Aspects of the Sequencing of Carbohydrates and Oligonucleotides by Matrix-assisted Laser Desorption/Ionization Post-source Decay

The possibility of structural elucidation of carbohydrates and oligonucleotides by matrix‐assisted laser desorption/ionization followed by post‐source decay (MALDI‐PSD) is investigated. Spectra of both classes of compounds exhibit the better signal‐to‐noise ratios for the intact species in the negative‐ion mode, but the most informative spectra for structural elucidation by PSD are obtained in the positive‐ion mode. A novel fragmentation mode is demonstrated on a model sialylated complex type N‐linked oligosaccharide. Prompt fragmentation in the positive‐ion mode can remove the sialic acids at high laser irradiance if a suitable matrix is used. The remaining sugar backbone is then characterized by post‐source decay. A modified oligonucleotide is analyzed with 3‐hydroxypicolinic acid as the matrix and ammonium tartrate to displace sodium. Enough structural information is obtained to verify the modification.