Practice question

In glycoprotein and glycan mass spectrometry, oxonium fragment ions diagnose monosaccharide composition during tandem analysis. Cleavage of glycosidic bonds under collision-induced dissociation produces characteristic low-mass reporter ions. An intense signal near m/z 204 corresponds to dehydrated sialic acid oxonium ion, often observed for N-acetylneuraminic acid containing glycoconjugates under positive ionization. Its presence alerts investigators to terminal sialylation, important for cell recognition, serum half-life, and immune modulation. Detection of this fragment helps assign sialylated N-glycans and differentiate them from neutral mannose, fucose, or glucose containing structures.