Protonation in electrospray mass spectrometry
The term “wrong-way-round ionization” has been used in studies of electrospray ionization to
describe the observation of protonated or deprotonated ions when sampling strongly basic or
acidic solutions (respectively) where such ions are not expected to exist in appreciable
concentrations in solution. Study of the dependence of ionization of the weak base caffeine on
the electrospray capillary potential reveals three distinct contributors to wrong-way-round
ionization. At near-neutral pH in solutions of low ionic strength, protonation of caffeine results
from the surface enrichment of electrolytically produced protons in the surface layer of the
droplets from which ions are desorbed. For solutions made strongly basic with ammonia,
gas-phase proton transfer from ammonium ions can create protonated caffeine. These two
mechanisms have been discussed previously elsewhere. For solutions of high ionic strength at
neutral or high pH, the data suggest that discharge-induced ionization is responsible for the
production of protonated caffeine. This mechanism probably accounts for some of the
wrong-way-round ionization reported elsewhere. (J Am Soc Mass Spectrom 2000, 11,
961–966) © 2000 American Society for Mass Spectrometry
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