Forensic Chemistry 13 (2019) 100154 Contents lists available at ScienceDirect Forensic Chemistry journal homepage: www.elsevier.com/locate/forc Isotope ratio mass spectrometry in forensic science applications T ⁎ Mayara P.V. Matosa, Glen P. Jacksonb,c, a Department of Biology, West Virginia University, Morgantown, WV, United States b Department of Forensic and Investigative Science, West Virginia University, Morgantown, WV, United States c C. Eugene Bennett Department of Chemistry, West Virginia University, Morgantown, WV, United States HIGHLIGHTS • Review of forensic applications of isotope ratio mass spectrometry publications since 2009. • Includes details on good practices for calibration, quality assurance and uncertainty assessments and reporting. • Review is organized into 10 different forensic science disciplines. ABSTRACT This review highlights advances in the application of isotope ratio mass spectrometry (IRMS) to materials of interest to the forensic community. We give special attention to the natural abundance differences in the stable isotopes of carbon, nitrogen, hydrogen and oxygen because they are the most abundant andinformative elements in typical forensic casework. We also present some of the recent developments in normalization, error reporting and data analysis, which together provide a more scientific and robust foundation for admissibility in court. The review focusses on research published in the last decade and is organized into 10majorsections, including human provenancing, wildlife forensics, environmental forensics, seized drugs, ignitable liquids, explosives, food forensics, poisoning, questioned docu- ments and miscellaneous applications. 1. Introduction IRMS to forensic casework. FIRMS has been organizing triennial con- ferences, providing forensic practitioner approvals, conducting inter- The ability to unravel the source of chemically indistinguishable laboratory proficiency tests [2,14], presenting guidance on suitable substances has made isotope ratio mass spectrometry (IRMS) an indis- reference materials (RMs), and offering a “Good Practice Guide for pensable tool in a variety of scientific disciplines, and especially in IRMS” (GPG) to assist practitioners with the essential principles of the forensic science [1–3]. There are many excellent reviews and monologs instrument operation, quality assurance, troubleshooting and data on the theory, development, and application of stable isotope analysis management [15]. The FIRMS Network often publishes a special issue (SIA) in different forensic science areas of study [4–13]. Herein, we do in an analytical or forensic science journal following each triennial not attempt to be so comprehensive. Instead, we focus on the most conference, and these special editions are a rich source of forensically- recent, novel and impactful forensic applications, including new aspects relevant IRMS literature for anyone interested in the topic. of data analysis and reporting. In the last decade, IRMS, in combination Relating to the FIRMS Network, Carter and Fry [16] conducted a with improved chromatographic techniques, has provided advances in series of inter-laboratory tests within FIRMS Network members to first diverse areas of forensic science, including sample preparation techni- assess the variance, quality control and quality assurance protocols of ques that have enabled more reliable or meaningful discrimination different laboratories, and then to guide improvements in there- between samples. The review is organized into ten major topical areas producibility of measurements between laboratories. Problems related of forensic interest. Most of the citations are for applications that in- to homogeneity in sample preparation, choice of reference materials volve source provenance and sample classification, but some articles and internal standards, as well as data reporting relative to delta (δ) report on better understanding the fractionation of isotopes within or scale, are among the identified deficiencies that required improvements between certain substrates. to make IRMS results fit-for-purpose. Only a minority of labs were Since its foundation in 2002, the Forensic Isotope Ratio Mass outside the expected range of the results. More recently, another inter- Spectrometry Network (FIRMS) has spearheaded the application of laboratory assessment was performed to compare the bulk isotope ⁎ Corresponding author at: Department of Forensic and Investigative Science, West Virginia University, Morgantown, WV, United States. E-mail address: [email protected] (G.P. Jackson). https://doi.org/10.1016/j.forc.2019.100154 Received 28 January 2019; Received in revised form 3 March 2019; Accepted 3 March 2019 Available online 06 March 2019 2468-1709/ © 2019 