Analysis of Oligosaccharides Isomers Using Ion Mobility-Quadrupole-Time of Flight Mass Spectrometry

Oligosaccharides play fundamental functions including protein folding and intercellular communication in living creatures. Due to isomer diversity caused by variety of monosaccharides units, connection types and sites, differentiating oligosaccharides isomers becomes a challenging task. The conventional structure revealing methods have the defects of time-consuming and complex sample pretreatment, so ion mobility has emerged as a promising technique for biological macromolecule fast analysis in the past decade. It brings a new dimension which contains information for ion shape and charge. Ion mobility combined with liquid chromatogram and mass spectrometry provides three-dimensional separation, improving both peak capacity and resolving power. In this work, two series of oligosaccharides were analyzed using ion mobility-quadrupole-time of flight tandem mass spectrometry (IM-Q/TOF MS). Glycans were diluted in acetonitrile-water-formic acid (49.95∶49.95∶0.1, V/V/V) to 100 μmol/L, then mixed with same concentration NaCl or CaCl₂ solution, and the final concentration was 50 μmol/L. For IMS research, 5 μL of sample was ejected into sample loop, and ionized by electrospray ion source at positive mode. Glycan metal ion adducts were separated in drift tube by uniform electric field and collision with buffer gas molecule based on their shape and charge. Eventually, ions were detected by time of flight mass spectrometry. Two series of oligosaccharides isomers have arrival time distribution difference from 0.15 ms to 0.66 ms, so some pairs of isomers can be partially distinguished. Moreover, features like M+Na⁺ in IM-MS spectra was derived from polymers like nM+Na⁺ through collision inside the quadrupole region, which had same drift time distribution of polymers. In addition, MS/MS behavior of oligosaccharides metal adducts was discussed at optimized collision energy. Furthermore, CCS values of oligosaccharides metal ion adducts was calculated using single field calibration method by Agilent tune mix. CCS trend line of oligosaccharides was plotted, and it’s capable for predicting CCS values of larger glycans by linear fitting. In general, qualitative analysis of oligosaccharides isomers is achievable using LC-IM-Q/TOF MS method, identification of glycan isomers in real complex sample should be discussed in the future.