Charge Characteristics and Fragmentation of Polycarboxylic Acids by Electrospray Ionization-Multistage Tandem Mass Spectrometry

摘要

The majority of dissolved organic matter (DOM), both natural and anthropogenic in water is dominated by polycarboxylic acid structures. When these polycarboxylic acids are subjected to either positive or negative ESI/MS, a mixture of molecular aggregates, multiply-charged species, and cationic adducts (Na~+, K~+, NH_4~+) result. These ions obscure the molecular-weight distributions of DOM. ESI/MS/MS studies of polyacrylic acid found that multiple charges formed every 8-10 carbon atoms in this linear polymer, and that singly charged ions could be distinguished from multiply charged ions in the MS/MS spectra by ion loss differences caused by water loss (18) and carbon dioxide loss (44) where n is the number of charges. ESI/MS/MS studies of glycyrrhizic acid and Suwannee River fulvic acid found that multiply-charged ions fragmented into singly charged ions that could be structurally elucidated by progressive fragmentation by ESI/MS/MS. Methylation minimized both multiple charges and molecular aggregate ions so that molecular weight distributions could be estimated by negative ESI/MS provided there were ionizable alcohol groups in the polycarboxylic acid molecules.