PURPOSE: To enable qualitative determination with high accuracy by preparing a sample from a serum sample by required 3-stage processings.

CONSTITUTION: The serum sample such as deprotein soln. is processed in a stage for processing said sample with a high-performance liquid chromatograph using an ion exchange column, by which the org. acidic materials of various peak components detected by a UV absorption inspection instrument are drawn. The drawn materials are then again subjected to the drawing process in a separating and drawing stage using an isokinetic electrophoresis apparatus to obtain the drawn materials contg. less extraneous components and having high purity. The high-purity sample contg. less extraneous components is prepd. again from the separated and drawn materials by a stage for removing the solvent from the drawn materials and processing the residue by an esterifying agent such as trimethyl silylating agent. The org. acidic materials contained in the serum are quantitatively determined with high accuracy when this sample is analyzed with a gas chromatogram mass spectrometer.