Electron transfer ionization in matrix assisted laser desorption/ionization mass spectrometry

Terthiophene and anthracene, both of which in positive mode form only molecular radical cations (M+.) upon laser irradiation (lambda = 337 nm) at near-threshold laser powers, were used as matrices for matrix-assisted laser desorption/electron-transfer ionization of metallocenes (1,2-diferrocenylethane, ferrocene, and decamethylferrocene) and a methylene-bridged bisphenol, 2,2'-methylenebis-(6-tert-butyl-4-methylphenol). In the mass spectra of these matrix/analyte combinations, the formation of protonated molecules was not observed. Instead, each analyte formed a molecular radical cation (A(+.)) when either matrix was used. Experiments utilizing anthracene-d(10) as the matrix confirmed the formation of only the analyte molecular radical cation, In addition, the molecular radical cation of ferrocene-not the protonated molecule-was produced when 2,5-dihydroxybenzoic acid was used as the matrix, indicating that a matrix commonly used to form protonated polar analytes can, in addition, function as an electron-transfer MALDI matrix.