A peak at m/z = 291 in MS suggests presence of:
Mass spectrometric analysis of N- and O-glycans relies on diagnostic oxonium ions produced by glycosidic bond cleavage under collision-induced dissociation. N-acetylneuraminic acid, known as sialic acid or Neu5Ac, forms a prominent oxonium ion near m/z 291 in positive mode, often dehydrated to 274. By contrast, fucose gives 147, hexose 163, N-acetylhexosamine 204, and sulfated sugars other masses. Detection of 291 therefore indicates terminal sialylation, a modification regulating cell recognition, serum half-life, immune evasion and pathogen binding, routinely monitored in biotherapeutic glycan profiling and cancer biomarker studies.
Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.