Analyzing method for azo compound

A method for analyzing a plurality of kinds of azo compounds in a sample, comprising:

• a first analyzing step in which a first analysis is performed for a target sample to detect a plurality of azo compounds in the sample, using a liquid chromatograph in which a liquid chromatograph with a reverse phase column, the liquid chromatograph being coupled with a mass spectrometer;
• and
• a second analyzing step in which a second analysis is performed for the target sample to detect a plurality of azo compounds which are contained in the sample, using a liquid chromatograph in which a liquid chromatograph with a reverse phase column, the liquid chromatograph being coupled with a mass spectrometer,
• wherein:
• the first analyzing step is a gradient analysis using, as two mobile phases, a first mobile phase which is either an aqueous ammonium acetate solution or an aqueous ammonium formate solution, and a second mobile phase which is a mixture of an organic solvent and either an aqueous ammonium acetate solution or an aqueous ammonium formate solution;
• and
• the second analysis is a gradient analysis using, as two mobile phases, a first mobile phase which is an aqueous solution of ammonia or ammonium salt with pH greater than 7.0, and a second mobile phase which is an organic solvent.