Provided is a method for quantifying a specific component contained in a measurement sample. The method includes: obtaining a measurement spectrum at each of a plurality of points in time by analyzing the measurement sample with chromatography; deriving an index value at each point of the plurality of points in time, by applying a filter for extracting the specific component, to the measurement spectrum at the point of the plurality of points in time; obtaining a chromatogram by arranging one or more index values at respective one or ones of the plurality of points in time; and quantifying the specific component based on a peak of the chromatogram.

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.