, Sample 5, shot from a distance of 12 inches, was the only sample to show Pb

Peaks. These peaks were identified at 368.346 and 405.781 nm, but at relatively low

intensities in comparison to the Pb peaks in the other samples analyzed. This finding

suggests that the soot and GSR produced from the firearm when fired did not have time

to disperse around the bullet entrance relative to the distance of the firearm from the T-

shirts. From the analysis of the first set of T-shirts, it would be difficult to detect evidence

of bullet wipe, therefore it would be more productive to apply the standard chemical test

for GSR at distances ranging from 1’ to 12’.

25

Table 2

Lead Spectral Lines of T-shirt Samples and Controls

Pb Lines Wavelength 280.02nm 283.31nm 363.96 nm 368.35n

m 405.78nm

T-shirt Samples Sample 1 (1”) Sample 2 (3”) Sample 3 (6”) Sample 4 (9”) Sample 5 (1’) X X Sample 6 (1’) X X X Sample 7 (3’) X X X X Sample 8 (6’) X X X X X Sample 9 (9’) X X X X X Sample 10 (12’) X X X PbSWC Bullet Head

Sample: head-01 X X X X X PbSWC Bullet Jacket

Sample: jacket-01

X X X

T-shirt Control Sample: T-shirt-01

Cardboard Control Sample: Cardboard-01

*Yellow area: Spectral line may be due to background noise.

26

Table 3

Barium Spectral Lines of T-shirt Samples and Controls

Ba Lines Wavelength 455.40nm 493.41nm 553.55nm T-shirt Samples Sample 1 (1”) X X X Sample 2 (3”) X X X Sample 3 (6”) X X X Sample 4 (9”) X X X Sample 5 (1’) X X X Sample 6 (1’) X X X Sample 7 (3’) X X X Sample 8 (6’) X X X Sample 9 (9’) X X X Sample 10 (12’) X X X PbSWC Bullet Head Sample: head-01 X X X PbSWC Bullet Jacket Sample: jacket-01 X X X T-shirt Control Sample: T-shirt-01 Cardboard Control Sample: Cardboard-01

27

Figure 8. Ba Composition of T-shirt Samples 1-5 (1” to 1’)

The second set of T-Shirts, Samples 7 through10, were shot from a distance of 1,

3, 6, 9, and 12 feet, and the data generated from these spectra had slight variance in the

presence of lead and barium prominent peaks, as portrayed in Figure 9. Analysis of this

set of T-shirts showed both prominent Lead and Barium peaks. The spectra of T-shirt

Sample 6 contained Pb peaks at 36.96, 368.35, and 405.78 nm and Barium peaks at

455.40, 493.41, and 553.55 nm. T-shirt Sample 7 contained spectral peaks of Pb at 280.2,

283.31, 363.96, and 404.78 nm and peaks of Ba at 455.40, 43.4, and 553.55 nm. T-shirt

Samples 8 and 9 contained all of the prominent Pb and Ba peaks detected by LIBS. The

spectral data for T-shirt sample 10, shot from 12 feet, only showed 3 of the 5 prominent

Pb peaks which were 363.96, 368.35, and 405.78 nm.


28

Figure 9. Pb and Ba Composition Comparison of T-shirt Samples 6-10 (1’to 12’)

All of the T-shirt samples were compared to the Pb-SWC head and jacket of the

bullet, T-shirt, cardboard, and scissors. Based on the spectral data generated, it was

determined that the T-shirt control, cardboard, and scissors (negative controls) did not

make a contribution to the Pb and Ba lines detected in Sampled 6 through 10 since the

spectral data of these controls did not contain any of the prominent barium and lead peaks

detected by LIBS. The spectra of the head of the Pb-SWC bullet, which serves as a

positive control of the experiment, contained all the prominent Pb peaks at 280.2, 283.31,

363.31, 363.96, 368.35, and 405.78 nm and all of the prominent Ba peaks at 455.40,

493.41, and 553.55 nm. The spectral data of the jacket of the Pb-SWC bullet, which also

served as a positive control for analysis, contained all of the prominent lean and barium

peaks except for two of the Pb peaks at 283.31 and 363.96 nm. In comparison to the Pb-


29

SWC bullet control, it was determined that the lead and barium lines found in Samples 6

through 10 were an indication of bullet wipe.

