Reverse flow injection method for field determination of nitrate in estuarine and coastal waters using a custom-made linear light path flow cell and the vanadium reduction method

Abstract

Real-time determination of nitrate (NO3−) concentration is critical for scientific research and water quality monitoring. However, few methods have been reported for underway determination of NO3− concentration in seawater. In this study, a reverse flow injection system coupled with a custom-made linear light path flow cell and vanadium reduction was developed for the spectrophotometric determination of NO3− in seawater. Sulfamic acid was used to remove the initial NO2− content in samples prior to NO3− reduction. A custom-made linear light path flow cell was constructed using simple components. The reagent concentration was optimized and the effects of organic nitrogen and salinity on NO3− determination were investigated. Method detection limits were 0.02 µM and 0.03 µM for freshwater and seawater, while the working range reached a maximum of 6 µM and 7.5 µM, respectively. The upper limits of the working range could be extended by using a shorter flow cell or applying a lower reaction temperature. The recoveries varied from 88.0% to 106.0% for spiked samples with different matrices. The analytical results obtained using the proposed method were found to exhibit good correlation with the results generated using two reference methods. Furthermore, the proposed method was successfully applied to the underway analysis of NO3− concentration in the Jiulong River estuary and Xiamen coastal seawater (Fujian, China). Overall, these results indicate that the newly developed method has high potential for use in environmental monitoring.