Determination of Nitrate in Seawater with Valve-Free Continuous Flow Analysis

A valve-free continuous flow method and its corresponding instrument were established, with only two multi-channel pump for delivering the sample and reagent, without any injection or solenoid valves and sample loop for selecting and adding the sample or reagent. Nitrate was reduced to nitrite with Cu-Cd reductant column, and then detected with spectrophotometric detector.The proposed method was suitable for determination of nitrate at normal level in most of estuary and coastal seawaters. With the optimum parameters, the linear range and detection limit were 5-180 mu M and 0.27 mu M, respectively. The nitrate samples (10 and 80 mu M) were continually measured for 11 times, and the relative standard deviations were 1.4% and 1.3%, respectively. The recovery of real samples at different salinity was ranged from 99.4% to 106.1%. There were no significant differences in the analytical results between the proposed method and a reference method, i. e., flow injection analysis. In comparison with flow injection analysis, the method and instrument were less cost and easy to operate,and was suitable to be applied in general laboratories and field for continuous monitoring. The method was successfully applied to measure nitrate of seawater samples in Xiamen western harbor and underway monitor nitrate in the Jiulongjiang estuary.