Simultaneous determination of phosphate and nitrite at nanomolar to micromolar concentrations in seawater using a syringe pump-based automated analyzer coupled with a long pathlength capillary flow cell

Phosphate and nitrite, two essential nutrients in marine ecosystems, exhibit dynamic concentration distributions due to complex biogeochemical processes. Accurate quantification of these nutrients requires analytical methods with both high sensitivity and a broad dynamic range. Here, we present an automated syringe pump-based analyzer coupled with a long pathlength liquid waveguide capillary cell (LWCC), incorporating a robust automated dilution program, for simultaneous determination of phosphate and nitrite in seawater across nanomolar and micromolar concentrations. The effects of reagent concentrations and sample salinity were evaluated. A quantitative model for assessing potential bias in trace phosphate determination introduced by arsenate was established. With a 2.5-m LWCC, the system achieves detection limits of 0.56 nmol/L for phosphate and 0.38 nmol/L for nitrite. By leveraging the automated dilution capability, the linear range of the system extends to 6 μmol/L for phosphate and 2.2 μmol/L for nitrite without manifold reconfiguration. The method demonstrates high throughput (10 samples/h for two analytes), precision (relative standard deviations of 0.37–2.1% across the dynamic range, n = 7), and trueness validated against reference methods and materials. Application to oligotrophic seawater profiles revealed distinct vertical patterns of phosphate and nitrite, demonstrating the unique capability of this method to resolve biogeochemical gradients inaccessible to conventional methods with limited detection limits or dynamic ranges.