Charge reduction electrospray

L. M. Smith, Corona Discharge in Charge Reduction Electrospray Mass Spectrometry, Anal. Chem. 2000, 72, 5158. 

Frey, B.L. Lin, Y Westphall, M.S. Smith, L.M. Controlling gas-phase reactions for efficient charge reduction electrospray mass spectrometry of intact proteins. J. Am. Soc. Mass Spectrom. 2005,16, 1876-1887. 

CREMS Charge reduction electrospray mass spectrometry... 

Scalp, M. Westphall, M. S. Smith, L. M. Charge reduction electrospray mass spectrometry. Anal Chem. 2000, 72, 52-60. 

When experiments with electrospray producing H3N(CH2)pNH2+ ions were performed in this laboratory,56 it was found that doubly protonated ions could be obtained from methanol-water solutions only when p > 4. For p < 4 only the singly protonated ions were observed, even though the doubly protonated ions are known to be present in the solution. We attribute the failure to observe the doubly protonated ions with p < 4 to the occurrence of charge reduction by deprotonation. Probably methanol, whose gas-phase basicity is greater than that of water, is involved in the deprotonation. The diprotonated diammines, p > 4, could all be dehydrated down to the naked ion either in CID experiments or at higher temperature. 

The hydrated sulfate S04(H20)2 could be produced by electrospray in the gas phase,80 but neither the triply charged orthophosphate P04 nor the doubly charged HOPOj" were observed as the naked ion or the hydrate.81 CID of the hydrated sulfate led to simple desolvation down to n = r = 4. The decomposition of the r = 4 hydrate led to charge reduction by intracluster proton transfer ... 

We have previously considered the mechanism of electrospray ionization in terms of the charging of droplets containing analyte and the formation of ions as the charge density on the surface of the droplet increases as desolvation progresses. The electrospray system can also be considered as an electrochemical cell in which, in positive-ion mode, an oxidation reaction occurs at the capillary tip and a reduction reaction at the counter electrode (the opposite occurs during the production of negative ions). This allows us to obtain electrospray spectra from some analytes which are not ionized in solution and would otherwise not be amenable to study. In general terms, the compounds that may be studied are therefore as follows ... 

A characteristic feature of ESMS is the detection of multiply charged analytes. Macromolecules, such as proteins have multiple sites where protonation or deprotonation (the two most common charge inducing mechanisms in electrospray—other routes to charge induction include, ionization through adduct formation, through gas-phase reactions, and through electrochemical oxidation or reduction) occur. These are desorbed effectively in ESMS and... 

Muddknan, D.C. Cheng, X.H. Udseth, H.R. Smith, R.D. Charge-state reduction with improved signal intensity of oligonucleotides in electrospray ionization mass spectrometry. J. Am. Soc. Mass Spectrom. 1996, 7, 697-706. 

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