Research and Technology - Forensic Science Communications - April 2007
April 2007 - Volume 9 - Number 2
Research and Technology
Application of X-Ray Diffraction Techniques in Forensic Science
Johny T. Abraham
Junior Scientific Officer
Central Forensic Science Laboratory
S. K. Shukla
Central Forensic Science Laboratory
A. K. Singh
Materials Science Division
Defence Metallurgical Research Laboratory
An important part of forensic science is the use of analytical techniques to uncover facts from even the smallest traces of evidence picked up from the crime scene. X-ray diffraction is one such technique that can be used for the characterization of a wide variety of substances of forensic interest. However, because of the form and size in which crime samples occur, the amount of material available, and the evidential restrictions to preserve the samples, X-ray diffractometry is seldom used for the analysis and comparison of nonpowder samples. This paper uses case studies to illustrate the potential of X-ray diffractometry to enhance the evidential value of such samples. These cases show that the information obtained using X-ray diffractometry is unique and that the X-ray diffraction pattern also can be used directly as a signature of the material for comparison when the chemical structure of the material is complex.
Forensic science, by virtue of its broad coverage, benefits from the innovations and developments in the fields of science and technology. Therefore, any techniques that help in the investigation of crime can be suitably adapted for forensic science. X-ray diffraction (XRD) techniques have long been used for characterization of a wide variety of substances and are typically nondestructive methods of investigation. Among the various XRD techniques, X-ray powder diffraction is relatively simple and is commonly adopted for the analysis of powder samples in forensic science laboratories (Folen 1975; Ruffell and Wiltshire 2004; Thangdurai et al. 2005). But this technique has seldom been used for the analysis of nonpowder samples of forensic interest because of the form and size in which crime samples occur, the limited amount of material, and evidential restrictions to preserve the samples. The potential of XRD as a preliminary investigative tool for the forensic examination of nonpowder samples is illustrated in this work.
Successful characterization using XRD techniques requires that an amount of crystallinity be present in the substances. A substance whose atomic components (atoms, ions, and molecules) have a three-dimensional periodic order is designated crystalline. The periodic arrangement of atoms in the crystals is represented by a set of crystallographic planes, and XRD occurs by the scattering of X-rays from this set of planes. Each crystalline substance crystallizes in a particular crystal structure characteristic of that compound, giving rise to a unique XRD pattern. In the X-ray powder diffractometer discussed in this study, X-rays from a fine-focus tube pass through a soller slit (a set of equidistant parallel plates) and a divergence slit onto a flat powder specimen (Figure 1). The X-ray beam diffracted by the crystallites of the specimen converges at the receiving slit after passing through an antiscatter slit. After passing through the receiving slit, the diffracted beam passes through a soller slit, falls on the monochromator, and then reaches the detector. The detector is moved by a gear system with twice the angle speed of the specimen. Pulses from the detector are amplified, sorted, and counted using suitable electronics. The intensity of the diffracted beam along with its angle of diffraction is displayed on a chart recorder or display unit. In modern-day diffractometers, the measurement and evaluation of diffraction patterns have become faster and easier through computer automation. The digitization of data files by the International Centre for Diffraction Data (ICDD, formerly the Joint Committee on Powder Diffraction Standards [JCPDS]), containing standard crystallographic data of more than 40,000 diffractograms, has made the identification of an unknown substance user-friendly and less cumbersome.
Figure 1: Schematic diagram of an X-Ray Powder Diffractometer (image provided by PANalytical B. V. Reprinted with permission).
A steel sample holder of about 45 mm in diameter used in conventional X-ray powder diffractometry was used to mount the samples. Cloth pieces to be examined as a part of fabric examination were directly mounted on the sample holder without any wrinkles, and the edges were kept pressed to the holder using cellophane tape. Care was taken to keep the cellophane tape away from the path of the primary X-ray beam. The stains of the control lipsticks were collected as smears on microscope glass slides tailored to fit in the steel sample holder.
The XRD studies were carried out using a Philips X’Pert (PW 3040, PANalytical, Almelo, the Netherlands) Powder Diffractometer using CuKα radiation (40 kV, 50 mA), with the programmable divergence slit and receiving slit kept at 1 degree and 0.1 mm, respectively.
Case Study 1: Comparison of Textile Fibers
In crimes where cloth pieces are recovered as physical evidence, the pieces are subjected to routine fiber and fabric examination to determine whether the pieces share common origin or not. The analytical aspects of fiber examination are well documented (Fong 1989; Gaudette 1988). Among the various techniques discussed for the identification and comparison of fibers, microscopy and infrared microscopy are the most frequently used techniques (Robertson and Grieve 1999). Although X-ray powder diffractometry is available in forensic science laboratories to investigate explosives, building materials, minerals, and drugs as a part of forensic examination (Fischer and Hellmiss 1989), this technique is seldom used for discrimination of fibers in fabric samples. In this study, the potential of the XRD technique to characterize the constituent fibers of a fabric was explored.
