Applications of Vibrational Spectroscopy in Criminal Forensic
Edward G. Bartick
Handbook of Vibrational Spectroscopy John M. Chalmers and Peter R. Griffiths (Editors)  John Wiley & Sons Ltd, Chichester, 2002 Applications of Vibrational Spectroscopy in Criminal
Forensic Analysis

Edward G. Bartick
FBI Academy, Quantico, VA, USA
This is publication number 01-06 of the Laboratory Division of the Federal Bureau of Investigation. Names of commercialmanufacturers are provided for identification only and inclusion does not imply endorsement by the Federal Bureau ofInvestigation. INTRODUCTION TO FORENSIC
Individual characteristics are properties of evidence that canbe attributed to a common source with an extremely high ANALYSIS
degree of certainty. Class characteristics are properties ofevidence that can only be associated with a group and never Sir Arthur Conan Doyle is believed by many to have first popularized the application of forensic analysis through hisfictional character Sherlock Holmes, originally published Fingerprint and DNA evidence are accepted as having in 1887. This work is thought to have inspired many of individual characteristics. However, fibers or copy toners the early forensic scientists. One of these was Frenchman are identified by their class characteristics and, because Edmond Locard, who proposed that when two objects come of the large production of these materials, they cannot be into contact with one another, a cross-transfer of evidence individualized. The strength of fiber evidence depends on occurs.1,2 This statement is known as Locard’s Exchange the likelihood of those same type of fibers being randomly Principle and is the foundation for use of physical evidence located on the suspect. Common fibers such as blue or white to link or at least associate a suspect to a crime scene or cotton from jeans or shirts, respectively, have little eviden- a victim. Depending on the nature of the evidence, a wide tial value. But blue nylon-6,6 fibers with an unusual cross- range of analytical methods are used in forensic casework.
section would have more significant value, because there This article illustrates how vibrational spectroscopy is used are fewer in existence. Forensic scientists have thoroughly to identify or compare physical evidence in criminal foren- developed statistical values for DNA and fingerprint data to demonstrate individual characteristics. Statistics are more A broad definition of the term “forensic”, according difficult to apply to class evidence, but approaches to apply to ‘Webster’s New World Dictionary,’ is “suitable for them are being investigated. The information obtained by a law court or public debate”. With the application of vibrational spectroscopy is usually characteristic of classes forensic science, one must demonstrate in court that the of materials, but in some instances the identification of spe- evidence analyzed has relevance to the case in question.
cific components demonstrates an uncommon characteristic.
The significance of evidence related to a case is often If a contaminant on an evidential material is identified as determined by whether the physical evidence has individual a rare substance specific to the crime scene, the evidence would demonstrate a high likelihood of originating fromthe crime scene. Therefore, vibrational spectroscopy is used This is a US Government Work and is in the public domain to identify chemical properties that contribute in varying degrees to the evidential value in criminal forensic analysis.
Forensic Applications of Vibrational Spectroscopy GENERAL USE OF VIBRATIONAL
sample preparation, they are used at least as frequently as SPECTROSCOPY IN FORENSIC
Gas chromatography (GC) with IR detection (GC/IR) ANALYSIS
for analysis of mixtures has not found much applicationin forensic analysis. GC combined with mass spectrome- Infrared spectroscopy
try (MS), or GC/MS, superseded GC/IR, and the ultimatesensitivity benefits of MS for trace components have over- Several authors have provided general overviews for the use shadowed the use of GC/IR in forensic analysis. However, of infrared (IR) spectroscopy in forensic analysis.4–6 The GC/IR used as a separation and identification tool for large applications vary to cover a wide range of physical evidence samples has proven successful in drug analysis. Because IR in the form of bulk materials and micro-sized particles. His- analysis can be used to identify isomers, it can sometimes torically, dispersive IR spectrometers were generally found be used to identify isomeric forms that render a drug active useful for bulk samples such as drugs. Micro-sized sam- ples were analyzed with skilled patience in beam condenser The recent developments of single or multiple reflection, accessories. For example, the Royal Canadian Mounted horizontal ATR accessories for use in sample compartments Police (RCMP) Forensic Service Laboratories (FSL) used have been very useful for forensic analysis. Small liquid high-pressure diamond-anvil cells in beam condensers for or solid samples, less than a millimeter in diameter, can the analysis of automotive paint chips transferred from hit- be measured. Typically, diamond internal reflection ele- ments (IREs) are used for durability. Some IREs consist With the development of Fourier transform infrared of thin diamonds backed with other materials to reduce (FT-IR) spectroscopy, the application of IR to forensic cost. These ATR accessories offer the option of extended analysis became more prevalent because of the increased frequency ranges to near 200 cm 1 in spectrometers with speed and sensitivity of FT-IR. The development of dif- cesium iodide optics. The extended range can be used to fuse reflection (DR) accessories provided ease of sample nondestructively acquire information regarding paint pig- introduction for several forensic applications. Samples with ments and inorganic fillers in polymeric materials.
