In-Source Decay MALDI and High-Energy Collision-Induced Dissociation Mass Spectrometry of Alkali Metal-Adducted Underivatized Oligosaccharides.

In-source decay (ISD) and high-energy collision-induced dissociation (HE-CID) were explored to provide structural information on alkali metal-adducted linear and stacked oligosaccharides (oligosaccharides with increased flexibility due to linkage type). These oligosaccharides include isomeric tetrasaccharides, maltoheptaose, and several human milk oligosaccharides (HMOs). Matrix-assisted laser desorption ionization (MALDI) ion production efficiency, as well as the product ion intensities, and the number of product ions formed in ISD and HE-CID of these oligosaccharides were influenced by the matrix, the ionic radius of the metal ion used for adduction, and the affinity of metal ions for specific functional groups in the oligosaccharides. 2,4,6-Trihydroxyacetophenone (THAP) was the best matrix for HE-CID of oligosaccharides, 4-dimethylaminobenzaldehyde (DMABA) worked best for ISD of tetrasaccharides and pentasaccharides, while 2,5-dihydroxybenzoic acid (DHB) was the best matrix for ISD and HE-CID of long chain oligosaccharides. In general, the number of product ions formed followed the trend Li > Na > K > Rb > Cs, except for HMOs where Na ≥ Li > K > Rb > Cs occurred. The type of product ions formed and their intensities varied based on the position of the glycosidic bond linkage and the content of the monosaccharide. ISD and HE-CID produced diagnostic ions that could structurally differentiate isomers. Overall, HE-CID of alkali-metal adducted oligosaccharides produces intense glycosidic bond cleavages and low intensity cross-ring and internal cleavages. In contrast, ISD generates mainly cross-ring cleavages and internal cleavages at intensities higher than in HE-CID. In addition, ISD produced unique product ions that complement results from HE-CID.