Poly(ethylene glycol) cationization with alkali metals in matrix-assisted laser desorption ionization investigated with the solvent-free method.

Matrix-assisted laser desorption ionization (MALDI) was applied to determine the relative efficiencies for attachment of alkali cations to poly(ethylene glycol) polymers. The solvent-free sample preparation method was employed in order to discriminate the gas-phase polymer-cation complex formation mechanism from solubility and co-crystallization effects. We show that the cohesion energy of the cation precursor has a strong influence on the outcome of the MALDI measurements. This potential drawback is circumvented by the simultaneous use of alkali salts of similar lattice energy. The experiments include chain polymers of different length (PEG600 and PEG1000) and alkali cations of different size (Na+, K+, Cs+). We found that the attachment efficiencies decrease with growing cation size. This decrease is significantly more rapid for the shorter PEG600 polymers. These results corroborate the general trends observed in earlier studies, but correct the dependence on alkali cation size assessed in previous MALDI experiments with the conventional dried-droplet sample preparation method. [ABSTRACT FROM AUTHOR]