Chromatographic behaviour of oxidised porous graphitic carbon columns

Abstract

The chromatographic behaviour of porous graphitic carbon (PGC) columns, chemically oxidised with potassium permanganate, has been compared with that of untreated PGC columns. It is shown that the retention of dopamine, DOPAC and heparin disaccharide is significantly changed by the oxidation of the PGC column and that the oxidation of the column involves an oxidation of groups on the PGC material. For freshly oxidised columns, no clear peaks were obtained for any of the studied compounds during a period of approximately two days. After this period, the chromatographic peaks reappeared and were then found to be similar to those obtained with an non-oxidised PGC column. Redox titrations of suspensions of the PGC material in 10 mM sulfuric acid with potassium permanganate gave rise to well-defined titration curves for which the equivalence volumes were found to depend on the amount of PGC material used in the titration. This dependence clearly demonstrates the presence of approximately 0.1 mole% of oxidisable groups in the PGC material, i.e. an approximate surface coverage of 4 × 10⁻¹¹ moles groups cm⁻². It was also found that the redox potentials of an non-oxidised PGC suspension and that of a reduced PGC material increased with time, suggesting a slow oxidation of the PGC. The importance of using mobile phases with both sufficient pH and redox buffer capacities is stressed for PGC chromatographic columns.