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Forensic science is broadly defined as the application of science to matters of the law. Practitioners typically use multidisciplinary scientific techniques for the analysis of physical evidence in an attempt to establish or exclude an association between a suspect and the scene of a crime.
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Chapter 14
Forensic Applications of LIBS
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Richard R. Hark and Lucille J. East
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Forensic science is broadly defined as the application of science to matters of the
law. Practitioners typically use multidisciplinary scientific techniques for the
analysis of physical evidence in an attempt to establish or exclude an association
between a suspect and the scene of a crime. A wide variety of analytical methods
have been used for identifying a sample and for determining whether two or more
objects have a common origin. One of these techniques, laser-induced breakdown
spectroscopy (LIBS), is a versatile, comparatively low-cost adaptation of atomic
emission spectroscopy that has been applied successfully to the forensic analysis
of counterfeit currency, drugs, explosives, fingerprints, ink and paper, glass,
gunshot residue, hair, paint, soil, and wood. The method requires minimal sample
preparation, can simultaneously provide information on major, minor and trace
element composition, and has the potential to be used in the field in real time.
This chapter includes a brief overview of the kinds of evidence typically
encountered in forensic investigations and the rules associated with introducing
such evidence in a court of law (e.g., in the U.S. the Frye or Daubert standards
apply, but similar rules are extant in other countries). The types of physical evi-
dence that have already been analysed using LIBS, including examples from actual
casework, are reviewed. The relative advantages and disadvantages of the LIBS
technique for forensic analysis are presented, especially in comparison to other
analytical methods. The analysis of soil, explosives, nuclear and biological
materials is covered in detail in other chapters; therefore, forensic applications of
LIBS for these materials are mentioned only briefly. The last section outlines the
additional research and other steps required to more fully realize the potential of
LIBS for forensic applications.
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R. R. Hark (&)
Department of Chemistry, Juniata College, 1700 Moore Street, Huntingdon,
PA 16652, USA
e-mail: hark@juniata.edu
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L. J. East
Applied Spectra, Inc., 46661 Fremont Boulevard, Fremont,
CA 94538, USA
e-mail: ljeast@appliedspectra.com
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14.1
Forensic Analysis of Physical Evidence
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In criminal and civil law scientific evidence is often used to indirectly prove a fact
that is relevant to the case being tried. In many instances, there is no direct
evidence (e.g., an eyewitness account of the crime), so this is the only type of
proof available. When properly collected, appropriately analyzed and the results
correctly interpreted by a knowledgeable expert, physical evidence provides
trustworthy and objective facts about the matter under investigation. For example,
it may allow an inferential connection to be made between an individual or items
belonging to that person, and a specific location and time when a crime was
committed. The accumulation of several pieces of this type of circumstantial
evidence, which all point to same conclusion, can serve to prove beyond a rea-
sonable doubt the guilt or innocence of a suspect. The literature on the subject of
forensic analysis of physical evidence is substantial and is described in numerous
general [18] and specialized books [912].
A basic premise of forensic investigation is Locard’s Exchange Principle,
which states that a cross-transfer of materials takes place when a person or
object comes in contact with another person or object [2]. The well-known
criminalist
Paul Kirk explained why this concept is so vital in connecting a
suspect with a crime scene:
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Wherever he steps, whatever he touches, whatever he leaves, even unconsciously, will
serve as silent evidence against him. Not only his fingerprints or his footprints, but his hair,
the fibers from his clothes, the glass he breaks, the tool mark he leaves, the paint he
scratches, the blood or semen he deposits or collectsall these and more bear mute
witness against him. This is evidence that does not forget. It is not confused by the
excitement of the moment. It is not absent because human witnesses are. It is factual
evidence. Physical evidence cannot be wrong; it cannot perjure itself; it cannot be wholly
absent. Only its interpretation can err. Only human failure to find it, study and understand
it, can diminish its value [13].
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14.1.1
Purposes for Analyzing Forensic Evidence
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The analysis of physical evidence is performed for the purpose of identification or
comparison. The results of forensic analyses have both investigative and eviden-
tiary value, with the standards required for the latter being more rigorous as they
must stand up to scrutiny in a court case. Presumptive tests, such as the use of
Marquis Reagent for the identification of opium alkaloids, are often non-specific
but can used in the field to rapidly assign a material to a given class of substances
[14, 15]. The results of this kind of wet chemistry test must be validated by
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confirmatory analysis using, for example, mass spectrometry or infrared spec-
troscopy in a crime laboratory.
