Studies of Exchangeable Protons in the Matrix-Assisted Laser Desorption/Ionization Process

The mechanism of ionization in matrix-assisted laser desorption/ionization (MALDI) is examined by sequentially eliminating the possible proton sources in sinapinic acid and α-cyano-4-hydroxy cinnamic acid. Three derivatives of sinapinic acid are synthesized by substituting the carboxylic acid hydrogen and the phenolic hydrogen with methyl groups, to form a methyl ester, a methyl ether and a doubly methylated derivative. All three yielded MALDI mass spectra, including the doubly methylated derivative, where both the carboxylic acid hydrogen and the phenolic hydrogen were absent, demonstrating that neither the carboxyl nor the hydroxyl hydrogen are required as a source of a proton for MALDI. The results suggest a chemical ionization mechanism for the protonation of the analyte. Experiments were performed to determine if the methoxy hydrogens of the aromatic methyl ether of cinnamic acid derivatives could be a proton source, using the trideuteromethyl ether derivative of α-cyano-4-hydroxy cinnamic acid as a matrix. Data obtained with this derivative suggest that the source of the proton is not the methoxy methyl group, but rather the carboxylic group.