PURPOSE: To provide a method for extremely effectively separating the isomer of 3,4-dihydroxyphenylserine by liquid chromatography and provide a method for easily determining the optical purity of L-threo-3,4-dihydroxyphenylserine in high accuracy by using the separation method.

CONSTITUTION: This method for separating a 3,4-dihydroxyphenylserine isomer by liquid chromatography comprises the use of an alkyl-bonded silica gel surface-coated with a Cu (II) complex of N,S-dioctyl-D-penicillamine as the stationary phase and a solution consisting of an aqueous solution of ammonium acetate/methanol or acetonitrile incorporated with a proper amount of copper (II) acetate as the mobile phase.