Generation of large radical ions from oligometallocenes by matrix-assisted laser desorption ionization

Non-polar polymers containing ferrocene, ferrocenylnaphthalene, and ruthenocenylnaphthalene groups in their repeating units were studied by matrix-assisted laser desorption ionization (MALDI) time-of-flight mass spectrometry. Sample preparation for these polymers utilized tetrahydrofuran as solvent and several new matrices such as dithranol, 9-nitroanthracene, and quinizarin (all of these are anthracene derivatives). By comparing the mass spectra of oligometallocenes recorded with different matrices at different wavelengths (337 nm and 2.94 microns) and laser desorption ionization mass spectra recorded without matrix, it could be verified that the major analyte-related peaks in the MALDI mass spectra corresponded to the radical molecular ions. Radical ions have not been seen in the MALDI mass spectra of biopolymers such as proteins, peptides, and carbohydrates. Radical formation was demonstrated for samples in the mass range (1 kDa-13 kDa). Even in the presence of potential cationization sites such as methyl ester groups in the repeating units of some polyferrocene samples, MALDI mass spectra were dominated by radical ions of the analyte. Two possible mechanisms for radical formation in MALDI are discussed. Comparison of results with different matrices suggested that the distribution of masses observed in the mass spectra and characterized by the polydispersity index was independent of the matrix, but significant differences (5%) in the average molecular weights of the mass distributions were found.