CHROMATOGRAPHIC EFFECTS OF RESIDUAL AMINO-GROUPS ON LIQUID-CHROMATOGRAPHIC STATIONARY PHASES DERIVED FROM AMINOPROPYL-SILICA GEL

Low-capacity phenylazo-8-quinolinol-silica gel (QSG) stationary phases were made by two different procedures. The first began with an aminopropyl-silica gel (APSG), which was converted to nitrobenzamide-silica gel (NBSG) by amidization with p-nitrobenzoyl chloride. The second began with the direct preparation of the NBSG using nitrobenzamidesilane. Retention data for several organic analytes were obtained on APSG and on the NBSG and QSG phases made by both procedures. Anomalously high capacity factors were obtained for p-nitrobenzoic acid on the NBSG material obtained from APSG, indicating the presence of unreacted aminopropyl anion-exchange sites. Only after reamidization of the NBSG using more vigorous conditions did the retention data approach that on the NBSG made by the second method. Even then, the opposite order of elution of p-nitrobenzoic and benzoic acids on the two NBSG and QSG phases indicated that vestiges of amino groups remained on the materials made from the APSG. Several chromatographic and chemical characterization methods confirmed the presence of residual aminopropyl groups.