Establishing the identity of a material with a high degree of certainty requires
that a testing protocol first be developed using standard reference materials. The
results of one or more chemical or spectroscopic tests ideally should allow for the
unambiguous identification of a substance to the exclusion of all other possibilities.
Application of the same analytical scheme to an unknown material should permit a
forensic scientist to provide a positive identification, assuming that contaminants
associated with the environment from which the sample was collected do not
interfere with the results.
A comparison analysis seeks to determine if two samples have a common
origin. When two or more samples can be said to have originated from the same
source with confidence, then the evidence is said to possess individual charac-
teristics [2]. Individuation of evidence is a goal of paramount importance in
forensic investigation but this can be difficult to achieve in practice. The concept is
based on the following hypothesis: the detection of distinctive physical features or
uncommon combinations of trace elements by analytical methods with sufficient
discriminating power allows very similar samples to be distinguished with some
level of likelihood. Since it is not possible to verify exhaustively and empirically
that the chemical and/or physical profiles of two pieces of evidence match only
each other and no other object, forensic analysis must be interpreted on the basis of
statistical probability [16, 17]. A familiar application of this approach involves the
comparison of a sample of DNA found at a crime scene and that obtained from a
suspect [18]. A number of DNA regions or loci called short tandem repeats (STRs)
that are known to have the greatest variability among humans are sequenced for
each sample and the match probability is calculated by multiplying the occurrence
frequency of the genetic markers (the product rule).
Evidence that can be assigned with confidence to a group but not to a single
source possesses class characteristics [2]. Most physical evidence obtained from
crime scenes falls into this category. A single piece of matching class evidence
may provide a relatively weak link between a suspect and a crime scene, but the
presence of multiple types of class evidence can be very compelling.
Individual and class characteristics lie along a continuum based on the prob-
ability of class membership. For example, analysis of a suspicious white powder
contained in a letter that was mailed to a public official may confirm with a high
degree of certainty that it is talcum powder (class characteristic). The variety and
uniqueness of the components present in commercially available talcum powder
products would determine how difficult it is to match the evidence with a given
brand of talcum powder. Trying to connect the white powder with a specific bottle
of talcum powder found in a suspect’s home (individual characteristic) would be
the most challenging task of all. The chances that the talcum powder came from
the suspected source are increased if the identity and levels of trace elements are
identical in both samples, but this still does not guarantee a 100 % match prob-
ability. Such a result is one piece of data that must be evaluated in conjunction
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with other factors (fingerprints and other types of physical evidence, motive, etc.)
in order to draw conclusions about guilt or innocence.
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14.1.2
Types of Forensic Evidence
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While it is not possible to list every type of physical evidence that forensic sci-
entists may be asked to analyze, certain categories of materials are commonly
encountered at crime scenes; these include [2]
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1.
biological samples (blood, body organs, hair, fingernails, pollen, saliva, semen
and skin)
2.
documents (ink, toner and paper)
3.
drugs and their synthetic precursors (prescription and illicit)
4.
explosives and associated post-blast residues
5.
fibers (natural and synthetic)
6.
fingerprints (visible and latent)
7.
firearms, ammunition and powders associated with the discharge of a firearm
8.
glas
s
9.
impressions (tire markings, shoeprints and bite marks)
10.
paint
11.
petroleum products (accelerants and fire debris)
12.
plastic, rubber, and other polymers (bags and tape)
13.
serial numbers and tool marks
14.
soil and minerals
15.
wood and other vegetative matter.
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The quantity of a piece of evidence and the complexity of the matrix in which it
is found can vary widely. Trace evidence is the term used to refer to samples that
are normally examined with a microscope. Proper collection of samples and
subsequent sample preparation is vital, especially when dealing with very small
quantities. Though traditional techniques suitable for macroscopic samples (liquid-
liquid extraction, solid-phase extraction, and modifications of the purge-and-trap
approach) and microsample preparation methods (solid-phase and liquid-phase
microextraction) are commonly used, analytical methodology that does not require
any sample preparation is considered ideal [19]. With all forensic evidence it is
also essential that the collection, storage, transfer, analysis, and disposition of
evidence is monitored carefully to maintain the integrity of the chain of custody at
all times.
