Does Electrospray Ionization Produce Gas-Phase or Liquid-Phase Structures?

Abstract

Electrospray ionization of tyrosine from a 3:1 (v:v) CH₃OH/H₂O solution is found to afford an M − H ion which is a 70:30 mixture of phenoxide and carboxylate ions. This corresponds to the gas-phase equilibrium composition and not the liquid-phase proportions. In contrast, the carboxylate is produced as the dominant ion (∼95%) from anhydrous CH₃CN and CH₃CN/H₂O mixtures. The addition of small amounts of CH₃OH to the solvent, however, convert the M − H ion back into the gas-phase isomeric ratio. The isomeric structure therefore depends on the solvent system from which an ion is sprayed.