matte-finished surfaces could be analyzed with no samplepreparation. Samples such as illicit drugs that previouslyrequired extensive grinding to make KBr pellets required Raman spectroscopy
less preparation. Suzuki was the first to apply the DRmethod to forensics with the analysis of drugs, polymers, Recent technological advancement in Raman spectrometers wood and solvents.10 He continued with additional work on has provided a reason for exploring this method in foren- drugs,11 polymer foams12 and paints.13,14 Document analy- sic applications. While the applications have been slow to sis by DR has been reported for copy toners15,16 and inks.17 find their way into forensic laboratories, the advantages It was not until the 1990s that the use of FT-IR became are being recognized and Raman spectrometers are start- more regularly applied in forensic laboratories. The intro- ing to find use in forensic analysis.20 Fourier transform duction of lower cost spectrometers and microscopes paved (FT) Raman received early attention in the redevelopment the way for FT-IR use in forensic analysis. FT-IR micro- of Raman technology. The 1064 nm near-infrared (NIR) scope development is considered a milestone achievement excitation laser that is used with FT systems causes fewer for forensic analysis and is considered the most signifi- samples to fluoresce than the visible wavelength lasers cant recent advancement for microsample analysis.5 Sample previously used. However, with the lower energy, longer preparation to introduce specimens into microscopes is fre- wavelength laser, signal averaging and rapid scans pro- quently easier, because only a small portion of the sample vided by FT systems are required to attain the quality of is required for placement in the IR beam for transmis- spectra desired. Unfortunately, the lower scattering power sion spectroscopy. For example, to analyze paint from a of the longer wavelength excitation impedes application surface, all that is required is a sliver sliced from the to microsamples that are commonly required with foren- surface with a scalpel. Therefore, standard sized samples sic analysis, thus limiting the use of microscopes with FT are often reduced in size and analyzed in microscopes Raman. Dispersive Raman systems that use NIR lasers because of convenient sample preparation.18 The reflection ranging from 780 to 840 nm have greater scattering effi- techniques, reflection-absorption (R-A), specular reflection, ciency. Combined with sensitive charge coupled device diffuse reflection (DR), and internal reflection spectroscopy (CCD) detectors and improved monochromators, these sys- (IRS), frequently referred to as attenuated total reflection tems have more general use compared to FT Raman systems (ATR),19 provide additional ease of sampling in IR micro- and have thus taken the forefront for a wide range of scopes. Because reflection methods require little or no sample analysis. Additionally, fiber optic probes are used Applications of Vibrational Spectroscopy in Criminal Forensic Analysis on dispersive systems to sample through glass bottles and plastic bags, making noninvasive sampling possible. Theseprobes contribute to the application of small, rugged Ramanspectrometers designed specifically for field use. Portabil-ity and ease of sampling are attractive features that areincluded in these Raman systems that can be used alongwith an array of other analytical instruments to be carried to crime scenes where the analysis of potentially hazardousmaterials is conducted without risk of transporting them POLYMER ANALYSIS
General polymers
Vibrational spectroscopy is applicable to a wide range of physical evidence. Because polymers are so common, they Figure 1. Cruise ship homicide case. Spectra of rubber particle
frequently play an evidentiary role in criminal cases. Poly- evidence from sweat pants from both (a) the victim and (b) the meric materials such as fibers, paints and adhesive tapes are suspect, (c) running track and (d) calcite reference.
frequently analyzed to identify characteristic information Particles from the track material were prepared in the same regarding their composition. Physical and chemical infor- way, and the spectra of all three samples were compared mation on these materials is stored in computer databases as shown in Figure 1. The three spectra of the rubber- to help determine the manufacturer or, supplier, or simply ized material matched closely. The material was heavily to discriminate between many similar samples of material.
