Competition between metal cationization and protonation/reduction in MALDI process: An example of riboflavin

In order to better understand effects of metal cationization on protonation and reduction in MALDI process, mass spectra of riboflavin complexes with different alkali metal ions generated by a series of laser shots at a fixed target spot were consecutively recorded with the matrix of 2,5-dihydroxybenzoic acid (DHB). Progressive changes in the spectra, including the decrease of reduction extent for the metalized molecule ions and the increase of relative intensities of metalized species compared to protonated species, were observed with the increase of laser shots. Covariance mapping analysis was also performed for the data. Results show that different metalized ions are positively correlated each other; while metalized ions and protonated species are negatively correlated. These results not only reflect competition between metalization and protonation/reduction in laser plume, but also show that the winner of it might be changed during the process of sample consumption by laser irradiation.