Multistage Reactive Transmission-Mode Desorption Electrospray Ionization Mass Spectrometry

B American Society for Mass Spectrometry, 2015 J. Am. Soc. Mass Spectrom. (2015) 26:1494Y1501 DOI: 10.1007/s13361-015-1171-5 RESEARCH ARTICLE Multistage Reactive Transmission-Mode Desorption Electrospray Ionization Mass Spectrometry Kevin C. Peters, Troy J. Comi, Richard H. Perry Department of Chemistry, University of Illinois, Urbana, IL 61801, USA Abstract. Elucidating reaction mechanisms is important for advancing many areas of science such as catalyst development. It is often difficult to probe fast reactions at ambient conditions with high temporal resolution. In addition, systems involving reagents that cross-react require analytical methods that can minimize interaction time and specify their order of introduction into the reacting system. Here, we explore the utility of transmission mode desorption electrospray ionization (TM-DESI) for reaction monitoring by directing a microdroplet spray towards a series of meshes with micrometer-sized openings coated with reagents, an approach we call multistage reactive TM-DESI (TMn-DESI, where n refers to the number of meshes; n=2 in this report). Various stages of the reaction are initiated at each mesh surface, generating intermediates and products in microdroplet reaction vessels traveling towards the mass spectrometer. Using this method, we investigated the reactivity of iron porphyrin catalytic hydroxylation of propranolol and other substrates. Our experimental results indicate that TMn-DESI provides the ability to spatially separate reagents and control their order of introduction into the reacting system, thereby minimizing unwanted reactions that lead to catalyst deactivation and degradation products. In addition, comparison with DESI-MS analyses (the Zare and Latour laboratories published results suggesting accessible reaction times <1 ms) of the reduction of dichlorophenolindophenol by L-ascorbic acid suggest that TM1-DESI can access reaction times less than 1 ms. Multiple meshes allow sequential stages of desorption/ionization per MS scan, increasing the number of analytes and reactions that can be characterized in a single experiment. Keywords: Ambient mass spectrometry, Transmission-mode desorption electrospray ionization, Reaction monitoring, Mechanisms, Time scale, Catalysis, Iron porphyrin Received: 25 February 2015/Revised: 16 April 2015/Accepted: 18 April 2015/Published Online: 20 June 2015 Introduction C haracterizing reaction mechanisms is important for developing new catalysts, drugs, and materials that address scientific and socioeconomic problems[1]. However, it is often difficult to obtain detailed molecular mechanistic information under normal operating conditions because intermediates have short lifetimes (typically less than ~1 s), exist in a complex and dynamic matrix involving multiple reaction pathways, and have low concentrations [2–5]. These difficulties are significantly more pronounced for homogenous multi-catalytic systems [6], which show great promise for rapidly achieving one- Electronic supplementary material The online version of this article (doi:10.1007/s13361-015-1171-5) contains supplementary material, which is available to authorized users. Correspondence to: Richard Perry; e-mail: pot complex transformations that improve efficiency, selectivity, and enantiomeric purity. Advancing these areas of research demand analytical technologies that can capture fleeting intermediates in real-time at ambient conditions, as well as provide the ability to separate reagents and specify their order of introduction to minimize unwanted side reactions between incompatible reagents during analysis and to facilitate step-wise elucidation of reaction mechanisms. Electrospray ionization mass spectrometry (ESI-MS) methods are one of the primary approaches for obtaining realtime information about solution-phase molecular species formed in the course of a reaction with high sensitivity, speed, and selectivity [7]. The recent introduction of ambient mass spectrometric techniques [8–15] such as desorption electrospray ionization (DESI)[16] developed by Cooks and co-workers has revolutionized analytical chemistry in the past decade, allowing chemical analyses with minimal sample preparation. A recent advance developed by Zare and co-workers K. C. Peters et al.: Multistage Reactive Transmission-Mode DESI involves using DESI to capture short-lived solution-phase reaction intermediates (<1 ms) [17–23] at ambient conditions, while minimizing sample preparation times, carry over effects, and experimental complexity compared with ESI configurations. This elegant discovery has opened the possibility for the development of new types of ambient ionization sources and applications for characterizing fast solution-phase processes [4, 24–32]. Transmission-mode DESI (TM-DESI)[33–38], developed by Brodbelt and co-workers, is an ambient MS method that involves directing an electrostatically charged solvent spray at a mesh having micrometre-sized open areas on which analytes of interest are deposited. TM-DESI requires small sample volumes and minimal source optimization prior to analysis, making it easily amenable to high-throughput analyses[33, 36, 39, 40]. TM-DESI has been previously used for direct analysis of samples containing analytes such as peptides and small organic molecules [33]. In addition, the mesh can be functionalized for selective extraction of analytes from complex matrices followed by TM-DESI-MS [35, 40]. Despite its simplicity, the utility of TM-DESI for probing chemical reactivity has not yet been explored. Herein, we describe a TM-DESI-based ionization source that employs two meshes in series (M1 and M2) (Figure 1), referred to hereafter as multistage reactive transmission-mode DESI (TMn-DESI, where n represents the number of desorption stages; conventional TM-DESI configuration has n=1) for characterization of reaction mechanisms. Using TM2-DESIMS, we studied iron (Fe) porphyrin-catalyzed hydroxylations such as Fe tetra(pentafluorophenyl)-porphyrin (Fe-TPFPP, 1) hydroxylation of substrate propranolol 2 (proposed mechanism shown in Scheme 1 [41]), previously analyzed by ESI Fourier transform ion cyclotron resonance MS[42, 43]. Experimental results show that TM2-DESI provides the ability to spatially separate reagents and to specify their order of introduction into microdroplet reaction vessels, reducing off-path processes such as oxidant-mediated hydroxylation of 2 (Scheme 1). In addition, comparison of TM1-DESI-MS and DESI-MS analyses of the chemical reduction of dichlorophenolindophenol (DCIP) by L-ascorbic acid (L-AA; Figure 3; reaction previously analyzed by liquid DESI-MS[44]) showed that TM1-DESI can access reaction times less than 1 ms. These capabilities, coupled with the high-throughput features of TM-DESI, demonstrate a unique ambient ionization approach for chemical analyses at ambient conditions. Multiple meshes increase the number of analytes and reaction steps that can be c (...truncated)