The simultaneous determination of inorganic nitrogen species (nitrite, nitrate, and ammonium ions) in seawater samples was examined by ion chromatography with the addition of EDTAs [ethylenediamine-
N,
N,
N',
N'-tetraacetic acid, trisodium salt (EDTA · 3Na) or ethylenediamine-
N,
N,
N',
N'-tetraacetic acid, tetrasodium salt (EDTA · 4Na)] to the samples. Nitrite and nitrate ions were separated on a polymer type of anion-exchange column with a high exchange capacity using 1.0 M sodium chloride + 5 mM phosphate buffer (pH = 5.0) as an eluent, and were detected by UV at 225 nm. Ammonium ion was detected at 710 nm as an indophenol blue derivative by a post-column reaction in the presence of dichloroisocyanurate and 1-naphthol. The addition of EDTAs (EDTA · 3Na and EDTA · 4Na) to seawater samples was effective to remove the interference by magnesium, calcium, and strontium ions in seawater samples. Also, it did not interfere with the sensitive determination of inorganic nitrogen species. The addition of EDTA · 3Na to real seawater samples was effective for the determination. Calibration curves by the peak-area method for all ions in 35‰ artificial seawater showed good linearity (
R2≥0.995) in the range of 0 – 2 mg L
−1. The detection limits (
S/N = 3) of nitrite, nitrate, and ammonium ions were 0.3 μg L
−1, 1.8 μg L
−1, and 1.9 μg L
−1, respectively. Also, the repeatability (1 mg L
−1,
n = 5) was below 0.23% (retention time), 1.70% (peak area), and 1.50% (peak height). The present method was applied to the simultaneous determination of inorganic nitrogen species in seawater samples with addition of EDTA · 3Na. The recovery rate by the analysis of spiked samples was in the rage of 98 – 105%.
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