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14.1.3
Rules for the Admissibility of Forensic Evidence
in Court: The Frye and Daubert Standards
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The utility of any analytical technique is gauged by certain operational perfor-
mance characteristics. These empirically derived figures of merit typically include
factors such as accuracy, precision, sensitivity, selectivity, and dynamic range. The
applicability of a particular method for the analysis of an item of physical evidence
is determined by its figures of merit. For example, measuring the index of
refraction of two samples of glass is often sufficient to verify that they do not come
from the same source. However, elemental analysis using micro-X-ray fluores-
cence spectroscopy (XRF) or laser ablation-inductively coupled plasma-mass
spectrometry (LA-ICP-MS) may be necessary to provide sufficient discrimination
between very similar samples, with the number of elements analyzed, the limit of
detection (LOD), and the precision of the measurements all being important fac-
tors in the quality of the results [20].
The development of new analytical technologies is constantly providing
potential tools that can be applied to forensic investigations. In order to ensure due
process under the law, rules have been developed for the admissibility of scientific
evidence and expert testimony in court. In 1923, the case of Frye v. United States
[21] addressed the issue of admissibility of polygraph evidence. The U.S. Federal
Court of Appeals determined that polygraph testing could not be admitted as
evidence because its reliability was not broadly accepted in the scientific com-
munity. This landmark case established the Frye standard, which requires that a
scientific technique ‘must be sufficiently established to have gained general
acceptance in the particular field in which it belongs [21].
Though the Frye standard is still used in some states in the U.S. (California,
Illinois, Kansas, Maryland, Minnesota, New Jersey, New York, Pennsylvania, and
Washington) most jurisdictions follow the Daubert standard. In Daubert v.
Merrell Dow Pharmaceuticals, Inc., the Supreme Court ruled that ‘the Federal
Rules of Evidence, and not Frye, provide the standard for admitting expert sci-
entific testimony in a federal trial [22]. Rule 702, which governs expert testi-
mony, does not imply‘that ‘general acceptance’ is a necessary precondition to the
admissibility of scientific evidence’’ [23]. Daubert tasks the trial judge to serve as
a ‘gatekeeperto determine whether the testimony of an expert witness incor-
porates scientifically valid methodology and reasoning that is relevant to the
questions being addressed in the case. This occurs prior to start of a trial and takes
the form of a Daubert hearing. The Court outlined five non-exclusive criteria that
could be considered in determining the admissibility of expert testimony:
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1.
Whether the theory or technique can be (and has been) tested.
2.
Whether the theory or technique has been subject to peer review and publication.
3.
The technique’s known or potential rate of error.
4.
The existence and maintenance of standards for the control of the technique’s
operation.
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5.
The general acceptance of the theory or technique by the relevant scientific
community [22] (the only criterion that was relevant under Frye).
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The Daubert standard subsumes Frye and allows for a more liberal interpre-
tation of what evidence is admissible. The Court recognized that ‘widespread
acceptance can be an important factor in ruling particular evidence admissible
and that techniques that are known to the scientific community but garner only
minimal support are appropriately viewed with incredulity. However, it was also
noted that
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‘publication (which is but one element of peer review) is not a sine qua non of admis-
sibility; it does not necessarily correlate with reliabilitysince ‘well-grounded but
innovative theories [may] not have been published. Some propositions, moreover, are too
particular, too new, or of too limited interest to be published. But submission to the
scrutiny of the scientific community is a component of ‘good science’, in part because it
increases the likelihood that substantive flaws in methodology will be detected. The fact of
publication (or lack thereof) in a peer reviewed journal thus will be a relevant, though not
dispositive, consideration in assessing the scientific validity of a particular technique or
methodology on which an opinion is premised’ [22].
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Though the foregoing discussion briefly outlined the standard for admissibility
of forensic evidence in the United States, the Daubert standard has influenced the
creation or revision of similar rules in other countries.
