High-frequency underway analysis of ammonium in coastal waters using an integrated syringe-pump-based environmental-water analyzer (iSEA).

Accurate methods and related robust analytical instruments for sensitive shipboard determination of ammonium in coastal waters are highly desirable for both oceanographers and environmental scientists. 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 ammonium in coastal areas. The chemistry is based on a modified indophenol method using o-phenylphenol. The effects of reagent concentrations and sample temperatures were evaluated. The detection limit was 0.15 µM with a 3-cm Z-flow cell, and the linearity was as high as 200 µM. The relative standard deviations at different concentrations (2, 10, and 20 µM) were 2.2%, 0.33%, 0.32% (n = 11). For n = 288 and without any stoppage during repeated analysis for 24 h, the relative standard deviation was 0.85%. The sample throughput was 12 h-1. The effects of salinity and five organic nitrogen compounds were evaluated and showed no interference using the proposed protocol for ammonium analysis. Between results obtained by reference and the present methods, there were no significant differences in the measurements of reference materials and different aqueous samples (n = 51). The analyzers worked well in the transect of 420 km during 7 cruises. A total of 716 analyses were performed automatically on board, demonstrating the capability of iSEA in automated real-time mapping of ammonium distribution in a shipboard laboratory.