Some of the available databases will be described as part C–O antisymmetric stretch near 1450 cm 1, and narrow of the analyses mentioned in the forthcoming pages. Other out-of-plane and in-plane bends near 880 and 710 cm 1, general polymeric materials found as evidence do not fall respectively.21 To determine the force required to embed into a particular category and must be studied on a case- the particles in the pants, a fabric similar to that of the by-case basis without the aid of comparison with similar pants was rubbed across the removed track piece at differ- ent degrees of pressure. The investigators found that it was The following is a case example where a polymeric mate- necessary to rub the fabric with significant pressure, such as rial became important evidence. Early one morning, on a in the case of dragging a person, to cause the transfer and luxury cruise ship off the coast of California, a man reported embed the material in the fabric. They determined that the to the captain that his wife had been blown overboard. Con- transfer of particles suggested an altercation between the sidering the minimal wind conditions, the captain became individuals, and, therefore, this evidence weighed heavily suspicious and alerted the law enforcement authorities. Sev- and resulted in a homicide conviction. The combined com- eral hours after the overboard report, the woman’s body parison of evidential materials by microscopic examination, was recovered by the US Coast Guard. The clothing that IR analysis, and physical testing played a significant role both the man and woman were wearing at the time of the incident was forwarded to the FBI Laboratory. Thewoman’s running pants appeared soiled with an orangesubstance on both legs. Careful examination of her pants Copy toners
with a stereo microscope revealed orange-colored, rubberyparticles. Examination of the man’s running pants also Questioned documents involving fraud and threatening let- revealed rubbery particles that appeared identical. On the ters are often produced on printers, copy machines and ship’s deck, at the location of the overboard incident, there facsimile machines. The machine model identification of was an orange-colored, rubberized running track. A por- this common office equipment has been achieved through tion of the track material was removed from the ship and comparison of the resins of the toners used as ink. These forwarded to the laboratory for analysis. The particles were “copy toners” have been studied for forensic analysis as removed from both pairs of running pants and flattened in a a class of polymeric material. An example where copy compression cell to make them sufficiently thin to perform toner analysis was used to produce an investigative lead transmission analysis using an FT-IR microscope system.
was in a case involving a copied address label. A packaged Forensic Applications of Vibrational Spectroscopy bomb, mailed to a corporate executive, had an address label baseline flattened spectrum in Figure 2(b) is typical of a that appeared to be an enlarged copy of the company’s styrene/acrylate copolymer. Significant variations in the IR return address logo typically used on company envelopes.
spectra are produced by the polymeric resins which contain Investigators suspected that the bomb had been mailed by numerous additives that vary in type and quantity. A visual an employee with access to internal supplies and that the comparison of the case sample spectrum was made with the person had copied an envelope using equipment within the 62 spectra of the model types in the building. One Kodak company. There were over 200 copy machines, involving model type matched closely with the case sample spec- 62 different copier models, located throughout the facil- trum. The spectrum was also searched in an IR database ity. It was important for the investigators to know the of copy toner resins categorized based on over 800 copier copier model used to narrow the area of the investigation to and printer models.23 The search software narrowed the employees with convenient access to a copier model of the toner type to a group containing 24 models of machines.
type used to print the label. Sample pages were prepared By careful scrutiny of the peaks, it was possible to narrow from each of the 62 models and forwarded to the FBI Lab- the spectra to six Kodak models in the database. The Kodak oratory for analysis. In the laboratory, the samples were copier model from the corporate building was included in prepared for IR analysis using a heat transfer technique the computer search. Therefore, the results of the visual to remove the toner from the documents. The preparation inspection and the computer search of the spectra corrob- technique involves heating the back of the paper with a orated. Two-thirds of the binders contained in the spectral soldering iron at a specified temperature and smearing the database consist of the styrene resins plus additives to pro- toner onto aluminum foil attached to a glass microscope vide desired properties in particular copy machines. Other slide. Spectra were obtained with an FT-IR microscope types of binders used are phenolic and polyethylene resins.
