Characterization of the molecular mass distribution of pullulans by matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry using 2,5‐dihydroxybenzoic acid butylamine (DHBB) as liquid matrix

Abstract

The performances of several matrices were investigated for the accurate determination of the molecular mass distributions of pullulans by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS). The ionic liquid matrix (ILM) 2,5-dihydroxybenzoic acid butylamine (DHBB) gave better and more reliable results than the crystalline matrices 2,5-dihydroxybenzoic acid (DHB) and 2,4,6-trihydroxyacetophenone (THAP). With the ILM it was possible to obtain spectra of pullulans up to more than 100 kDa, the highest molar mass reported thus far. Owing to the known advantages of liquid matrices providing better spot-to-spot reproducibility, an almost noise-free spectrum and constant baselines were obtained when working under optimized conditions. In particular, the extent of in-source fragmentation occurring with this group of fragile polymers was considerably and decisively reduced. Thus, a more reliable representation of the true oligomer and polymer distributions is experimentally attainable, especially for distributions with small polydispersity values. The maximum error in the measured distribution associated with fragmentation was estimated by model calculations describing the changes in the polymer distribution upon different probabilities of fragmentation events. These simulation results indicated that the data obtained by MALDI-TOFMS using the liquid DHBB matrix were of high reliability. In particular, the average value of the distributions, M(w), and the polydispersity were obtained with predicted uncertainties of between 3 and 15% depending on the width of the distribution and the mass of the polymers.