ATOMIC SPECTROSCOPY FOR FORENSIC APPLICATIONS 1 (LA/ICPMS), modern atomic spectroscopy has proven Atomic Spectroscopy for itself to be a compelling and evolving tool in the Forensic Applications investigation of forensic evidence. Diana M. Grant and Charles A. Peters FBI Laboratory, Washington, USA 1 INTRODUCTION Forensic science may be defined as the application of scientific methods to problems that are addressed 1 Introduction 1 in a courtroom setting. Given that the results of an 2 Development of Atomic Spectroscopy 2 examination have the potential to affect a verdict at trial, 3 Applications 3 the law provides that physical evidence may be examined 3.1 Natural Products 4 by both prosecution and defense experts. The size of a 3.2 Drugs of Abuse 5 forensic specimen can thus present a sampling challenge 3.3 Suspected Poisonings 6 for analysts, particularly when a requested examination is 3.4 Lead Applications 8 destructive. Preservation of collected evidence is always 3.5 Gunshot Residue Analysis 10 a consideration prior to examination. The analyst must 4 Metals Analysis 11 ask the question: ‘‘Has the evidence been altered since or 4.1 General Applications 11 as a result of its collection?’’ Another issue that arises is 4.2 Precious Metals 12 the order in which examinations are performed so as not to preclude subsequent analyses. 5Glass 13 Evidentiary samples are generally analyzed for rela- 6 Laser Ablation 15 tional value to a suspect, victim, or crime scene. Specimens Acknowledgments 17 may readily associate a subject with a crime scene or .1,2/ Abbreviations and Acronyms 17 victim, as with DNA or fingerprints. More often, how- ever, evidence from a victim or crime scene may be used Related Articles 17 as a circumstantial link to known materials found on a References 18 suspect or in his environment. For this type of specimen, the associative link will be comparative. That is, the relationship between a known Forensic science is a discipline that evolved from the and the questioned evidence will be assessed as to the interaction between law enforcement and the scientific degree of similarity. Whether the association is formed on community. The typical operating budget of laboratory the basis of color, texture, diameter, physical impressions, facilities created to support law enforcement is often far or compositional similarities at the elemental level, the below what is necessary for practical modernization. forensic examiner’s written opinion will assess the degree Hence, routine analyses that require low maintenance of comparability between the submitted known and .2/ and can be readily performed with practical experience questioned specimens. were quickly instituted as standard procedures in many Forensic scientists are often asked to determine which such laboratories. As an example, atomic absorption examinations are best suited to the collected evidence (AA) spectroscopy has served the forensic community for in a given case. Dialog between the scientist and the over 40 years and continues to work effectively for such evidence contributor is then necessary to establish what diverse applications as gunshot powder residue analysis questions are being asked and if answers are reasonably and toxicological examinations in suspected heavy metal achievable given the limitations of sample size and the poisoning cases. laboratory’s analytical capabilities. After all known or These and other traditional forensic applications have available analytical tools are employed to address the also utilized other forms of spectrochemical analysis such question(s) posed to the laboratory, a solid scientific as neutron activation. The advantage of the various forms foundation should be established upon which a legal of atomic spectroscopy over these other methods lies in opinion may be rendered. An ideal scenario would allow the practical considerations of accessibility and cost as that any necessary instrumentation would be readily much as in sampling requirements and detection limits. accessible, specimen size would be plentiful, and the From well established AA methods to the increasing condition of the evidence would be pristine. number of forensic applications being developed for laser External contamination and minimal sample size are ablation/inductively coupled plasma mass spectrometry just two examples of the problems often encountered Encyclopedia of Analytical Chemistry, Online © 2006 John Wiley & Sons, Ltd. This article is © 2006 US Government in the US and © 2006 John Wiley & Sons, Ltd in the rest of the world. This article was published in the Encyclopedia of Analytical Chemistry in 2006 by John Wiley & Sons, Ltd. DOI: 10.1002/9780470027318.a1110 2 FORENSIC SCIENCE with forensic evidence. As with any object in limited 