ANION CHROMATOGRAPHY USING OCTADECYLSILANE REVERSED-PHASE COLUMNS COATED WITH CETYLTRIMETHYLAMMONIUM AND ITS APPLICATION TO NITRITE AND NITRATE IN SEAWATER

A procedure is described for the separation of inorganic anions using two types of C18 reversed-phase columns (polymer-coated and -uncoated silica packing materials), which were coated prior to treatment with 1 mM cetyltrimethylammonium (CTA+) in water (H2O) and H2O-methanol (MeOH) mixtures at 20-degrees-C. It is suggested from the determination of the anion-exchange capacity by three methods that the sorbed CTA+ exerts as an anion-exchange site. The mobile phase for anion separation was composed of an aqueous solution of 0.1 M sodium chloride-5 mM sodium phosphate buffer (pH 5.8). The correlation between anion-exchange capacity and the retention of anions was good for both columns. The advantages of the CTA+-coated columns and the mobile phase are discussed. Ultraviolet (UV, 225 nm) and amperometric (AMP, +1.0 V, glassy carbon (GC) working electrode vs Ag/AgCl) detection of an ion chromatographic system, which consisted of a column with polymercoated packing material (coated with 1 mM CTA+ in a H2O-(80)-MeOH(20) mixture; exchange capacity, 0.25 mequiv/column), could be successfully applied to the direct determination of nitrite and nitrate in seawater, without interference by a large amount of Cl-, SO4(2-), and Br-. The determination was carried out by the peak area method because of the decrease in peak height with salinity. The detection limit was 4-mu-g/L (UV) and 2-mu-g/L (AMP) for nitrite and 8-mu-g/L (UV) for nitrate.