Method for determining origin of amino acid

A method for measuring a stable isotope ratio of an amino acid, the method comprising:

• (a) measuring a stable carbon isotope ratio (δ13C ) of the amino acid, comprising:
• separating and purifying the amino acid in a first sample containing the amino acid by hydrophilic interaction liquid chromatography, as a first pretreatment, so as to obtain a first pretreated sample;
• and
• measuring the δ13C of the amino acid contained in the first pretreated sample by elemental analysis-stable isotope ratio mass spectrometry (EA-IRMS), wherein the δ13C is not measured by gas chromatography-stable isotope ratio mass spectrometry (GC-IRMS);
• and
• (b) measuring a stable nitrogen isotope ratio (δ15N) of the glutamic acid, comprising:
• pretreating a second sample containing the amino acid so as to obtain a second pretreated sample, as a second pretreatment, the second pretreatment comprising:
• cation-exchanging the second sample, with a cation-exchange column, and then
• derivatizing the amino acid in the second sample, so that the second pretreated sample is obtained;
• and
• measuring the δ15N a stable nitrogen isotope ratio (δ15N) of the amino acid contained in the second pretreated sample by GC-IRMS, wherein the δ15N is not measured by EA-IRMS.