Textile fibers are a mixture of crystalline and amorphous regions. The extent to which either region predominates within the fibers largely determines the overall properties of the fiber. The measurement of the degree of crystallinity provides useful data while characterizing fibers using X-ray diffractometry. Because no previous study using the XRD technique was identified by the authors, a database of X-ray diffractograms of known fibers was created for comparison and identification of unknown fiber diffractograms.
Sample pieces from fabrics of known fiber constituents were mounted on the sample holder, and the diffraction pattern of each type of fiber was studied. The selected fibers included commonly used fibers such as wool, cotton, polyester, and nylon, and the diffractograms obtained for these fibers are depicted in Figure 2. Figure 2 demonstrates that each type of fiber has its own characteristic diffraction pattern. In wool fibers, no characteristic diffraction pattern is obtained, which can be attributed to the amorphous structure of wool fibers.
Figure 2: X-Ray Diffractograms of Textile Fabrics (a) Wool (b) Cotton (c) Polyester and (d) Nylon
The diffractograms of fabrics with more than one fiber were also studied as part of the database creation. In blended fabrics, the constituent fibers are present in varying percentages. Figure 3 displays the diffractograms of two blended fabrics having 58 percent cotton and 42 percent polyester and 33 percent cotton and 67 percent polyester, respectively, compared with the diffractograms of unblended cotton and polyester fabrics. Similarly, Figure 4 depicts the diffractogram of a cotton-nylon-blended fabric (labeled “cotton-rich”) compared with the diffractograms of unblended fabrics. Figures 3 and 4 (Figures 3b, 3c, and 4b) demonstrate that the diffractograms of blended fabrics are different from one another and also from unblended fabrics with respect to the amount of their constituent fibers.
Figure 3: X-Ray Diffractograms of (a) Unblended Cotton; (b) 58% Cotton, 42% Polyester; (c) 33% Cotton, 67% Polyester; and (d) Unblended Polyester
Figure 4: X-Ray Diffractograms of (a) Unblended Cotton (b) Cotton-Nylon Blend (Cotton-Rich) and (c) Unblended Nylon
Fabric samples of varying brand and color were also analyzed (Figure 5). Whereas Figures 5a and 5b represent the diffraction patterns of white cotton fabrics of two common brands, Figure 5c depicts the diffractogram of a brown-colored cotton fabric. The variation due to brand and color is reflected only in the intensity and width of the peaks, with the diffractograms remaining characteristic of the constituent fiber. It is well known that large crystallites give rise to sharp peaks, whereas the peak width increases as crystallite size decreases (Cullity 1978). The variation in the peak intensity and width is likely due to inherent differences in the cotton fibers.
Figure 5: X-Ray Diffractograms of (a and b) White Cotton Cloth Samples of Two Different Brands and (c) Brown Cotton Cloth Sample
To demonstrate the utility of the created database, one of the cloth pieces recovered from a crime scene and a control piece of cloth were analyzed using the X-ray diffractometer, and the diffractograms obtained were compared with each other and with the database. Figure 6 demonstrates the similarity of the diffractograms of the crime and control samples. Comparison with the diffractograms of the known fabric samples (Figure 2) revealed that the diffractograms of the control and crime scene cloth samples were similar to that of unblended polyester fiber.
Figure 6: X-Ray Diffractograms of (a) Crime Scene and (b) Control Fabric Pieces
The study demonstrates that X-ray diffractometry can be used in the initial screening of samples without any tedious sample preparation and comes in handy when a large number of samples are to be examined. The diffractograms of fibers are very simple, with fewer than five diffraction lines, and allow for easy memorization of the profiles. Finally, because the samples can be recovered intact without any loss, the samples can be subjected to other routine fiber examination techniques to discriminate the constituent fibers.
Case Study 2: Identification of Lipstick Stains
Lipstick stains found on fabrics, glass panes, etc., can be of considerable value in crime scene investigation if a comparison with a control lipstick can be successfully carried out. In this study, the ability of XRD to characterize lipstick stains on cloth materials for initial screening was explored. The study was initiated when a garment (suspected to be used to wipe off excess lipstick from lips) bearing a lipstick stain was forwarded to the laboratory along with a control lipstick for source correspondence. Because no previous study using the XRD technique was identified by the authors, a database was created containing X-ray diffractograms of lipstick stains from known sources.
A large variety of lipsticks are available in the world market, and it is impossible and costly to procure samples of all of the brands to build a comprehensive database. The alternative is to create an instant database by collecting samples of popular brands, including the questioned brand, from the locality where the crime occurred. The number of control samples collected also can be minimized if only colors similar to the questioned sample are collected. The restriction in using such instant databases is that for conclusive identification of the lipstick brand, additional examination techniques must be adopted, a common practice employed in forensic science laboratories.
