Matrix/analyte ratio influencing polymer molecular weight distribution in matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) has been used to characterize poly(L-lysine) polymers and unique oligomer peptides, like 10-, 15- and 25-mer [LyS](n) oligolysine peptides. Several matrices have been tried in order to find optimal conditions, but only alpha-cyano-4-hydroxycinnamic acid gave analytically useful spectra. The synthetic oligomers and their mixtures gave good quality spectra, showing protonated and cationized molecules, including doubly charged species. The polymers, analogously, gave a wide distribution of single- and double-cationized peak series. The polymer distributions observed indicate the presence of significant suppression effects. The concentration (matrix/analyte ratio) was found to influence the results significantly; distributions shifting to higher masses when higher polymer concentrations were used. This effect was studied in detail using the synthetic ('monodisperse') oligolysine peptides. It was found that the relative intensities change by over an order of magnitude in the 0.1-10 pmol/mu L concentration range (typical for most proteomic analyses). The results indicate that concentration effects need to be considered when MALDI-MS is used for quantitative purposes.