performing an amidation reaction that amidates a lactone structure included in the glycan through contacting the sample with a reaction solution that has a pH of higher than 10;

adding an acidic solution to the reaction solution after the amidation reaction; and

purifying the sample contained in the reaction solution by using a carrier for hydrophilic interaction chromatography after the acidic solution is added to the reaction solution, wherein:

the amidation reaction amidates the lactone structure included in the glycan by aminolysis based on an interaction between an amino group and a lactone;

the reaction solution comprising at least one selected from the group consisting of ammonia, a primary amine, and a salt thereof;

wherein the concentration of ammonia, a primary amine, or a salt thereof in the reaction solution is 0.5 M or more;

a time during which the sample is in contact with the reaction solution for the amidation reaction is shorter than 30 minutes;

performing a lactonization reaction that lactonizes at least a part of sialic acids included in the glycans before the amidation reaction, wherein in the lactonization reaction, at least one selected from the group consisting of α2,3-sialic acid, α2,8-sialic acid, and α2,9-sialic acid among the sialic acids is lactonized;

an operation that reacts the sample with a dehydration condensation agent is not performed after the addition of the reaction solution to the sample, wherein: the reaction solution does not include a dehydration condensation agent that is reacted with the lactones, and wherein only contacting the sample with the amidation reaction solution is performed for the amidation reaction;

pH of the reaction solution after the acidic solution is added to the reaction solution is 10 or lower; and

the amidation reaction is performed in a liquid phase.