CHARACTERIZATION OF ADSORBENTS BY HIGH-PERFORMANCE LIQUID-CHROMATOGRAPHY USING AROMATIC-HYDROCARBONS - POROUS GRAPHITE AND ITS COMPARISON WITH SILICA-GEL, ALUMINA, OCTADECYLSILICA AND PHENYLSILICA

The retention characteristics of 52 aromatic hydrocarbons, mostly alky lbenzenes, were measured on porous graphite using methanol as the mobi le phase. There is a linear dependence of log k' on the number of carb on atoms, (i) for polymethylbenzenes whose methyl groups are all in or tho positions and (ii) for n-alkylbenzenes above butylbenzene. As with alumina and silica gel, the ortho effect for graphite was also observ ed, but graphite shows much higher ct values (the increase in log k' f or addition of a carbon atom) than any of the other packing materials studied. The superior selectivity of graphite is in line with its crys talline structure, consisting of a flat, crystalline array of hexagona lly arranged carbon atoms. In terms of optimum eluent and retention gr aphite behaves like a reversed-phase material, but in terms of selecti vity towards isomeric compounds it behaves like the oxide adsorbents s ilica gel and alumina.