Simultaneous Determination of Inorganic Nitrogen Species in Seawater by Ion Chromatography - Utilization of a Gas-diffusion Method for the Determination of Ammonium Ion

Simultaneous and selective determination of inorganic nitrogen species [nitrate (NO2-), nitrate (NO3-), ammonium ion (NH4+)] in seawater by using ion chromatography (IC) was examined. Nitrite and nitrate ions in artificial seawater were separated and detected by using an anion-exchange column of the polymer type with a high ion-exchange capacity, eluent of 1 mM NaOH + 1 M NaCI, and a UV detector (225 nm). On the other hand, ammonium ion was detected by using a gas diffusion system and spectrophotometric method (wavelength, 550 nm using cresol red solution and HEPES buffer as a reaction solution) as post-column method. Thus, the simultaneous and selective detection of three components was possible. The calibration curves by a peak area method of each ion in 35 parts per thousand artificial seawater showed good linearity (r(2) = 0.999 or more) in the range of 0.02 to 2 mg L-1. The repetition accuracy (each ion concentration being 1.0 mg L-1, n = 5) was less than 0.33 % (retention time), 1.75 % (peak area), and 1.73 % (peak height). The detection limit values (S/N = 3 estimated from each ion concentration 0.05 mu g L-1) of nitrite ion, nitrate ion and ammonium ion were 1.1 mu g L-1, 5.2 mu g L-1 and 12 mu g L-1, respectively. This method was applied to the simultaneous determination of inorganic nitrogen species in real seawater samples; the recovery rate by the standard addition method was in the range of 98 to 105 % for all samples.