Isotope ratio mass spectrometry Zeland Muccio and Glen P. Jackson * First published as an Advance Article on the web 14th November 2008 DOI: 10.1039/b808232d Isotope Ratio Mass Spectrometry (IRMS) is a specialized technique used to provide information about the geographic, chemical, and biological origins of substances. The ability to determine the source of an organic substance stems from the relative isotopic abundances of the elements which comprise the material. Because the isotope ratios of elements such as carbon, hydrogen, oxygen, sulfur, and nitrogen can become locally enriched or depleted through a variety of kinetic and thermodynamic factors, measurement of the isotope ratios can be used to differentiate between samples which otherwise share identical chemical compositions. Several sample introduction methods are now available for commercial isotope ratio mass spectrometers. Combustion is most commonly used for bulk isotopic analysis, whereas gas and liquid chromatography are predominately used for the real-time isotopic analysis of specific compounds within a mixture. Here, highlights of advances in instrumentation and applications within the last three years are provided to illustrate the impact of this rapidly growing area of research. Some prominent new applications include authenticating organic food produce, ascertaining whether or not African elephants are guilty of night-time raids on farmers’ crops, and linking forensic drug and soil samples from a crime scene to a suspected point of origin. For the sake of brevity, we focus this Minireview on the isotope ratio measurements of lighter-elements common to organic sources; we do not cover the equally important field of inorganic isotope ratio mass spectrometry. Introduction Isotope ratio mass spectrometry (IRMS) is a technique which finds increasingly widespread use in disciplines such as archae- ology, medicine, geology, biology, food authenticity, and forensic science. The histogram plot in Fig. 1 shows the number of publications per year containing the research topic ‘Isotope ratio mass spectrometry’ using SciFinder Scholar 2006 (searched on May 3, 2008) and reflects the rapid growth in applications since the introduction of commercially-available instrumentation approximately ten years ago. The fastest growth is arguably in forensic applications, where the ability to differentiate substances by their geographical origins provides information that is diffi- cult or unattainable by any other technique. Disciplines which stand to benefit from IRMS are those which require the ability to accurately and precisely measure variations in the abundance of isotopic ratios of light elements such as 13 C/ 12 C, 18 O/ 16 O, D/ 1 H, 15 N/ 14 N, and 34 S/ 32 S. The ratios of these isotopes are always measured relative to an isotopic standard in order to eliminate any bias or systematic error in the measure- ments. These standards are, or can be linked to, internationally recognized standards such as Vienna Pee Dee Belemnite (VPDB) Zeland Muccio Zeland Muccio is a Ph.D. student in the Department of Chemistry and Biochemistry at Ohio University, Athens, OH. Her primary research is on developing novel forensic applications using gas chromatography- isotope ratio mass spectrometry (GC-IRMS). Zeland holds an Associate’s degree in Law Enforcement, Bachelors degrees in Business Administration and Chemistry and a Masters degree in Management Information Systems. Glen Jackson Glen P. Jackson is an Assistant Professor in the Department of Chemistry and Biochemistry at Ohio University, Athens, OH. His research interests include ion trap development, forensic applications of isotope ratio mass spectrometry and the construction of miniature, portable mass spectrometers. He also teaches workshops on Forensic Applications of Mass Spectrometry. Center for Intelligent Chemical Instrumentation, Department of Chemistry and Biochemistry, 175 Clippinger Laboratories, Ohio University, Athens, OH 45701-2979, USA. E-mail: jacksong@ohio.edu; Tel: +1 740 593-0797 This journal is ª The Royal Society of Chemistry 2009 Analyst, 2009, 134, 213–222 | 213 MINIREVIEW www.rsc.org/analyst | Analyst