Supercharging in electrospray ionization: effects on signal and charge

Multiply charged ions are ideally suited to tandem mass spectrometry, where fragmentation efficiency and pathways are typically a strong function of charge. Addition of either of two compounds, in-nitrobenzyl alcohol (m-NBA) or glycerol, to electrospray solutions results in an increase in the number of charges that can be added to gas-phase protein cations. Overall electrospray ionization (ESI) signal is not adversely affected by adding these compounds. Thus, these charge enhancers work by increasing the absolute number of higher charge state ions. This ability to enhance charge appears to be related to the high surface tensions of these compounds. Electrospray droplets consisting of solvents with higher surface tension require additional charging at the droplet surface in order to undergo Rayleigh fission. During the ion formation process, the higher density of charge at the droplet surface translates into higher charge states of the gas-phase analyte ion. Addition of m-NBA also enhances formation of high charge states of negative ions. For the synthetic polymer, poly(ethylene glycol) (PEG), addition of m-NTBA results in an increase in the charge states by increasing the cationization of the polymer. In contrast, addition of glycerol results in a decrease in the charge states, presumably because it competes for sodium ions due to its high sodium affinity. Addition of 1-20% m-NBA to electrospray solutions can enhance the formation of higher charge state ions with no reduction in overall electrospray signal for a wide variety of analyte ions. This appears to be an ideal compound for enhancing high charge states for MS/MS and other experiments for which high charge states are desired. (C) 2002 Elsevier Science B.V. All rights reserved.