by R-A. With this method, the IR beam passes through The regions boxed off in Figure 2(b) contain small bands the sample and is reflected from the aluminum foil to the from the additives that provide the differentiating spectral detector via the microscope optics.22 Figure 2(a) shows features of this toner resin. Because the building contained the original spectrum of the toner from the bomb pack- only eight examples of that particular Kodak model, the age label. This spectrum is sloped due to scattering from results of this analysis permitted the investigators to narrow the carbon black particles used for the copy image. The their search to personnel working in limited locations of the Figure 2. Poly(styrene:acrylate) resin copy toner spectrum from an address label on a bomb package: (a) original uncorrected spectrum;
(b) flattened spectrum showing boxed regions where additive absorptions can be observed.
Applications of Vibrational Spectroscopy in Criminal Forensic Analysis building. Thus, a suspect was determined in considerably position or environment within the comonomer structure as less time than if the company’s entire personnel required a methacrylate, methyl methacrylate or vinyl acetate. Over 20 variations of acrylics can be identified by IR.30 Thus, IRis a very useful tool in providing information that furtherdiscriminates fiber types to enhance the evidential value of Raman spectrometry promises to complement IR analysis Textile fibers are often transferred between clothing dur- of textile fibers due to the ease of sampling and the addi- ing personal contact in violent crimes such as rape andhomicide. The foremost method of analysis for this type tional information it provides. Light microscopy analysis of evidence is visual light microscopy, though IR is very requires fiber samples to be mounted in a liquid medium, useful to further specify fiber type. Fibers from the clothing under coverslips, on a glass slide. Because glass absorbs of the victim and suspect are screened microscopically for strongly in the IR, the fibers must be removed and cleaned similarity, closely comparing physical and optical proper- prior to IR analysis. Raman analysis, on the other hand, ties of the fibers. These properties may vary significantly has been successfully performed on single fibers mounted due to the color, shape, texture, and chemistry of textiles.
on glass microscope slides,31,32 thus avoiding the need to Polarized light microscopy is used to determine the generic remove the fibers from the slide mount. The additional sam- classification of the polymer type, and IR microscopic anal- ple preparation time is not required and, once mounted, ysis plays an important role by identifying subclasses of the chance of fiber loss is minimal. Figure 4 illustrates synthetic fibers.24,25 A spectral library of 83 polymeric fiber the process of obtaining a nylon-6 fiber spectrum using types, obtained by transmission spectroscopy on flattened microscopic Raman analysis. Figure 4(a) includes spectral single fibers, was developed to aid forensic examiners in contributions from the fiber, Permount mounting medium the identification of fiber composition.26–29 IR is particu- and glass from the coverslip. Figure 4(b) shows the result larly useful for subclassifying acrylic fibers that are seen of Permount subtracted from the original spectrum. The frequently as a wool alternative in sweaters and readily nylon-6 spectrum remains in Figure 4(c) after the glass transfer between individuals during contact. These fibers contributions are subtracted. The baseline is flattened for consist of at least 85% polyacrylonitrile28 plus copoly- the final spectrum shown in Figure 4(d). The signal-to- mers and ionic end-groups to enhance dyeing properties.
noise ratio is less than obtaining a spectrum without being Figure 3 shows spectra of the three most common acrylic mounted under a coverslip, but it is sufficient to identify copolymers. The band shape of the C–O stretch in the the fiber by the prominent characteristic bands as labeled.
region between 1300 and 1000 cm 1 varies depending on its The band assignments of the major peaks are labeled in Figure 3. IR spectra of acrylic copolymer fibers: (a) poly(acrylonitrile:vinyl acetate); (b) poly(acrylonitrile:methyl acrylate);
(c) poly(acrylonitrile:methyl methacrylate).
