Solvent Molecules Undergo Homolytic Cleavage and Radical Recombination Processes during Negative-Mode Electrospray Ionization: Adduct Formation with Antimony(III)-Tartrate Dianion

Abstract

Negative-ionization mode electrospray ionization-mass spectrometry (ESI-MS) analysis of antimony(III)-tartrate in frequently used solvent systems, ACN/H(2)O and MeOH/H(2)O, revealed that the antimony(III)-tartrate dianion associates to solvent reaction products generated by radical formation and their subsequent recombination during the negative-mode electrospray process. A systematic increase and decrease in negative spray capillary voltage (SCV) from normal operational voltage ranges of a conventional quadrupole ion trap instrument during these analyses showed initially unobserved adduct ions to correspondingly increase and diminish in relative ion intensity. The identity of the adducted species, including products such as H(2)O(2), NCCH(2)CH(2)CN, and CH(2)(OH)(2), were confirmed by performing similar experiments with deuterated and nondeuterated solvent mixtures. Relative intensity dependence of these adducted ions was monitored as the volume composition of each solvent system was changed. It was clearly observed that the relative intensity of {[Sb(2)-tar(2)][H-O-O-H]}(2-) and {[Sb(2)-tar(2)][NC-CH(2)-CH(2)-CN]}(2-) adduct ions increased with the volume percent of H(2)O and CH(3)CN, respectively. Similarly, an increase in volume percent of CH(3)OH increased the relative intensity of {[Sb(2)-tar(2)][H-O-CH(2)-O-H]}(2-) adducted ions. On the basis of this evidence, it was proposed that homolytic cleavage of C-H bonds for CH(3)CN and CH(3)OH molecules, and O-H bonds for H(2)O molecules, produces a series of radicals during negative-ionization mode ESI, and subsequent self-recombination or cross-recombination of these radicals then occurs to form the neutral solvent products, which are observed in the mass spectra as [Sb(2)-tar(2)]-adducted ionic species. These findings provide new insight into processes, which are relevant to understanding the mechanism of electrospray ionization, a widely used technique.

New structural insight for antimony(III)-tartrate

“…Enantioselective molecular recognition properties of tartar emetic in solution and in the gas phase have often been explained based on this particular geometry [5]. However, explicating our observations [7][8][9][10], particularly the proton-assisted enantioselective character of tartar emetic was not trivial using this particular structure alone. Also, in several instances, it was mentioned that the anti-parasitic and anti-bacterial activity of tartar emetic is still not fully understood [2,3].…”

“…When a proton was not attached, the antimony(III)-L-tartrate dianion failed to show enantioselective molecular recognition properties. The use of ESI-MS was crucial for visualizing this disparity in enantioselective biomolecular recognition capacity between the proton-bound and unbound antimony(III)-D,L-tartrate complexes.Additionally, a serendipitous finding from our chiral biomolecular recognition studies was the observation of antimony(III)-tartrate's capability to adduct with unusual solvent reaction products that were proposed to be generated during negative ionization mode electrospray [10]. Studies on adduct ion formation between dianionic antimony(III)-L-tartrate and species like hydrogen peroxide and succinonitrile have provided new insight into the processes that are relevant to the fundamental understanding of electrospray ionization.…”

“…Additionally, a serendipitous finding from our chiral biomolecular recognition studies was the observation of antimony(III)-tartrate's capability to adduct with unusual solvent reaction products that were proposed to be generated during negative ionization mode electrospray [10]. Studies on adduct ion formation between dianionic antimony(III)-L-tartrate and species like hydrogen peroxide and succinonitrile have provided new insight into the processes that are relevant to the fundamental understanding of electrospray ionization.…”

Multistage Reactive Transmission-Mode Desorption Electrospray Ionization Mass Spectrometry

“…Other spray-based ionization sources (e.g., continuous-flow ESI, stopped-flow ESI, and DESI) would require additional tubing, pumps, microdroplet emitters, or valves to achieve high-throughput analyses of complex reactions similar to TM Figure S10; viz., applying a voltage increased sensitivity). In addition, ion signals suggesting formation of reactive oxygen species [57] or other unwanted byproducts were not observed (Supplementary Figure S10 + was observed at m/z 270.008 (Figure 3a) with intensity of~4% relative to [4a + H] + at m/z 267.993. Assuming that the concentration of DCIP in the secondary microdroplets is low compared with L-AA, the observed reaction time in microdroplets (t) for TM 1 -DESI (t TM ) and DESI (t DESI ) can be approximated using pseudo-first-order kinetics [58] (see SI) Table S1), which suggests that t TM and t DESI have the same order of magnitude of less than 1 ms (t DESI was experimentally estimated by Latour and coworkers [59,60]).…”

Paper-Based Electrochemical Cell Coupled to Mass Spectrometry

“…SSI was used to avoid voltage-induced reaction pathways such as the formation of reactive oxygen species, which can occur with water (H 2 O) or acetonitrile (CH 3 CN) as ESI solvents [82]. When a solution of Cys (MW = 121 gmol -1 ; 10 mM in 1:1 water:methanol (H 2 O:CH 3 OH)) was used as the microdroplet spray and KCl (10 mM) as the electrolyte added to the paper substrate, application of a potential difference (ΔV) = 4 V across electrodes E1 and E2 yielded mass spectra (Figure 2b and c) + (m/z 284.994).…”