Infrared ion spectroscopy: New opportunities for small-molecule identification in mass spectrometry - A tutorial perspective. Author: Martens J1, van Outersterp RE2, Vreeken RJ3, Cuyckens F3, Coene KLM4, Engelke UF4, Kluijtmans LAJ4, Wevers RA4, Buydens LMC5, Redlich B2, Berden G2, Oomens J6 Affiliation: <sup>1</sup>Radboud University, Institute for Molecules and Materials, FELIX Laboratory, Toernooiveld 7, 6525 ED, Nijmegen, the Netherlands. Electronic address: jonathan.martens@science.ru.nl. <sup>2</sup>Radboud University, Institute for Molecules and Materials, FELIX Laboratory, Toernooiveld 7, 6525 ED, Nijmegen, the Netherlands. <sup>3</sup>Drug Metabolism &amp; Pharmacokinetics, Janssen R&amp;D, Beerse, Belgium. <sup>4</sup>Department of Laboratory Medicine, Translational Metabolic Laboratory, Radboud University Medical Center, Nijmegen, the Netherlands. <sup>5</sup>Radboud University, Institute for Molecules and Materials, Chemometrics, Heyendaalseweg 135, 6525AJ, Nijmegen, the Netherlands. <sup>6</sup>Radboud University, Institute for Molecules and Materials, FELIX Laboratory, Toernooiveld 7, 6525 ED, Nijmegen, the Netherlands; van't Hoff Institute for Molecular Sciences, University of Amsterdam, 1098XH, Amsterdam, Science Park 908, the Netherlands. Electronic address: j.oomens@science.ru.nl. Conference/Journal: Anal Chim Acta. Date published: 2020 Jan 6 Other: Volume ID: 1093 , Pages: 1-15 , Special Notes: doi: 10.1016/j.aca.2019.10.043. Epub 2019 Oct 24. , Word Count: 223 Combining the individual analytical strengths of mass spectrometry and infrared spectroscopy, infrared ion spectroscopy is increasingly recognized as a powerful tool for small-molecule identification in a wide range of analytical applications. Mass spectrometry is itself a leading analytical technique for small-molecule identification on the merit of its outstanding sensitivity, selectivity and versatility. The foremost shortcoming of the technique, however, is its limited ability to directly probe molecular structure, especially when contrasted against spectroscopic techniques. In infrared ion spectroscopy, infrared vibrational spectra are recorded for mass-isolated ions and provide a signature that can be matched to reference spectra, either measured from standards or predicted using quantum-chemical calculations. Here we present an overview of the potential for this technique to develop into a versatile analytical method for identifying molecular structures in mass spectrometry-based analytical workflows. In this tutorial perspective, we introduce the reader to the technique of infrared ion spectroscopy and highlight a selection of recent experimental advances and applications in current analytical challenges, in particular in the field of untargeted metabolomics. We report on the coupling of infrared ion spectroscopy with liquid chromatography and present experiments that serve as proof-of-principle examples of strategies to address outstanding challenges. Copyright © 2019. Published by Elsevier B.V. KEYWORDS: Biomarker discovery; Inborn errors of metabolism; Infrared ion spectroscopy; Liquid chromatography; Molecular structure identification; Untargeted metabolomics PMID: 31735202 DOI: 10.1016/j.aca.2019.10.043