Forensic Applications of Vibrational Spectroscopy Figure 4. Raman spectra of a nylon-6 fiber mounted under a coverslip on a glass microscope slide: (a) original spectrum, (b) Permount
mounting medium subtracted; (c) glass subtracted; (d) baseline flattened with the major characteristic bands labeled.
accordance with Hendra et al.33 Raman spectra differ from IR spectra because the selection rules for Raman vary fromIR, thereby producing complementary information. The IR spectroscopy of paints has been useful in forensic analy-sis since the 1960s. Automotive, architectural, art, marine, information obtained by Raman is at times more definitive aircraft, tool, and other types of paints may become evi- in determining the polymeric structure. Since Raman spec- dence in a variety of crime scene scenarios.36 After visual troscopy demonstrates spectral response from dyes, the light microscopy, IR analysis offers the most information dye information can be useful. However, the dye spectral in forensic paint examination. The organic binders are fre- features can interfere with identification of the polymeric quently identified with IR, and both organic and inorganic composition of fibers. A protocol to determine whether to pigments can often be identified. Scientists from the RCMP use Raman or IR when analyzing dyed and undyed fibers, have been classifying automotive paints based on chemical grouped by generic class, is currently being established composition since the 1970s.7–9 The original analysis was in the FBI Laboratory. To further characterize the fibers, performed with the use of high-pressure diamond-anvil cells dye spectral features could provide information regarding in beam condensers on dispersive IR spectrometers. Since the dye type. Studies have been conducted using surface then, the RCMP and other analysts have changed to using enhanced Raman spectroscopy (SERS) to study dye compo- the less cumbersome low-pressure compression diamond sition using silver colloid substrates.34,35 While the spectral cells with beam condensers in FT-IR systems. Inorganic features are significantly enhanced by SERS, this method pigment components in paints have revealing spectral fea- requires that the fibers are removed from the glass slides, tures at the lower wavenumbers. Beam condensers are resulting in an additional step and a chance of fiber loss.
used rather than FT-IR microscopes to overcome the lim- Raman analysis has demonstrated certain advantages over ited frequency range of mercury cadmium telluride (MCT) IR, but the strengths and limitations of both are still under detectors used in IR microscopes. For paint analysis, the study to determine where each of the methods can be used extended range to near 200 cm 1 is obtained with CsI optics to obtain the greatest information with the most convenient and a standard deuterated triglycine sulfate (DTGS) detector Applications of Vibrational Spectroscopy in Criminal Forensic Analysis Physical and chemical information obtained by light Researchers are beginning to demonstrate the usefulness microscopy and IR analysis on automotive paint, used to of Raman analysis for organic and inorganic pigment potentially identify make, model and year of vehicles, is identification in paint.38,39 Figure 5 compares IR and readily searched in a database. The computerized library, Raman paint spectra of a yellow acrylic melamine enamel developed by the RCMP, is widely used by forensic lab- automotive paint. The IR spectrum in Figure 5(a) clearly oratories throughout North America.37 Necessitated by the shows the resin binder features. The N–H stretch near international nature of the automotive industry, European, 3350 cm 1, the C–H stretches near 3000 cm 1, the CDO Japanese and Australian forensic laboratories will soon be stretch near 1730 cm 1, the C–N stretch near 1540 cm 1, contributing to this database in order to provide a more and the typical C–O envelope from 1300 to 1000 cm 1 are comprehensive collection. Scientists from forensic working observed in the IR. Of particular interest to paint analysis groups of the listed countries are contributing automotive are any contributions by pigments. The weak, broad band at paint samples and data from their respective nations and 868 cm 1 appears to be contributed by chrome yellow, as shown in the reference spectrum (Figure 5(b)). However, Paint types other than automotive do not usually dis- because of the band’s comparatively low intensity and lack play as much diversity in chemistry, color and layer of detail in the paint spectrum, it would be difficult to structure. Therefore, a wider variety of analytical instru- positively identify chrome yellow by this method alone.
ments are often used to characterize these paints. Along The peaks labeled at 659, 425 and 357 cm 1 are rutile, with microscopy and IR spectroscopy, other methods fre- a crystal form of titanium dioxide. Figure 5(d) is the quently used are pyrolysis GC/MS and inorganic analysis Raman spectrum of the yellow auto paint. The major by scanning electron microscopy with energy dispersive X- peaks at 843 and 365 cm 1 match up with the Raman ray spectroscopy (SEM/EDX) or X-ray diffraction (XRD) spectrum of chrome yellow shown in Figure 5(c). Peaks at 611 and 446 cm 1 are contributed by rutile, as shown in 3600 3200 2800 2400 2000 1800 1600 1400 1200 1000 Figure 5. IR and Raman spectra of yellow acrylic melamine enamel auto paint with pigments: (a) IR spectrum of the auto paint; (b) IR
spectrum of chrome yellow pigment; (c) Raman spectrum of chrome yellow pigment; (d) Raman spectrum of the yellow auto paint;
(e) Raman spectrum of rutile. (Spectra provided by E. Suzuki, Washington State Patrol, Forensic Laboratory, Seattle, WA.)
