Formation and fate of ion pairs during MALDI analysis: anion adduct generation as an indicative tool to determine ionization processes.

Stimulated by recent experiments, which verified the preservation of the analyte solution charge state upon incorporation in the host matrix crystals, investigations are reported focusing on the role of analyte and counter ions in the matrix-assisted laser desorption/ionization (MALDI) process. These counter ions are only visible in the MALDI mass spectra under certain conditions, i.e., if inter-ionic proton transfer followed by evaporation of the neutrals is prevented, as in the case of metal cations. However, ion pairs can also survive the MALDI process if anions of very low gas phase basicities are used. By this means the intermediates of ion production in MALDI can be visualized. Depending on the amount of energy transfer to the analyte, which is mainly controlled by the matrix, different grades of adduct generation are observed. The analyte-, matrix- and polarity-dependant adduct distribution substantiates the hypothesis that multi-ion pairs are incorporated in the MALDI crystals and that ionization is essentially accomplished by charge separation processes. Moreover, the adduct distribution--and most probably also the charge separation efficiency--was found to be caused mainly by competition of different anionic species for coordination at the positively charged analyte sites. Furthermore, the results point to a less efficient charge separation with increasing number of ion pairs, which might be one major reason that mainly singly charged ions are obtained with MALDI.