Two different colors (pink and brown) of six commonly used brands of lipstick (including that of the control lipstick) were selected for this study. The brands selected were Blue Heaven, Lakme, Arche, Avon, Personi, and Yarker. Stains of the selected brands were obtained as smears on separate microscope glass slides by swiping the lipsticks. Glass slides were selected because they do not produce any characteristic XRD peaks because of their amorphous nature. The lipstick stains on the glass slides were then analyzed using the X-ray diffractometer. The corresponding diffractograms are depicted in Figure 7, which demonstrates that the diffractograms were different for each brand.
Figure 7: X-Ray Diffractograms of Various Brands of Lipstick on Glass Slides: (a) Blue Heaven (b) Lakme (c) Arche (d) Avon (e) Personi and (f) Yarker
Lipstick consists of wax, oil, organic dye, and inorganic pigment (Choudhry 1991; Russell and Welch 1984). A comparison of the diffractograms with the ICDD database using search/match software could reveal the chemical structure of each of the selected brands. But, in the case of multiphase materials such as lipstick, identification of the individual phase based on peak position and intensity is complex and requires expertise. Further, a mixture of crystallographic modifications can occur because of variations in the brand, batch, and color of the samples, demanding a more comprehensive database, making the study time-consuming and costly.
Another approach is to compare two diffractograms in reference to peak position and intensity because each substance gives rise to a unique XRD pattern. Because the aim of this study was to compare the lipstick stain on the garment with the control lipstick, the second approach was adopted, and the XRD patterns obtained (Figure 7) were used directly as signatures of the material for comparison with the questioned sample.
A word of caution while carrying out such comparisons: the stains are being analyzed along with the substance on which the stains are adhering. Hence a systematic displacement of the diffraction peaks toward smaller or larger values can occur with reference to the diffractogram of the control stain. To demonstrate this effect, a simulated fabric sample with a lipstick stain from a known source was analyzed. Figure 8 shows simultaneous displacement of peaks characteristic of lipstick. An upward displacement of the peak at 21 degrees is due to the shouldering effect of the peak on the characteristic peak of the cotton fiber at 22.5 degrees.
Figure 8: X-Ray Diffractograms of (a) a Simulated Lipstick Stain on a Cloth Piece and (b) a Control Lipstick Stain on a Glass Slide
Using the X-ray diffractometer, the portion of the garment bearing the questioned lipstick stain was then analyzed along with another portion of the same garment free of any stains. The diffractograms obtained with respect to the stained and unstained garment pieces along with the control lipstick stain on the glass piece are depicted in Figure 9. Figure 9 demonstrates the superimposition of the diffractogram of the lipstick stain on that of the garment. The similarity of the diffractogram of the lipstick stain on the garment with that of the control lipstick stain can also be observed from the figure. When the diffractogram of the unstained garment piece was compared with the fiber database, it was found similar to that of the unblended cotton fabric. Once the samples are classified with reference to their probable source, a detailed study including microspectrophotometry and elemental analysis (Choudhry 1991) can be performed for a conclusive opinion.
Figure 9: X-Ray Diffractograms of (a) an Unstained Cloth Piece (b) a Control Lipstick Stain on a Glass Slide and (c) a Lipstick Stain on a Cloth Piece
The majority of physical evidence materials recovered from crime scenes are generally crystalline or semicrystalline in nature. XRD is one of the well-established methods of investigation for the identification of crystalline substances. Accordingly, analysis of such sample evidence using X-ray diffractometry can aid criminal investigations. But the majority of the evidentiary materials recovered may not be in the form of powder or a flat solid disc, the conventional type of sample studied using an X-ray powder diffractometer. This is one of the reasons investigation using XRD techniques is not yet a part of routine analysis in forensic science laboratories. However, as described in this study, it is possible to apply the XRD technique with minimum alteration, making sample preparation simpler and, typically, nondestructive. As a part of fabric examination, XRD can be used as an initial screening tool to sort the fabric pieces based on the type of fiber. The diffractograms of fibers are very simple, with fewer than five diffraction lines, and even allow for easy memorization of the profiles. This will not only help the forensic scientist to discriminate the type of fiber with minimum time and expertise but also can limit the examination to one or two instrumental techniques for further discrimination of fibers.
The technique also helps to preserve the fabric when it contains general stains of evidential value. As described in the study, diffractograms of the stain superimposed on that of the fabric help in simultaneous analysis of the stains as well as the material bearing the stain. In the case of evidentiary materials such as lipstick stains that contain a mixture of different chemical substances, analysis using the ICDD database file is complex and requires expertise to identify the individual phases. In such cases, the XRD patterns can be used directly for comparison because they are unique and can be treated as the signature of the materials being analyzed. Thus the XRD technique can be used as a nondestructive method of analysis and is especially suited for the determination of evidentiary materials that must not be destroyed.
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