Forensic Applications of Vibrational Spectroscopy Figure 5(e). With Raman, the high scattering efficiency of permits acquisition of spectral features of the inorganic some pigments, relative to those of binders, helps to easily fillers commonly found in duct tape adhesives and less determine the pigment components, as interfering binder frequently found in the film backing. Figure 6(a) shows an atypical duct tape backing spectrum containing cal-cium carbonate (calcite). The calcite filler has a latticeband21 near 315 cm 1 that would not have been observed without the extended frequency range capabilities. The C–Oasymmetric stretching band21 near 1450 cm 1 underlies the Pressure-sensitive adhesive tapes play an important role in C–H bending band near 1460 cm 1. The C–O out-of-plane forensic analysis as evidenced by their diverse uses. Elec- bend can be observed near 880 cm 1. The spectrum in trical tapes can be used in wiring electronic devices to Figure 6(b) is more typical of polyethylene backing show- bombs, duct tapes in binding victims of violent crimes, and ing only the C–H bands without the filler features. Because other tapes in wrapping packages containing drugs, bombs fillers are not typically used in the backings, analysis of or other threatening material. By carefully characterizing this unusual duct tape evidence could provide increased tapes submitted as evidence, they can be compared with known tapes in a suspect’s possession or they can be stud- To more fully characterize tapes, other analytical meth- ied to develop investigative leads when the brand can be ods are used. The physical characteristics are observed and established. IR spectroscopy can be useful with ATR acces- measured with the unaided eye and microscopically, and sories in determining the major organic components of both inorganic composition is determined by SEM/EDX and the adhesives and backings. In the past multiple reflection XRD analysis. Physical characteristics, such as the yarn accessories were used, but more recently single reflection counts and weave type on the fabric reinforcement within ATR accessories for the IR microscopes have been used duct tapes, may quickly narrow down the brand possi- to acquire spectra of small, uncontaminated areas on the bilities. XRD can provide further information about the tapes. While this is convenient, the frequency range is lim- inorganic components like distinguishing between anatase ited by the MCT detectors used on the microscopes which and rutile crystalline forms of titanium dioxide extenders.
cut off near 700–650 cm 1. A study has demonstrated the TiO2 is frequently used in duct tape adhesives and some usefulness of a single-reflection, horizontal sample com- manufacturers prefer a specific crystalline structure, thus partment accessory using a diamond interface backed with identification of the mineral type can assist with identify- KRS-5.40 This accessory provides a spectral range from ing the manufacturer. All of the information obtained by the 4000 to 260 cm 1when used with cesium iodide optics various analyses has recently been placed in a searchable in the spectrometer. The extended range below 400 cm 1 database for quick comparison of tape properties.41 Figure 6. IR spectra of duct tape backing film by ATR: (a) typical polyethylene backing; (b) backing containing calcite.
Applications of Vibrational Spectroscopy in Criminal Forensic Analysis IR has been used for the analysis of both licit and illicit drugs for many years.4,42 The computerized drug libraryproduced at the Georgia State Crime Laboratory (GSCL) is a standard in forensic analysis. Currently, it containsover 2000 spectra of drugs and related chemicals. Samples Figure 8. Reduction reaction of ephedrine to methamphetamine.
