Silver Cation Affinities of Monomeric Building Blocks of Polyethers and Polyphenols Determined by Guided Ion Beam Tandem Mass Spectrometry

摘要

Matrix-assisted laser desorption/ionization (MALDI) has become increasingly important for the characterization of synthetic polymers. Proton transfer ionization is the primary ionization pathway observed and utilized for the analysis of biological polymers, resulting in the formation of protonated analyte molecules. In contrast, metal cationization is the major ion formation process for synthetic polymers, resulting in the formation of metal cationized analyte molecules. The mechanism of metal cationization in MALDI is still poorly understood, but is believed to occur primarily in the gas phase after desorption has occurred. Therefore, accurate absolute metal cation affinities for MALDI matrices and synthetic polymers could provide useful information for understanding and optimizing MALDI MS analyses of synthetic polymers. Most synthetic polymers having heteroatoms exhibit a strong affinity for sodium or potassium salts. Apolar synthetic polymers possessing no heteroatoms can be successfully ionized after the addition of silver or copper salts. Therefore, Na~(+), K~(+), Cu~(+), and Ag~(+) have been widely used in MALDI MS analyses of synthetic polymers. In this study, absolute silver cation affinities of 10 monomeric building blocks of polyethers and polyphenols are determined by guided ion beam tandem mass spectrometry. The ligands examined in the present study include: phenol (P), 2-, 3-, and 4-methylphenol (2MP, 3MP, and 4MP), 2-, 3-, and 4-hydroxy phenol (2HP, 3HP, and 4HP), and 2-, 3-, and 4-hydroxy benzoic acid (2HBA, 3HBA, and 4HBA). The kinetic-energy-dependent cross sections for collision-induced dissociation (CID) of these 10 Ag~(+) (ligand) complexes are analyzed using an empirical threshold law to extract absolute 0 and 298 K bond dissociation energies (BDEs) after accounting for the effects of multiple collisions, kinetic and internal energy distributions of the reactants, and unimolecular dissociation lifetimes.