The simultaneous and selective determination by ion chromatography was examined for inorganic nitrogen species [nitrite (NO2−), nitrate (NO3−), ammonium (NH4+) ions] in seawater. NO2− and NO3− in artificial seawater were separated and detected by using an anion-exchange column of the polymer type with a high ion-exchange capacity, an eluent of 1 mM NaOH + 1 M NaCl, and a UV detector (225 nm). On the other hand, NH4+ was detected by using a gas-diffusion system and a fluorometric detection method (410 nm and 470 nm for excitation and emission wavelengths, respectiverly, using o-phthalaldehyde (OPA) solution and potassium phosphate buffer as a reaction solution) as a post-column method. The calibration curves by the peak-area method for each ion in 35‰ artificial seawater showed good linearity (r2 = 0.994 or more) in the range of 0.01 to 2 mg L−1. The repeatability (each ion concentration 1 mg L−1, n = 5) was 0.25 % or less (retention time), 0.72 % or less (peak area), and 0.99 % or less (peak height). The detection limit values (S/N = 3, estimated from each ion concentration 1 mg L−1) of NO2−, NO3−, NH4+ were 0.4 μg L−1, 1.6 μg L−1, and 3.7 μg L−1, respectively. This method was applied to the simultaneous determination of inorganic nitrogen species in real seawater samples and the recovery rate by the standard addition method was in the range of 94 to 110 % for all samples.