prepared in standard 13-mm KBr pellets have been used forinclusion of drug spectra in the library. However, recently stereoisomers (d and l forms). IR is more effective than MS the GSCL successfully applied ATR to drug analysis.43 in differentiating diastereoisomers. Thus, the diastereoiso- Horizontal ATR sample compartment accessories with three mers ephedrine and pseudoephedrine, which are precur- reflections provide sufficient sensitivity to acquire spectra of sors for methamphetamine, are identifiable compounds by approximately 400 ng of lysergic acid diethylamide (LSD) this method. Figure 8 shows the reduction reaction of as a film cast from chloroform (Figure 7). The region the ephedrine to methamphetamine. Figure 9 illustrates between 2400 and 1800 cm 1 was blanked to remove the the comparison of the gas phase spectra of these com- uncompensated diamond absorption produced by the IRE.
pounds. In Figure 9(a) and (b), the region between 1300 The ATR spectra of drugs can be successfully searched and 1000 cm 1 shows subtle but consistently different spec- in the original transmission spectral library in spite of the tral features of the diastereoisomers ephedrine and pseu- intensity differences in the peaks. This method is beginning doephedrine. The spectra of amphetamine and metham- to gain acceptance in laboratories around the USA. The phetamine, shown in Figure 9(c) and (d), respectively, FBI Laboratory has started an ATR database of drugs for do not have the OH stretching band near 3600 cm 1, computer spectral searching. Recently, a drug library of 455 because the reduction reaction removes the OH attached spectra by ATR was produced at the Illinois State Police to the carbon next to the phenyl group. The spectra of amphetamine and methamphetamine differ in the entire GC combined with IR (GC/IR) simplifies the analy- region below 1700 cm 1. The most pronounced difference sis of drug mixture samples typical of those associated lies with the NH deformation band near 1600 cm 1, which with clandestine laboratories and is a standard procedure is significantly greater in intensity for the primary amine, of the Drug Enforcement Administration (DEA) labora- amphetamine. Once these components are identified, cap- tories. For the analysis of methamphetamine and related illary electrophoresis (CE) is required to determine the compounds, the DEA is required to identify the optical optical stereoisomer (d or l enantiomer) present. Since the Figure 7. Spectrum of 400 ng of LSD by ATR. (Spectrum provided by Robert Ollis, Georgia Bureau of Investigation Crime Laboratory,
Decator, GA.)
Forensic Applications of Vibrational Spectroscopy methods are often required with mixtures of components, prior to spectroscopic methods to identify the components.
Due to the sensitivity often required, particularly for post-blast residues, GC/MS is frequently applied to explosivesanalysis. Liquid chromatographic methods, ion chromatog-raphy and CE are also frequently applied because of theseparation capabilities of these methods.
ATR has been successfully applied to plastic explosives in bulk form and after extraction. Keto47 demonstrated anextraction method for the determination of C-4 militaryexplosive. In his method, the explosive hexahydro-1,3,5- trinitro-1,3,5-triazine (RDX), the plasticizer and the rubber binder from C-4 are separated selectively by solvent extrac- tion and filtration. The extracts are cast as a film on an ATRcrystal for IR analysis. Bartick and Merrill48 have shownhow the development of a database of bulk plastic explo- sives can be used to successfully identify the general plastic explosive type and even the manufacturer. A library ofpure explosives components can be searched to potentially identify the explosive material. Figure 10 shows the explo- sive component RDX, identified from a questioned plasticexplosive mixture. A library search of the plastic explosive Figure 9. Clandestine laboratory mix separated by GC/IR.
library identified the questioned explosive as C-4.
Spectra of: (a) ephedrine; (b) pseudoephedrine; (c) amphetamine;(d) methamphetamine. (Spectra provided by Henry Blum, DEA Often, unknown materials are found at crime scenes and, for safety considerations, it is important to determinethe chemical composition prior to handling and bringing most potent and sought-after methamphetamine is the d the material to the laboratory for analysis. Early studies enantiomer, the DEA analyzes the mixtures to confirm the with Raman spectroscopy of trace explosives were done presence of d-methamphetamine. Either the l-ephedrine or by Lewis et al.49–52 to determine the basic requirements of the d-pseudoephedrine enantiomer may be used to produce a field portable system. Successful results were obtained d-methamphetamine. CE separates all eight structures of for all samples with a 1064-nm laser FT system, and most ephedrine, pseudoephedrine and methamphetamine for the samples were successfully analyzed on a dispersive system final identification of the existing d or l enantiomer versions.
using 632.8 nm excitation. Lewis et al. considered the best Raman spectroscopy is beginning to attract interest both potential for field systems to be a compromise that would in the laboratory and for field drug analysis. Many drugs areexcellent Raman scatterers, and therefore lend themselvesto rapid analysis with direct laser beams, fiber optic probesand microscopes. The application of fiber optic probes provides the ability to obtain spectra for drug samplescontained in plastic bags or bottles, thereby making fieldanalysis simple. Several field portable Raman spectrographshave become available on the market. These instrumentshave been compared for such features as frequency range,resolution, laser excitation and portability in terms of power requirements, size and weight.45 While still in an early stage, this field approach appears to have great promise.
