The occurrence of electrochemical processes during the operation of an electrospray ionization (ESI) source is well established. In the positive ion mode, electrons are drawn from the ESI metal capillary to a high voltage power supply. These electrons are the product of charge-balancing oxidation reactions taking place at the liquid/metal interface of the ion source. In a recent study, (Anal. Chem.2001, 73, 4836-4844), our group has shown that the introduction of a ground loop can dramatically enhance the rate of these oxidation processes. Such a ground loop can be introduced by connecting the sample infusion syringe (or the liquid chromatography column, in the case of LC-MS studies) to ground. The magnitude of the ground loop current can be controlled by the electrolyte concentration in the analyte solution, and by the dimensions of the capillary connecting the syringe needle and the ESI source. Using ferrocene as a model system, it is demonstrated that the introduction of such a ground loop can significantly enhance the signal intensity of analytes that form electrochemically ionized species during ESI. However, analytes that form protonated molecular ions, such as reserpine, also show higher signal intensities when a ground loop is introduced into the system. This latter observation is attributed to the occurrence of electrolytic solvent (acetonitrile and/or water) oxidation processes. These reactions generate protons within the ion source, and thus facilitate the formation of [M + nH](n+) ions. Overall, this work provides an example of how the careful control of electrochemical parameters can be exploited to optimize signal intensities in ESI-MS.

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Examination of electron transfer and proton transfer reactions of lumiflavin and proton transfer reactions of the lumiflavin radical anion by Fourier transform ion cyclotron resonance mass spectrometry is described. From the equilibrium constant determined for electron transfer between 1,4-naphthoquinone and lumiflavin the electron affinity of lumiflavin is deduced to be 1.86 ± 0.1 eV. Measurements of the rate constants and efficiencies for proton transfer reactions indicate that the proton affinity of the lumiflavin radical anion is between that of difluoroacetate (331.0 kcal/mol) and p-formyl-phenoxide (333.0 kcal/mol). Combining the electron affinity of lumiflavin with the proton affinity of the lumiflavin radical anion gives a lumiflavin hydrogen atom affinity of 59.7 ± 2.2 kcal/mol. The ΔG298 deduced from these results for adding an H atom to gas phase lumiflavin, 52.1 ± 2.2 kcal/mol, is in good agreement with ΔG298 for adding an H atom to aqueous lumiflavin from electrochemical measurements in the literature, 51.0 kcal/mol, and that from M06-L density functional calculations in the literature, 51.2 kcal/mol, suggesting little, if any, solvent effect on the H atom addition. The proton affinity of lumiflavin deduced from the equilibrium constant for the proton transfer reaction between lumiflavin and 2-picoline is 227.3 ± 2.0 kcal mol(-1). Density functional theory calculations on isomers of protonated lumiflavin provide a basis for assigning the most probable site of protonation as position 1 on the isoalloxazine ring and for estimating the ionization potentials of lumiflavin neutral radicals.

The proton-bound dimer of acetonitrile and ethanol, (CH 3 CN)(CH 3 CH 2 OH)H + , exhibits three unimolecular reactions on the microsecond time scale: two simple bond cleavage reactions to form CH 3 CNH + + CH 3 CH 2 OH and CH 3 CH 2 OH 2 + + CH 3 CN, and the loss of water to form CH 3 CNCH 2 CH 3 + . The latter process is preceded by the isomerization of the proton-bound dimer to a second isomer, (CH 3 CNCH 2 CH 3 )(H 2 O) + . The competition between the simple dissociation reactions and the isomerization reaction was modeled with ab initio calculations and RRKM theory to obtain relative energies for the reaction surface. The 0 K binding energy of the (CH 3 CN)(CH 3 CH 2 OH)H + complex was calculated to be 152 kJ mol -1 at the G2(MP2,SVP) level of theory (relative to the dissociation products CH 3 CNH + and CH 3 CH 2 OH). The isomerization barrier for the proton-bound dimer was estimated to be 22 kJ mol -1 lower than CH 3 CNH + + CH 3 CH 2 OH. The greater polarizability of the ethyl group is believed to stabilize this transition structure over that found for the (CH 3 CN)(CH 3 OH)H + ion (which was previously estimated to lie 6 kJ mol -1 below CH 3 CNH + and CH 3 OH).

Mass spectrometry and ab initio calculations have been employed to investigate the unimolecular decompositions of proton-bound dimers consisting of acetonitrile with n-and i-propanol. Common to both systems is a competition between dissociation of the hydrogen bond in the proton-bound dimer and isomerization to (CH 3 CNR)(H 2 O) + [R = CH 2 CH 2 CH 3 and CH(CH 3) 2 ]. The minimum-energyreaction pathways for the isomerization in these two systems, as well as those for the dimers containing methanol and ethanol, are presented and compared. The dominant isomerization pathway for these ions is an internal S N 2 reaction that proceeds via a stable intermediate CH 3 CN•••ROH 2 + ion [R = CH 3 , CH 2 CH 3 , CH 2 CH 2 CH 3 and CH(CH 3) 2 ]. The mass spectra for the four butanol-containing dimers (n-, s-, i-and t-butanol) follow similar behavior.