Different behavior of dextrans in positive-ion and negative-ion mass spectrometry

A mass spectrometric (MS) comparative study of dextran samples using two different ionization techniques (matrix-assisted laser desorption/ionization (MALDI) and electrospray ionization (ESI)) in both positive- and negative-ion modes is reported. The experiments were carried out with two polydisperse dextrans (1000 and 8800 Da) and isomaltotriose. In the positive-ion mode, the expected alkali metal ion adducts of dextrans were observed in both techniques. In contrast, the expected preferential formation of deprotonated molecules M - H](-) was not confirmed in negative mode MALDI time-of-flight (TOF) MS, where the series of ions M(x)- H +42](-) or M(x+1)- H - 120](-), coming either from some addition or fragmentation, were observed. In both ionization techniques, these ions formed the main distributions of dextrans in the negative-ion mode. It seems that the negative molecular ions formed from the alpha1 --> 6 linkage of polyglucose oligomers easily decompose, and the product ions M - H - 120](-) markedly dominate. The fragmentation experiments and especially the investigation of the fundamental role of the nozzle-skimmer potential in ESI-MS supported our explanation of the observed behavior because its higher values caused higher fragmentation. The experiments with isomaltotriose excluded any addition of 42 Da during the MS procedures, which is not distinguishable from the loss of 120 Da in the case of polydisperse dextrans. MALDI-TOFMS was found to be more sensitive for the detection of higher oligosaccharides and ESI-MS more useful for structural studies.