As with many types of forensic evidence, explosives produce unique IR spectra, thus making IR useful for identification Figure 10. ATR spectrum of a C-4 plastic explosive mix and
of the major components in bulk explosives.46 Separation RDX library pick for the explosive component.
Applications of Vibrational Spectroscopy in Criminal Forensic Analysis use a NIR 785-nm laser on a dispersive spectrometer.
(Decator, GA), Henry Blum of the Drug Enforcement Cheng et al.53 have done additional studies that include Administration Laboratory (Washington, DC) and Dr. Ed- imaging Semtex plastic explosive deposited in fingerprints ward Suzuki of the Washington State Patrol Crime Lab- on aluminum foil. RDX and pentaethythritol tetranitrate, oratory (Seattle, WA). All these gentlemen contributed the explosive Semtex components, were isolated in the their expertise and spectra for figures used in writing this prints, and spectra were obtained with a microscope system.
Currently, the FBI Laboratory uses an echelle dispersiveRaman spectrograph that operates with a 785-nm laser anda fiber optic probe that has become successful for field ABBREVIATIONS AND ACRONYMS
analysis.54 An explosives library has been developed foruse with this instrument.55 Current developments in portable FT-IR instrumentation also show promise for field analysis of explosives. Recently, SensIR Technologies (Danbury, CT) introduced a portable instrument referred to as the TravelIR, that uses a single reflection ATR arrangement for sample analysis.56 Liquid and solid sample analysis is easily conducted in the field. A preliminary study of explosive samples has been conducted at the FBI Laboratory’s FSRU. IR and Raman field methods are expected to complement one another.
1. E. Locard, Police J., 1, 177 (1928).
Vibrational spectroscopy is used throughout forensic lab- 2. E. Locard, Am. J. Police Sci., 1, 276 (1930).
oratories for many applications. IR analysis with FT-IRinstrumentation has many applications in the areas such as 3. R. Saferstein, ‘Criminalistics, An Introduction to Forensic Science’, 7th edition, Prentice Hall, Upper Saddle River, NJ polymers, drugs and explosives. With the exception of field analysis, where methods have just started to develop, IR hasreached a level of maturity in forensic applications. Raman, 4. E.M. Suzuki, ‘Forensic Applications of Infrared Spectro- scopy’, in “Forensic Science Handbook”, ed. R. Saferstein, on the other hand, is just beginning to develop. Both lab- Prentice-Hall, New York, 71– 195, Vol. III (1993).
oratory and field methods are anticipated to flourish in the 5. E.G. Bartick and M.W. Tungol, ‘Infrared Microscopy and next few years. The greatest developments are expected its Forensic Applications’, in “Forensic Science Handbook”, for field analysis using portable Raman spectrometers. The ed. R. Saferstein, Prentice-Hall, New York, 196– 252, Vol. III instruments will require small size designs providing the performance needed by specific sample types, and cost 6. C. Roux, P. Maynard and M. Dawson, Chem. Aust., 11 (1999).
will be a major factor. The greatest challenge, with both 7. P.G. Rogers, R. Cameron, N.S. Cartwright, W.H. Clark, J.S.
Raman and IR field analysis, lies with the development of Deak and E.W.W. Norman, Can. Soc. Forensic Sci. J., 9, 1
the software so that nontechnical law enforcement officials can quickly, conveniently, and accurately use the instru- 8. P.G. Rogers, R. Cameron, N.S. Cartwright, W.H. Clark, J.S.
ments to get the pertinent investigative information. Of Deak and E.W.W. Norman, Can. Soc. Forensic Sci. J., 9, 49
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