Cement Samples

Cement samples 2 through 5, were shot from the distance of 3, 6, 9 and 12’, and

the data generated varied tremendously, making it difficult to track a trend or pattern of

detection based on distance. The cement block was not shot at a distance of 1’ due to

safety precautions taken to avoid injuries that could occur with shooting cement at such a

close range. Spectral data from cement sample 2, shot from 3’, had only one prominent

barium peak present at 553.55 nm. The data retrieved from the spectra of cement sample

3, shot from 6’, only indicated the presence of barium at 455.40 nm and 553.55 nm.

Cement Sample 4, shot from 9’, showed lead spectral lines at 280.2, 363.96, 368.35, and

405.78 nm and one barium spectral line at 553.55nm. Completely opposite to cement

sample 4, sample 5, shot from 12’, showed only one prominent lead peak at 280.2 nm and

two barium peaks at 455.40 and 553.55 nm.

The controls used for the analysis of the cement samples were the Pb-SWC bullet

controls, a cement control, and a flat head control. As previously stated the Pb-SWC

bullet controls contain the primary lead and barium peaks in spectral data to indicate the

presence of bullet wipe. The cement control spectral data contained 1 lead peak at 280.2

nm and 3 barium peaks at 455.40, 493.41, and 553.55 nm, as shown in Figure 10 and

Table 4 and 5.

30

Table 4

Lead Spectral Lines of Cement Samples and Controls

Pb Lines Wavelength 280.02nm 283.31nm 363.96 nm 368.35nm 405.78nm

T-shirt Samples Sample 2 (3’) Sample 3 (6’) Sample 4 (9’) X X X X Sample 5 (12’) X

PbSWC Bullet Head

Sample: head-01 X X X X X PbSWC

Bullet Jacket

Sample: jacket-01

X X X

Cement Control Sample:

cement-01 X

Flat-head Control Sample: flat-011

31

Table 5

Barium Spectral Lines of Cement Samples and Controls

Ba Lines Wavelength 455.40nm 493.41nm 553.55nm

Cement Samples Sample 2 (3’) X Sample 3 (6’) X X Sample 4 (9’) X Sample 5 (12’) X X

PbSWC Bullet Head Sample: head-01 X X X

PbSWC Bullet Jacket Sample: jacket-01 X X X Cement Control

Sample: cement-01 X X X Flat-head Control Sample: flat-011 X X X

32

Figure 10. Elemental Composition Comparison of Cement and Cement Control, Prominent Pb and Ba Peaks

The flat head screwdriver spectral data did not have any of the lead or barium

lines; therefore use of the flat head to chip away pieces of the cement did not contribute

to the lead and barium peaks detected in the cement sample. However, since the cement

control contained the same prominent spectral lines lead peak at 280.19 and barium lines

at 455.40, 493.41, and 553.55 nm as some of the cement samples , the barium lines in

particular detected in sample 2, 3, and 5 cannot be used to indicate the occurrence of

bullet wipe. As shown in the graph overlay of a cement sample and the cement control in

Figure 10, it is difficult to differentiate between contribution of spectral barium lines

from the cement block itself and the contact of the bullet with the surface of the cement.

The spectral data looks identical in comparison aside from varied peak intensity.


33

Wood Samples

The spectral data from the wood samples showed a much more consistent trend in

lead and barium detection in comparison to the other samples analyzed. Wood Samples 1

through 5 were shot from a distance of 1, 3, 6, 9, and 12’. Indicated in Table 6 and 7,

sample 1 of the wood sampled contained all of the prominent lead peaks at 280.19,

283.31, 363.96, 368.35, and 405.78 nm and all of the prominent barium peaks at 455.40,

493.41, and 553.55 nm. Samples 2 and 4, shot from a distance of 3 and 9’, contained all

of the prominent lead and barium peaks except for one lead peak at 283.31 nm. Sample 3

of the wood samples, shot from a distance of 6’, contained all of the prominent lead and

barium peaks except for the lead lines at 280.2 and 283.31 nm. The only sample of the

wood samples that did not contain all of the primary barium spectral lines was sample 5,

which was shot from a distance of 12’. Sample 5 only contained the barium spectral peak

of 553.55 nm.