The provisions of the Daubert standard not only establish guidelines for the
introduction of new methods of forensic analysis, they also form the basis for
challenging some long-accepted techniques in court [24]. Many of these chal-
lenges relate to the fact that for certain sub-disciplines of forensic science (e.g.,
fingerprint, bite and tool mark analysis) the‘technique’s known or potential rate
of erroris not well established. In 2009, a report by the National Research
Council made numerous recommendations for strengthening the practice of
forensic science in the United States [25]. Consequently, efforts are being made to
place all areas of forensic analysis on a more scientifically sound footing (e.g.,
[26]). In 2013, it was announced that a National Commission on Forensic Science
will be created to draft proposals for the U.S. attorney general and the Justice
Department [27]. This action is especially important as public expectations of
forensic science have become exaggerated and distorted. The so-called ‘CSI
Effect’ refers to the influence that the popular television program and similar
shows have in raising expectations about forensic capabilities and reliability
beyond what is realistically possible [28].
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14.1.4
Analytical Techniques for Physical Evidence
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The majority of analytical techniques used for analysis of physical evidence in
over 400 crime laboratories in the United States have been borrowed from other
scientific disciplines and adapted for forensic purposes [10]. Examples of some of
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Table 14.1 Analytical techniques commonly used by forensic scientists for investigation of
physical evidence
General technique type Examples
Microscopic analysis Atomic force microscopy (AFM)
Optical microscopy
Polarized light microscopy (PLM)
Scanning electron microscopy (SEM)
Transmission electron microscopy (TEM)
Elemental analysis Atomic absorption (AA) and atomic emission spectroscopy (AES)
X-ray fluorescence spectroscopy
Particle-induced X-ray emission (PIXE)
Inductively coupled plasma-mass spectrometry (ICP-MS)
Laser ablation-inductively coupled plasma-mass spectrometry (LA-
ICP-MS)
Neutron activation analysis (NAA)
Mass spectrometry Desorption electrospray ionization mass spectrometry (DESI)
Direct analysis in real time mass spectrometry (DART)
Ion mobility spectrometry (IMS)
Isotope ratio mass spectrometry (IRMS)
Matrix-assisted laser desorption ionization time-of-light mass
spectrometry (MALDI-TOF)
Secondary ion mass spectrometry (SIMS)
Molecular spectroscopy Fluorescence-spectroscopy
Fourier transform infrared spectroscopy (FTIR)
Nuclear magnetic resonance spectroscopy (NMR)
Raman spectroscopy
Ultraviolet/visible spectroscopy (UV/Vis)
X-
ray diffraction (XRD)
Separation techniques Capillary electrophoresis (CE)
Gas chromatography (GC)
Ion chromatography (IC)
Liquid chromatography (LC)
Paper chromatography
Thin layer chromatography (TLC)
Thermal analysis Differential thermal analysis (DTA)
Differential scanning calorimetry (DSC)
Pyrolysis gas chromatography (PGC)
Thermogravimetric analysis (TGA)
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the common tools utilized by the forensic scientist for the chemical and physical
analysis of evidence are listed in Table 14.1. Additional methods, as well as
various hyphenated techniques, have also been developed; thus, the list should not
be considered exhaustive.
Standard operating procedures (SOPs) have been established for processing
different types of evidence, many of which include the use of more than one
technique. In some cases, the approach involves comparing the spectrum, chro-
matogram, or other result generated from a piece of evidence against a
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computerized library built from samples of known origin. Forensic databases exist
for automobile paint (PDQ), ballistics (NIBIS), DNA (CODIS), fingerprints (IA-
FIS), glass, and shoeprints [2, 29]. In addition to the instrumental techniques listed
in Table 14.1, forensic chemistry also encompasses the use of a wide variety of
wet chemical methods such as color- or fluorescence-based presumptive tests for
the identification of blood, drugs, or semen at crime scenes [30] and the use of
chemical reagents for the visualization of latent fingerprints [31].
Though there are many approaches available for forensic analysis of physical
evidence, no single method does everything equally well. There is an ongoing need
to develop new forensic tools that balance the desire to optimize analytical figures
of merit with other factors such as cost per analysis, ease of use, instrument
portability, amount of sample consumed, sample preparation time, and throughput.
