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A new on-column derivatization method based on the redox activity of porous graphitic carbon (PGC) packing materials was presented for enhancement of separation selectivity of HPLC. Two PGC packing materials were used as the solid redox agents as well as the stationary phase, and their redox activities were investigated using trans-1,2-diaminocyclohexanetetraacetate (DCTA) complexes of some metal ions as probe compounds. It was found that the redox property of PGC was modified by treating them with a solution containing a reducing agent, sodium sulfite or hydroxylammonium chloride. The original PGC packings oxidized Co(II)-DCTA to Co(III)-DCTA during elution, while the PGC treated with a reducing agent showed reduction activity converting Co(III)-DCTA to Co(II)-DCTA. These two cobalt complexes do not form their individual chromatographic zones but migrate as a single zone of their mixture on the PGC column contrary to the chromatographic behavior on a C18 bonded silica, on which Co(II)-DCTA and Co(III)-DCTA can be separated. Treatment of the PGC column with a reducing agent solution transforms the oxidative activity of the original PGC packing to a reductive one from the upper part of the column, so that the retention time of the cobalt complex can be controlled by changing the volume of the reducing agent solution to be used for treatment of the PGC column. The selective separation and determination of cobalt in a reference manganese nodule sample by the developed method was demonstrated.