Electrochemical processes in a wire-in-a-capillary bulk-loaded, nano-electrospray emitter.

Experiments are described that illustrate solvent oxidation, emitter electrode corrosion, and analyte oxidation in positive ion mode nano-electrospray mass spectrometry using a wire-in-a-capillary, bulk-loaded nano-electrospray emitter geometry. Time-lapsed color photography of pH and metal specific indicator solutions within operating nano-electrospray emitters, as well as temporal changes in the ions observed in the nano-electrospray mass spectra, are used to probe these reactions, judge their magnitude, and study the time dependent changes in solution composition and gas-phase ion signal brought about as a result of these electrochemical reactions. The significance of these observations for analytical applications of nano-electrospray mass spectrometry are discussed.

Ion/ion proton-transfer kinetics: implications for analysis of ions derived from electrospray of protein mixtures.

Protein ions of different mass and charge but similar mass-to-charge ratios are shown to undergo significantly different rates of differential neutralization, defined as the rate of change of charge with time, upon initiation of reactions with oppositely charged ions in the quadrupole ion trap. Overlapping charge state distributions arising from mixtures of ions of dissimilar charge are separated on the mass-to-charge scale at short reactions times. It is also demonstrated that the time frame for near total neutralization, defined as charge reduction to the 1+ ion, is relatively insensitive to initial charge state. It is shown, for example, that the (M + 11H)(11+)-(M + 22H)22+ ions derived from horse skeletal muscle apomyoglobin yield the (M + H)+ ion as the major ion/ion reaction product over the same reaction period that largely converts doubly protonated bradykinin to the singly protonated species. Less than 25% of the bradykinin ions are expected to be totally neutralized when roughly 7% of the myoglobin ions are expected to be totally neutralized. The phenomenon of significantly different initial differential neutralization rates for ions of dissimilar charge, and the relative insensitivity to ion charge for total neutralization, can be used to advantage in strategies for protein ion mixture analysis.

Observation of gas-phase molecular dications formed from neutral organics in solution via qemical electron-transfer reactions by using electrospray Ionization Mass Spectrometry.

The first observation of organic dications formed by multiple electron loss in electrospray mass spectra is reported. The dications of ß-carotene, canthaxanthine, cobalt(II) octaethylporphyrin, and nicke(II) octaethylporphyrin were created in solution via chemical electrontransfer reactions and detected in the gas phase by electrospray ionization mass spectrometry (ES-MS) using a flow-injection experiment. The analytes were injected into a flowing solvent-oxidant stream (10 µL/min) composed of dried methylene chloride containing ≈ 0.1% by volume trifluoroacetic acid and 0.1% by volume antimony pentafluoride (SbF5). The dications created in this oxidizing solvent system were preserved for detection by rapidly transferring them from the reactive solvent-oxidant system to the gas phase, where, in the absence of the solvent system, they were "long-lived" and amenable to mass analysis. This work demonstrates means to produce ions novel to ES-MS and means to detect and study by ES-MS species that are short-lived in solution. In addition, this work shows that electrospray ionization can potentially be used to generate gas-phase dications for mass spectrometric study that are difficult to produce directly from gas-phase neutrals by other ionization techniques (e.g., M(2+) from ß-carotene).

Determination of daughter ion formulas by multiple stages of mass spectrometry.

The ability to obtain daughter ion formulas via comparison of MS(n) spectra of parent ions containing only (12)C with those of parent ions with one (13)C (from the natural (13)C abundance) is shown for cases in which isobaric interferences with the (13)C-containing ion preclude the use of the conventional tandem mass spectrometric approach. This method allows the presence of isobaric daughter ions to be ascertained, and unexpected, complex dissociation pathways to be identified. A three-dimensional quadrupole ion trap is used for these experiments. Its high tandem mass spectrometry efficiency makes possible this type of experiment.