Cationization Processes in Matrix-assisted Laser Desorption/Ionization Mass Spectrometry: Attachment of Divalent and Trivalent Metal Ions

Abstract

In this paper, divalent and trivalent metal ions have been used as cationization reagents in matrix-assisted laser desorption/ionization (MALDI) experiments. Analytes studied include polyethylene glycol, polyethylene oxide dimethyl ether, cyclic polyethylene oxide and α-cyclodextrin. Independent of the valence of the metal ions and the nature of the analyte molecules studied, singly charged quasimolecular ions were generated in all cases. These quasimolecular ions were shown to be generated in different forms depending on the type of the di- or trivalent metal salts and the chemical properties of the analyte molecules. For Cu2+ ions, the quasimolecular ions were formed by the adduction of the reduced metal ions, i.e. Cu+. For Pb2+, Cd2+, Ba2+, Sr2+ and La3+ ions, singly charged quasimolecular ions were formed by adduction of the metal ions with either simultaneous deprotonation of the analyte molecule or adduction of an additional deprotonated matrix moiety onto the analyte molecule. Of these two choices, deprotonation of the analyte molecule is always preferred if labile protons, such as those in hydroxyl groups, are available in the analyte molecule. A model of ion formation from MALDI based on solid-state molecular pre-arrangement is used to account for the formation of such complex singly charged quasimolecular ions. © 1997 John Wiley & Sons, Ltd.