For example, a typical crime scene may provide dozens of pieces of trace evidence
that ideally should be analyzed to determine their elemental composition [7, 32,
33]. However, two problems that crime laboratories face as they attempt to process
a high volume of evidence are limited personnel and financial resources. Scanning
electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS), a typical
method for elemental analysis, is relatively time-consuming and consequently has
low sample throughput. Although laser ablation-inductively coupled plasma-mass
spectrometry is comparatively fast and gives excellent quantitative results, the cost
of outfitting a crime laboratory to do LA-ICP-MS can be prohibitively expensive.
An affordable analytical approach that rapidly delivers dependable results could
therefore find use in the forensic scientists’ toolbox. Laser-induced breakdown
spectroscopy (LIBS) is one technique with significant promise to meet these cri-
teria [34, 35].
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14.1.5
LIBS: An Emerging Tool for the Forensic
Community
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Laser-induced breakdown spectroscopy (LIBS) is a laser ablation atomic emission
technique that has been applied successfully to the elemental analysis of a wide
variety of materials, including items of specific forensic interest [3645]. In LIBS,
a pulsed laser generates a microplasma on the surface of a target, and a spec-
trometer (e.g., Czerny-Turner, echelle) coupled with a sensitive detector (e.g.,
CCD, ICCD, PMT) analyzes the light that is emitted as the plasma cools. A
representative LIBS spectrum of float glass showing atomic and ionic emission
lines used for quantitative analysis is shown in Fig. 14.1. Simultaneous identifi-
cation and quantification is possible because all elements have one or more
emission lines in the range from 190 to 900 nm, and emission intensity is pro-
portional to the concentration of the emitting species. LIBS experiments can
therefore provide quantitative data after calibration using standard reference
materials or through the use of a ‘‘calibration free approach (CF-LIBS) [46].
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Fig. 14.1 Representative LIBS spectrum of float glass resulting from an average of ten single-
pulse spectra taken at a single location. The spectrum was obtained in an Ar atmosphere using a
commercial LIBS instrument consisting of a broadband spectrometer and a Nd:YAG laser
operated at 1064 nm. Selected atomic and ionic emission lines utilized for forensic comparison
analysis are shown (modified from [47])
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Development of effective sampling methodology and optimization of instrumental
parameters is important to provide high quality results. Creation of robust libraries
of sample data combined with the application of multivariate chemometric
methods, such as Principal Component Analysis (PCA) and Partial Least Squares
Discriminant Analysis (PLSDA), provides the means to identify unknown mate-
rials with a high degree of discrimination based on the uniqueness of the spectral
fingerprint. LIBS can therefore be used to identify class and individual charac-
teristics of evidence.
LIBS instrumentation is relatively inexpensive (~$50,000 to $100,000), has
been ruggedized for use in industrial settings and harsh environments, and is
adaptable for direct or stand-off analysis. The method is straightforward, rapid
(seconds per analysis), inherently sensitive (low ppm range), requires little or no
sample preparation, consumes only small amounts of sample (ng), is capable of
stratigraphic profiling, and is amenable to use in the field. Although portable LIBS
systems have been described in the literature since at least the mid-nineties [48,
49], it is only recently that commercial units have become available. In what could
be called an historic example of ‘extraterrestrial forensic investigation’ a LIBS
instrument on the Curiosity rover is currently being used to look for evidence that
Mars may have had an atmosphere that was capable of supporting life [50]. LIBS
has even been featured on at least one popular television crime show [51].
LIBS compares favorably with other elemental analysis techniques listed in
Table 14.1 and offers several important advantages for forensic analysis of trace
evidence.
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1.
LIBS allows for simultaneous analysis of all elements in the periodic table with
a single laser pulse, including elements of low atomic mass that are not
accessible by some other analytical techniques.
2.
LIBS instrumentation is less expensive to acquire and costs less to operate than
many other techniques [52].
3.
The LIBS technique is comparatively easy to learn and is even amenable for
introduction in undergraduate laboratory courses [53, 54].
4.
Hyphenated techniques involving LIBS and other orthogonal spectroscopic
methods such as laser-induced fluorescence (LIF) [5557] and Raman spec-
troscopy [9, 5860] have been developed, as these techniques share many of the
same instrumental components.
