METHOD TO IDENTIFY THE GLYCOSIDIC LINKAGE OF SIALIC ACID IN GLYCOPEPTIDES
A separation time of an isomer of one or more isomers of a sialylated glycopeptide of a sample is calculated from a peak of a precursor XIC. Product ion intensities of the first group are summed at the separation time producing a first sum and product ion intensities of the second group are summed at the separation time producing a second sum using XICs of the first and second groups. A ratio of the first sum to the second sum is calculated. The ratio at the separation time is compared to predetermined ratio ranges that each corresponds to a combination of a selection from a set of the first linkage and the second linkage taken one or more times. One or more linkages of the sialic acid to the glycan of the isomer are identified from a combination found to match the ratio in the comparison.