New strategies for resolving oligosaccharide isomers by exploiting mechanistic and thermochemical aspects of fragment ion formation

Three complementary experimental approaches for elucidating human milk oligosaccharide (HMOs) isomers by Fourier Transform Ion Cyclotron Resonance mass spectrometry (FT-ICR) are described: tandem-MS disruption by double resonance to distinguish different fragmentation pathways, examination of fragment intensity ratios arising from differential alkali metal ion affinities and monitoring competitive fragmentation rates. The interpretation of the fragmentation pattern from a mechanistic and thermochemical point of view permits the assignment of not only pure isomers but, in some cases, mixtures of them. Methodologically the procedures are simple, reliable and rapid making unnecessary both the use of previous separation techniques and tedious chemical modifications of the HMOs. In principle, the rationale can be expanded to resolve other isomeric mixtures of biological nature.