The wood samples were also compared to control samples in order to

determine whether barium and lead peaks were from bullet wipe or from the elemental

composition of the control material. All of the samples contained at least 3 of the 5

prominent lead lines that were detected in the Pb-SWC bullet control spectra. The wood

sample spectra were also compared to a box-cutter control and a wood control. In

comparison of the wood sample spectra to the box cutter, matching peaks were found at

280.2 nm for lead and 455.40, 493.41, and 553.55 nm for barium. From these results, it

was determined that the lead line shown at 280.2 nm in the wood samples could have

come from the box cutter used to cut pieces of the wood and not from bullet wipe. The

34

spectral results from the wood control only showed a matching peak at the barium line of

553.55 nm, as depicted in Figure 11. This barium line was found in both the Pb-SWC

bullet control, wood control, and the box cutter so bullet wipe and elemental composition

of the sample and controls cannot be differentiated based on this barium spectral line

alone.

Table 6

Lead Spectral Lines of Wood Samples and Controls

Pb Lines Wavelength 280.02nm 283.31nm 363.96 nm 368.35nm 405.78nm

T-shirt Samples Sample 1 (1’) X X X X X Sample 2 (3’) X X X X Sample 3 (6’) X X X Sample 4 (9’) X X X X Sample 5 (12’) X X X X X

PbSWC Bullet Head

Sample: head-01 X X X X X PbSWC

Bullet Jacket

Sample: jacket-01

X X X

Wood Control Sample:

wood-011

Box-cutter Control

Sample: box-011

X

35

Table 7

Barium Spectral Lines of Wood Samples and Controls


T-shirt Samples Sample 6 (1’) X X X Sample 7 (3’) X X X Sample 8 (6’) X X X Sample 9 (9’) X X X

Sample 10 (12’) X PbSWC Bullet Head

Sample: head-01 X X X PbSWC Bullet Jacket

Sample: jacket-01 X X X Wood Control

Sample: wood-011 X Box-cutter Control Sample: box-011 X X X

36

Figure 11. Elemental Composition Comparison of Wood Sample and Wood Control,

Prominent Pb and Ba Peaks

Drywall Samples

The drywall samples were shot from a distance of 1’, 3’, 6’, 9’, and 12’, analyzed,

and then compared to their perspective control samples. The spectra from the drywall

sample, shot from a distance of 1’, indicated the presence of lead at peaks 363.96, 368.35,

and 405.78 nm and all 3 of the primary barium peaks at 455.40, 493.41, and 553.56 nm.

Drywall sample 2 and sample 4 were shot from a distance of 3 and 9’ and both of their

spectra indicated the same spectra lines as drywall sample 1 except it did not have the

lead spectral line at 363.957 nm. The spectral data retrieved for drywall sample three did

not give the expected results. Drywall sample 3, which was shot from 6’, only showed the


37

presence of 1 barium peak at 553.55 nm. The collection of sample 3 may have been

inadequate or there may not have been enough contact of the bullet with the surface area

of the drywall to detect bullet wipe. Sample 5 of the drywall samples only contained the

primary peaks of barium at 455.50, 493.41, and 553.55 nm. These spectral results did not

appear to portray a descending or ascending pattern of detection of bullet wipe.

The drywall samples were compared to both the paper and chalky, white gypsum

material drywall controls, as shown in Figure 12, and a box cutter control. The spectra of

the drywall paper control did not indicate the presence of any of the primary lead and

barium peaks. The lack of detection of lead and barium peaks in the drywall control

demonstrates that the elemental composition did not contribute to the presence of lead or

barium spectral lines in the experimental drywall samples. The other drywall control,

which consists of the white, chalky material, indicated the barium spectral line of 553.55

nm as shown in Table 8 and 9. The box cutter control showed similar peaks to the

drywall spectra at lines 455.40, 493.41, and 553.55 nm. The drywall samples, box cutter,

and drywall controls all indicated the presence of barium at 553.55 nm; therefore this

peak would not be used as a differentiating peak to identify bullet wipe on these drywall

samples. Also, since the box cutter contained the primary barium peaks of 455.40 and

493.41 nm, it was difficult to discern bullet wipe from the use of the box cutter as the

main contributor of these peaks.

