Sensitive analysis of metal cations in positive ion mode electrospray ionization mass spectrometry using commercial chelating agents and cationic ion-pairing reagents Edra Dodbiba, Chengdong Xu, Eranda Wanigasekara and Daniel W. Armstrong * Department of Chemistry and Biochemistry, University of Texas at Arlington, Arlington, TX 76019, USA Metals play a very important role in many scientific and environmental fields, and thus their detection and analysis is of great necessity. A simple and very sensitive method has been developed herein for the detection of metals in positive ion mode ESI-MS. Metal ions are positively charged, and as such they can potentially be detected in positive ion mode ESI-MS; however, their small mass-to-charge (m/z) ratio makes them fall in the low-mass region of the mass spectrum, which has the largest background noise. Therefore, their detection can become extremely difficult. A better and well-known way to detect metals by ESI-MS is by chelating them with complexation agents. In this study eleven different metals, Fe(II), Fe(III), Mg(II), Cu(II), Ru(III), Co(II), Ca(II), Ni(II), Mn(II), Sn(II), and Ag(I), were paired with two commercially available chelating agents: ethylenediaminetetraacetic acid (EDTA) and ethylenediaminedisuccinic acid (EDDS). Since negative ion mode ESI-MS has many disadvantages compared to positive ion mode ESI-MS, it would be very beneficial if these negatively charged complex ions could be detected in the positive mode. Such a method is described in this paper and it is shown to achieve much lower sensitivities. Each of the negatively charged metal complexes is paired with six cationic ion-pairing reagents. The new positively charged ternary complexes are then analyzed by ESI-MS in the positive single ion monitoring (SIM) and single reaction monitoring (SRM) modes. The results clearly revealed that the presence of the cationic reagents significantly improved the sensitivity for these analytes, often by several orders of magnitude. This novel method developed herein for the detection of metals improved the limits of detection (LODs) significantly when compared to negative ion mode ESI-MS and shows great potential in future trace studies of these and many other species. Copyright © 2012 John Wiley & Sons, Ltd. Ionized metal species are present in many different biological, ecological and industrial environments; as such they play a very important role in our lives. In many instances metals are found associated with many different organic ligands. For example, about one-quarter of all existing proteins require a specific metal to help not only fulfill their precise functions in biochemical reactions, but also to maintain their stable state. [1,2] Also, different oxidation states alters the specific role metal in a particular environment. For example, the oxide form of ruthenium (Ru), such as ruthenium(VIII) tetraoxide (RuO 4 ), is considered highly toxic, however ruthenium com- plexes containing Ru(II) and Ru(III) have been studied exten- sively and have shown great potential as anticancer agents when bound to certain ligands. [3–5] Knowing the correct oxi- dation state of Ru is also very important as Ru(II) is much more stable than Ru(III), and this does not only affect biologi- cal environments, but photochemical systems as well. [6] The metals examined in this study can have different oxida- tion states and are crucial to many different ecological and industrial systems. They are cobalt (Co), calcium (Ca), nickel (Ni), manganese (Mn), tin (Sn), silver (Ag), iron (Fe), magne- sium (Mg), and copper (Cu). A number of methods have been developed for the accurate detection and quantification of these metals. Among the most used methods to detect metals are atomic absorption spectro- metry (AAS), emission spectroscopies (ES), and inductively coupled plasma mass spectrometry (ICP-MS). [7,8] Of course, due to the high temperatures of these methods, speciation and the ability to determine oxidation state of the metals can be problematic. [7,9–12] Another technique used to detect metals and their organic complexes is electrospray ionization mass spectrometry (ESI-MS) which is a softer ionization technique, and can further provide complete characterization of a metal-organic sample. [10–16] The goal of this study is to detect anionic analytes (chelating agent + metal) in the positive ion mode at a higher sensitivity than the traditional negative ion mode ESI-MS, and to find the best ion-pairing reagent suitable for this task. In order to use this approach, the metal ions have to first associate with an anionic chelating agent. Chelating agents are organic molecules that complex with metals with different coordination geometries and strengths. Currently, there is a large selection of commercially available chelating agents. These complexes have been studied exten- sively by ESI-MS. [16–19] In this study, two well-known chelating agents were chosen and used for metal analysis. They are: ethylenediamine- N,N,N’,N’-tetraacetic acid (EDTA) and ethylenediamine- N,N’-disuccinic acid (EDDS). These multidentate ligands are aminocarboxylic acid compounds and as such they form negatively charged metal ion complexes. [10,16,20,21] Thus, they * Correspondence to: D. W. Armstrong, Department of Chem- istry and Biochemistry, University of Texas at Arlington, Arlington, TX 76019, USA. E-mail: sec4dwa@uta.edu Copyright © 2012 John Wiley & Sons, Ltd. Rapid Commun. Mass Spectrom. 2012, 26, 1005–1013 Research Article Received: 8 November 2011 Revised: 13 January 2012 Accepted: 4 February 2012 Published online in Wiley Online Library Rapid Commun. Mass Spectrom. 2012, 26, 1005–1013 (wileyonlinelibrary.com) DOI: 10.1002/rcm.6185 1005