Elsevier B.V. All rights reserved. M.P.V. Matos and G.P. Jackson Forensic Chemistry 13 (2019) 100154 analysis of honey coming from diverse metrological institutes [17]. The lead, such as insight into a person’s geographic origin or travel history. results derived from forensic stable isotope laboratories within the Assisting such investigations, many IRMS studies have examined the FIRMS Network [18], and all the participating laboratories, provided relationship between the isotopic composition of human tissues and the δ13C values for bulk honey that were within an acceptable range. The geographic origin, health status or dietary interventions of the subjects. method by which each laboratory calculated and reported their mea- Such studies are useful for both paleo-archeological purposes and surement uncertainty was a notable point of discussion, however, be- modern law enforcement investigations because both applications re- cause some laboratories report standard deviations instead of expanded quire the prediction of geographic origin of unidentified human re- measurement uncertainty, such as the 95% confidence interval. To mains [10,11,44–46]. align with international standards, the use of an expanded uncertainty A critical factor for strong geo-location inference is the establish- instead of a standard deviation is recommended for reported results. ment of reliable and high-resolution spatial maps for the precipitation To assist practitioners in establishing expanding uncertainties, Dunn and groundwater values for δ2H and δ18O [47,48]. Such isotope land- and coworkers [19] have suggested a simple and straightforward im- scapes, also known as Isoscapes [49], provide the framework for spa- provement to the Kragten spreadsheet published in the GPG [15], tiotemporal movement of humans and other organisms [2,47,50]. In which complements the existing approaches and guidelines used for 2014, δ2H and δ18O measurements of 349 US tap water samples were delta scale [20]. The new spreadsheet is quite practical, and it combines made available in the largest public dataset to date. The samples in- the different sources of isotope uncertainty to multiple samples inone cluded a broad geographic sampling and seasonal sampling [51]. Dr. spreadsheet. Note that analytical results can include human errors, Gabriel Bowen, from the University of Utah, has made the spatial which are outside the random errors that are captured in uncertainty- variation metadata for water isotopes fully accessible through the budgeting spreadsheets. All researchers and practitioners should take Water Isotopes Database (http://wateriso.utah.edu/waterisotopes/ preventative steps to promptly identify and mitigate sources of occa- index.html) and through a recently developed mobile phone applica- sional mistakes, thereby enhancing the reliability of any reported result tion called “wiSamples”. Publicly-available data like this are critical for [21,22]. the validation and acceptance of Isoscapes for modern human prove- One of the biggest challenges in IRMS is the selection and use of nancing, and some groups have successfully started using Isoscapes for appropriate—i.e. matrix matched—and well-characterized reference such applications [52]. As a word of caution to prospective researchers materials (RMs) [23]. The last few years have witnessed significant in this area, one has to be especially careful with the calibration and developments in RMs for calibration of δ2H, δ18O [24–27], δ13C, δ15N, interpretation of δ2H measurements of proteinaceous material like hair, and δ34S values [28–30]. One RM is a mixture of three glycine solutions muscle and feathers [53]. The number of exchangeable hydrogens that are recommended for establishing multiple-point calibration varies greatly depending on the protein composition, so it is critical to curves for the δ13C scale [29]. Another three sugar RMs (BEET-1, GALT- use matrix-matched standards to obtain accurate isotope ratio nor- 1 and FRUT-1) were developed for δ13C normalization of sugar-based malization [54]. materials, although it can also be used for other organic materials [31]. For geographical provenance determinations, biological samples The hair RMs USGS-42 and USGS-43 have been developed for mea- must be representative of the organism and be chemically robust to surements of hydrogen and oxygen stable isotopes, but are also ap- provide meaningful isotope ratios, which is why bone, teeth, hair and propriate for measurements involving C, N, and S of human and nails are especially useful matrices. Hair and fingernails have the added mammalian hair [28]. The use of (at least) a two-point calibration scale advantages that they store a chronological record of diet and health of a is now a minimum requirement for