38

Table 8

Lead Spectral Lines of Drywall Samples and Controls

Pb LinesWavelength 2

80.02nm 2

83.31nm 36

3.96 nm 3

68.35nm 4

05.78nm T-shirt Samples

Sample 1 (1’) X X XSample 2 (3’) X XSample 3 (6’) Sample 4 (9’) X XSample 5 (12’)

PbSWC Bullet Head

Sample: head-01 X X X X XPbSWC

Bullet Jacket

Sample: jacket-01

X X X

T-shirt Control Sample:

T-shirt-01

Cardboard Control Sample:

Cardboard-01 X

39

Table 9

Barium Spectral Lines of Drywall Samples and Controls


T-shirt Samples Sample 1 (1’) X X X Sample 2 (3’) X X X Sample 3 (6’) X Sample 4 (9’) X X X Sample 5 (12’) X X

PbSWC Bullet Head Sample: head-01 X X X

PbSWC Bullet Jacket Sample: jacket-01 X X X Drywall Control

Sample: paper-01 Drywall Control

Sample: drywall 2, w. X Cardboard Control

Sample: Cardboard-01 X X X *Yellow area: Spectral line may be due to background noise.

40

Figure 12. Elemental Composition Comparison of Drywall Sample and Control, Prominent Ba and Pb Peaks


41

CONCLUSION

From the spectral data generated from the T-shirt, cement, wood, and drywall

samples, it was determined that LIBS could be applied as a presumptive test for the

detection of bullet wipe for certain materials. The data generated for the T-shirt samples

overall indicated that barium is detected heavily in close range and lead is detected more

heavily at a longer range. The spectral data demonstrated that bullet wipe could be

detected at distances ranging from 1” to 12’.

From analysis of the cement sample, it was determined that cement contains

barium as one of its main components. The presence of barium in spectra cannot be used

as a definitive indication of bullet wipe since it is an elemental component of cement. The

spectral data generated from the analysis of cement was the most complex of all the

spectral data produced among all the samples analyzed. Further analysis of the cement

samples could consist of recording other trace elements.

The wood samples produced spectral data that gave an overall indication that

bullet wipe could be detected on wood material at distances ranging from 1’ to 12’.

Although a distinct pattern was not seen for the five samples, the wood samples in

general showed indications of bullet wipe based on its comparison to the wood control,

box cutter control, and Pb-SWC controls. The only spectral data that was not applicable

for the detection of bullet wipe was the barium peak at 553.548 nm. This peak, as

42

previously discussed, appeared in the spectral data of the wood samples, wood control,

Pb-SWC controls, and the box-cutter control, eliminating this peak as an integral part of

bullet wipe detection.

All of the drywall samples contained prominent barium lines; however the lead

lines were not prominent in the spectral data. The only lead lines that appeared in the

spectral data were at 363 and 368 nm. Since the box cutter was used for sample collection

and contained all of the prominent barium lines at 455, 493, and 553 nm, it was difficult

to determine whether the drywall or the box-cutter contributed to those peaks. Lead lines

would have to be present in a sample in order to indicate the presence of bullet wipe.

Overall, barium was the most prominent peak of existence in the spectral data of

the drywall samples. Barium lines were detected in all of the drywall samples at 455.403,

493.408, and 553.548 nm. Lead spectral lines were the least consistent in the drywall

sample spectra, however the presence of lead lines in the spectra would have to present

along with barium lines to indicate the presence of bullet wipe since the barium spectral

lines are not only present in the drywall sample spectra, but also in the drywall spectral

data.

After analysis of all the samples, controls, and tools used, it was determined that

the barium line at 553.548 nm was the most common amongst all of their spectral data.

Many of the tools used for collection of the samples contained barium, which made it

difficult to distinguish between bullet wipe and the tools as the contributor of the barium

lines. These factors would have to be further researched by comparing the relative

intensities of the peaks to each other to identify any distinct difference between them or

43

determining the probability that the tools used would have a grave effect on the elements

detected in the sample spectra and what percentage of contribution that would be.

44

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