Importance

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Abstract

The present invention includes: preparing a sample containing a peptide to be analyzed; adding a labeling compound which specifically reacts with a thiol group to the sample, and labeling a reduced cysteine residue contained in the peptide in the sample with the labeling compound; separating a sample containing the peptide labeled with the labeling compound by a chromatograph; executing product ion scan measurement with a peptide ion, as a precursor ion, generated by ionizing the peptide labeled with the labeling compound under a condition that a thiol group is not desorbed from a cysteine residue; and determining whether or not the peptide to be analyzed contains a reduced cysteine residue by determining whether or not a peak appearing at a specific mass-to-charge ratio corresponding to a structure of the labeling compound is present in a mass spectrum created based on data obtained by the measurement execution.

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First Claim