2 DEVELOPMENT OF ATOMIC supply, preservation of materials deemed to be evidential SPECTROSCOPY in nature is often a fundamental consideration in requests for analysis. Thus, an arguably nondestructive method The notion of coupling a plasma excitation source with a such as instrumental neutron activation analysis (INAA) multielement detection system in the mid-1960s ushered is quite useful for a wide range of applications in forensic in a new generation of workhorse instrumentation for the science assays..3–7/ However, INAA has not been readily modern analytical laboratory. Since then, industrial, envi- available for use by most forensic laboratories. The lack ronmental, and academic laboratories have demonstrated of convenient accessibility, required licensing for use of a wide applicability of plasma techniques to trace element such facilities, and regulatory waste disposal issues has studies..25–32/ Forensic laboratories have also published led to sustained interest in alternative methods of analysis studies of atomic spectrometry applications..33–36/ How- such as atomic spectroscopy. ever, as a discipline, forensic science seeks to apply AA spectroscopy has been well documented in forensic analytical techniques to evidence in a criminal mat- examinations involving suspected heavy metal poison- ter for the purpose of weighing the probative value of ing determinations in tissue samples, other biological that evidence. Therefore, only those techniques that materials, and for gunshot residue (GSR) analyses..8–19/ have been tested and validated by the scientific com- While INAA has the advantage of multielement capa- munity are suitable for presentation in a courtroom. bility for solid samples, which preserves the condition Such methods are thus no longer considered research of a specimen, AA is more readily accessible, and ulti- endeavors as much as they are new applications of estab- mately cheaper to use and maintain. The merits of both lished protocols, already discussed extensively in relevant methods were compared in early forensic application journals. papers..20,21/ Moreover, while all of the conventional atomic spec- Atomic spectroscopy most routinely involves dissolu- trometry techniques mentioned thus far have become tion of a solid matrix before analysis. Traditional AA well established in a variety of analytical problem solv- utilizes a flame or furnace system to atomize aque- ing arenas, AA spectroscopy is still the most popular ous samples. Later systems that allow for solid sample of these methods for routine forensic examinations..37/ atomization have improved sensitivity for a variety of Examples range from GSR analysis to toxicological assays matrices..22/ Most recently, plasma technology has grown for the presence of heavy metal poisoning in hair..38,39/ in popularity due to its efficiency in ionizing an aque- Therefore, a discussion of atomic spectroscopy techniques ous sample..23/ Modes of detection have also evolved to in forensic applications should begin with classical AA improve sensitivity for a variety of matrices with these protocols. systems. They range from photomultiplier tubes (PMTs) Slavin.40/ credits a 1930 publication by Muller and or electrodeless discharge lamps (EDLs) used in conven- Pringsheim as the initial discussion of AA instrumenta- tional AA instruments to the charge transfer devices tion. This paper described the use of AA spectroscopy in (CTDs) found in inductively coupled plasma atomic the determination of mercury, an element that remains emission spectrometry (ICPAES) instrumentation. Some important in current literature..41/ The first articles ICPAES instruments still rely on PMT technology for that addressed the use of AA spectroscopy in foren- wavelength recording and enhancement. These systems sic applications, however, did not appear until the early are still quite useful for some applications and can reach 1970s..10,12,13,42/ These articles, which pertained to GSR comparable detection levels in certain well-defined matri- analysis, represent early examples of the use of mod- ces. Further evolution has produced inductively coupled ern atomic spectrometers in forensic laboratories. Before plasma mass spectrometry (ICPMS) instrumentation, this series of articles, GSR literature concentrated almost which utilizes either a quadrupole or magnetic sector exclusively on wet chemistry techniques..43/ detection system for mass-to-electric charge ratio (m/z) Forensic assays have recently focused on the organic determinations. nature of evidence. The most current forensic science Limits of detection for these various atomic methods review article by Brettell and Saferstein.44/ illustrates can range from microgram per gram (µgg1) to fem- the number of articles that characterize