5.
LIBS instrumentation can be miniaturized and ruggedized so that portable
systems [48, 49] can be taken into the field for analysis of evidence at or near a
crime scene.
6.
LIBS can be used to analyze solids directly without the need to solubilize the
sample.
7.
LIBS provides high spatial resolution (10s to 100s of microns) so that even
minute samples can be analyzed. Stratigraphic analysis of layered materials
such as automobile paints chips is also possible, as a crater forms that pro-
gressively bores down into a sample with successive laser pulses [61, 62].
8.
As only a small amount of material is ablated with each laser pulse, the technique
is micro-destructive and allows for analyses to be repeated to verify the results.
9.
The LIBS method is very fast; a trained operator could reasonably be expected
to process 1020 samples in an hour.
10.
LIBS systems could be designed with autosampling capabilities to allow for
high throughput analysis of certain types of evidence, though this approach
has not yet been applied to forensic evidence [63].
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LIBS has many important benefits but, as with all analytical methods, there are
also drawbacks that must be considered when evaluating it as a tool for forensic
analysis [64].
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1.
The shot-to-shot variability of LIBS data often leads to levels of precision
(generally 5–20 % RSD) that are lower than some established methods used for
forensic analysis. There are a variety of reasons that can account for the
inconsistency of LIBS data.
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a. Instrumental—The Nd:YAG lasers that are typically used in LIBS systems
have uneven distribution of energy in each pulse, leading to differential cou-
pling of the laser energy to the sample surface from one shot to the next. Lasers
that operate on the femtosecond time scale do not experience this problem, but
the cost of such components is prohibitively high at the present time.
b. Matrix effectsSample inhomogeneities and differences in physical prop-
erties, such as reflectivity and hardness of the surface, lead to variations in the
line intensities when multiple spectra are collected from the same sample.
This effect has to do with the coupling between the laser pulse and the surface
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of the material. Because LIBS is a surface analysis technique, deposits such
as oxidation layers or fingerprint residue can interfere with the determination
of the underlying bulk composition. To address these issues several laser
pulses are often used to clean the sample surface before collecting tens or
hundreds of spectra distributed in a grid pattern. The problems associated
with poor precision can be diminished by data normalization [65] or by
ensemble averaging [66].
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2.
The accuracy of a LIBS measurement is compromised to a certain degree by the
composition of the chemical matrix. The emission intensity of one element is
influenced by the presence of other elements, which means that an element
present in equal concentration in two different samples will exhibit different
LIBS emission intensities [67]. It is therefore necessary to utilize standards or a
calibration-free approach [46] to obtain quantitative results. However, this
phenomenon can actually be an asset when attempting to match two samples to
determine if they have a common origin.
3.
Spectra from the same sample acquired on different LIBS instruments cannot be
assumed to match each other exactly; thus, a spectral library created using one
LIBS system is not necessarily transferable to another LIBS instrument. The
intensity of emission lines depends on the specific system configuration and the
components used (laser, spectrometer, detector, optics). It may be possible to
address this issue by employing calibration standards and an appropriate con-
version algorithm.
4.
As the sensitivity for LIBS is not as high as for some techniques, such as (e.g.,
LA-ICP-MS) it may not be possible to discriminate with high certainty between
similar samples. This limitation is attributable to self-absorption, spectral
interferences and emission line overlap, and the fact that elements with high
ionization potentials (e.g., F, Cl, and S) have inherently higher LODs.
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14.2 Forensic Applications of LIBS
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A number of researchers have recognized the advantages of using LIBS for
forensic analysis of physical evidence for investigative and evidentiary purposes.
The applications described in this section represent both preliminary efforts and in-
depth studies with the larger numbers of samples and more sophisticated error
analysis that is required for LIBS to stand up to the Frye and Daubert standards.
The few instances in which LIBS has played a role in a criminal investigation or a
court case are highlighted.
