H.C. Michelle Byrd and Sheng Lin-Gibson
Matrix-assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF MS) generally uses metal salts to cationize synthetic polymers, such as polystyrene. However, reduced analyte signal often results from complications during the metal ionization processes in the gas phase including competition between the analyte and matrix for cationization, self-clustering of the metal salts, and clustering of metal salts with matrix. One approach to increase the polymer ion signal is to apply more aggressive laser power; however, this may result in fragmentation of the polymer leading to molecular mass distribution (MMD) unrepresentative of the true distribution. Our laboratory has developed a covalent cationization method in which a phosphonium or an ammonium salt is covalently attached to one of the chain ends. This will assure singly charged analytes and the competition associated with metal ionization will be eliminated. Better spectra can be generated since the undesirable background noise can also be reduced. Furthermore, polymers, such as polyolefins, that cannot be cationized using traditional metal approach can be cationized using covalent cationization to produce successful MALDI spectra.
We demonstrate the covalent cationization method used to characterize polyolefins and polystyrenes. Furthermore, a comparison between covalent cationization method and traditional metal cationization for polybutadiene and polystyrene is made to gain a better understanding of the mechanisms that govern the desorption/ionization processes.