Chemical additives facilitate molecular weight characterization of underivatized dextran by high-resolution electrospray ionization mass spectrometry

Polysaccharides are essential resources for all organisms. Despite the increasing popularity and sophistication of biological macromolecule research, analytical approaches for polysaccharides are still urgently required due to their complex heterogeneity and diverse structures. One of the primary characteristics of polysaccharides is molecular weight, whereas precise mass results are still hard to obtain by current molecular weight characterization approaches. High resolution mass spectrometry (MS) with soft ionization techniques can provide accurate mass information for macromolecules, except for polysaccharides, due to the problem that most neutral polysaccharides have limited ionization efficiency. Focusing on the improvement of ionization, a practical method was proposed here in which chemical additives can facilitate the electrospray ionization (ESI) of neutral polysaccharides. Taking dextran as an example, effective ESI additives were examined and investigated with two distinct behaviors. Compared with an average molecular weight of 13 600 Da given by HPSEC-MALLS-RID, 16 866 Da was identified as the maximum mass by ESI-MS with formamide assistance. Large polysaccharide ions with a resolvable isotopic distribution were then analyzed by tandem MS directly to investigate the fragmentation features. Formamide was used as an effective ESI additive as well for other types of polysaccharides. The method proposed here provides an easy but reliable ionization way for further MS research on neutral polysaccharides.