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14.2.1
Glass
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Glass can be an important source of evidence and may allow investigators to
connect a suspect (or an automobile) with the scene of a burglary or a hit and run
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accident. Glass fragments from building windows, beverage containers, automo-
bile windows, headlights or mirrors, for example, are collected and evaluated in
the forensic laboratory. These samples can range from ordinary ‘‘soft’(soda lime)
glass to float glass (manufactured by floating molten glass on a bed of liquid tin),
tempered glass (treated by appropriate heating and cooling so that it shatters in
small pieces) or laminated glass (a plastic sheet sandwiched between two layers of
tempered glass; required for automobile windshields in the US). Obvious physical
properties such as color, thickness and texture can be observed visually. Tradi-
tionally, because glass breaks randomly, attempts are made to piece together
fragments. Often, however, fragments are too small or numerous to be able to
match them up (tempered glass, for example, shatters into very small pieces), and
the traditional physical techniques of measuring density, refractive index (bending
of light as it traverses the glass) [68] and dispersion (variation in the degree of
bending with wavelength of light) are used. Given the uniformity of current
technology for producing plate glass, it has become difficult to individuate glass
samples from using these methods.
Though glass is composed primarily of silica (SiO2), sodium oxide (Na2O), lime
(CaO), and other additives (e.g., MgO, Al2O3), material from different batches may
vary in composition of trace elements. Various chemical methods, including spark
source mass spectrometry [69], atomic emission spectroscopy (AES) [70, 71],
atomic absorption spectroscopy (AA) [72], neutron activation analysis (NAA)
[73, 74], XRF [71, 72], particle-induced X-ray emission spectroscopy (PIXE) [75],
SEM-EDS [75], inductively coupled plasma atomic emission spectrometry (ICP-
AES) [71, 72, 76], inductively coupled plasma mass spectrometry (ICP-MS) [77],
laser ablation inductively coupled plasma optical emission spectrometry (LA-ICP-
OES) [78], and LA-ICP-MS [79, 80] have been explored for discrimination of glass
samples. LA-ICP-MS could be considered the‘gold standardfor glass analysis as
it is minimally destructive, very sensitive, and requires no sample preparation. For
example, LA-ICP-MS analysis of 91 automobile glass samples allowed 99.3 % of
the 1,035 possible pairs to be distinguished [81].
There have been several reports on the use of LIBS to analyze glass (e.g., [82
84]) but a 2006 paper by Bridge et al. was the first to focus on forensic glass
analysis [47]. The performance of LIBS was compared with LA-ICP-MS for the
analysis of 23 automobile float glass samples. A Q-switched Nd:YAG laser
operating at 1064 nm and a broadband spectrometer were used to collect LIBS
spectra in an Ar atmosphere. Emission lines for Al, Ba, Ca, Cr, Fe, Mg, Na, Sn, Si,
and Ti, which were spectrally isolated and had no obvious shoulders or overlap-
ping peaks, were utilized. LA-ICP-MS data was gathered using 213-nm laser
ablation and typical experimental parameters. Ten averaged elemental ratios
obtained with LIBS and isotopic ratios provided by LA-ICP-MS, along with
refractive index (RI) values, were used to create discrimination matrices. The
combinations of RI with LIBS or LA-ICP-MS were able to distinguish
[96
% of
the samples at the 99 % confidence interval (CI).
In a follow-up study, LIBS spectra for disparate glass samples (automobile
headlight glass, brown beverage glass and automobile window float glass) were
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Fig. 14.2 LIBS spectra
taken from three different
types of glass: a automobile
headlamp glass, b brown
beverage container glass and
c automobile side window
float glass, spectrum taken on
the float side of the glass.
Each spectrum is the average
of ten single-pulse spectra
taken at one location on the
glass surface [85]
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shown to have recognizable differences (Fig. 14.2) [61, 85]. However, as shown in
Fig. 14.3, the LIBS spectra for float glass from three different makes of automobile
were not readily distinguishable by visual inspection. The use of chemometric
analysis (ANOVA and Tukey Honestly Significant Difference test) showed that a
combination of RI and LA-ICP-MS data gave an overall discrimination of 98.8 %,
whereas RI and LIBS data gave a somewhat lower discrimination of 87.2 % for 91
samples (1,122 pairwise comparisons). The precision of the LIBS results was
evaluated by repetitive analysis of a NIST SRM 610 glass standard [85]. The
relative standard deviation (%RSD) for emission peak intensity ratios on any