STUDIES OF EXCHANGEABLE PROTONS IN THE MATRIX-ASSISTED LASER DESORPTION IONIZATION PROCESS/

The mechanism of ionization in matrix-assisted laser desorption/ioniza tion (MALDI) is examined by sequentially eliminating the possible prot on sources in sinapinic acid and alpha-cyano-4-hydroxy cinnamic acid. Three derivatives of sinapinic acid are synthesized by substituting th e carboxylic acid hydrogen and the phenolic hydrogen with methyl group s, to form a methyl ester, a methyl ether and a doubly methylated deri vative. All three yielded MALDI mass spectra, including the doubly met hylated derivative, where both the carboxylic acid hydrogen and the ph enolic hydrogen were absent, demonstrating that neither the carboxyl n or the hydroxyl hydrogen are required as a source of a proton for MALD I. The results suggest a chemical ionization mechanism for the protona tion of the analyte. Experiments were performed to determine if the me thoxy hydrogens of the aromatic methyl ether of cinnamic acid derivati ves could be a proton source, using the trideuteromethyl ether derivat ive of alpha-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.