Simultaneous underway analysis of nitrate and nitrite in estuarine and coastal waters using an automated integrated syringe-pump-based environmental-water analyzer.

Methods for determining nitrate and nitrite have been comprehensively developed. However, there are few studies of simultaneous shipboard high-frequency monitoring of these two nutrients in estuarine and coastal area. In this study, a multipurpose integrated syringe-pump-based environmental-water analyzer (iSEA) was combined with an on-line filtration system for underway analysis of nitrate and nitrite in saline samples. Vanadium chloride was used instead of a toxic cadmium column to reduce nitrate to nitrite, which was measured on the basis of the classic Griess reaction. This fully automated analyzer had a limit of detection of 0.02 μmol L-1 for nitrite and 0.14 μmol L-1 for nitrate. The sample throughput was 12 h-1 for simultaneous measurement of nitrite and nitrate. With automated dilution, the calibration curve for nitrate was linear up to a concentration of 400 μmol L-1 (R2 > 0.999). The relative standard deviation of 24-h measurement (n = 288) of nitrite is 0.92% and that of nitrate is 1.4%. Both the reference solutions and samples of different salinities (range of 0-35) were measured (n = 85). According to the statistical t-test (P = 0.95), the results were insignificantly different from the results obtained using the reference method. After several cruise tests, the analyzer showed excellent spatial resolution for underway analysis of nitrite and nitrate in estuarine and coastal waters.