PURPOSE: To separate and analyze a fluorescent diastereomer, which is formed by reactng o-phthalaldehyde and a chiral compound having a SH-group with a specimen containing an optical isomer having an NH2-group under and alkaline condition, with high sensitivity, by subjecting the same to liquid chromatography.

CONSTITUTION: o-Phthalaldehyde and a compound prepared by protecting the NH2-group of a chiral compound having a SH-group by an acetyl group, for example, N-acetyl-L-cisteine are reacted with a specimen containing an optical isomer having an amino group under an alkaline condition with pH of about 10.4. By this method, the specimen (H2NR) forms a luminous compound, for example, as shown by formula I . When the specimen is a mixture of D and L or a racement, two kinds of diastereomers are mutually generated as represented by formulae II, III. These diastereomers are subjected to liquid chromatography to be optically split to detect fluorescent intensity or absorbancy of each of them. When the specimen contains only one of D and L, detection is performed without separating the same. By this method, qualitative analysis is enabled simply with high sensitivity.