A modified method for on-line determination of trace ammonium in seawater with a long-path liquid waveguide capillary cell and spectrophotometric detection

An automated colorimetric method for the on-line determination of trace ammonium in seawater was established using a flow injection technique coupled with a 2.5-m liquid waveguide capillary cell. Using low ammonium seawater as a carrier, a sample was injected into the carrier and mixed with reagents to form indophenol blue dye, which was monitored at a wavelength of 690 nm. Different strategies of reagent injection were investigated to obtain a lower reagent blank and a higher detection sensitivity. Experimental parameters were optimized using a univariate experimental design, and the matrix effect of seawater was preliminarily investigated. The proposed method had high sensitivity with a detection limit of 3.6 nmol·L− 1. The linearity was 10 to 500 nmol·L− 1 and the upper limit could be extended to 30 μmol·L− 1 by choosing a less sensitive detection wavelength or lower reaction temperature. The recoveries were between 95.0 and 104.3% and the relative standard deviation was 4.4% (n = 7) for an aged seawater sample spiked with 50 nmol·L− 1 ammonium. The sample throughput was 22 h− 1. The analytical results obtained with the proposed method showed good agreement with those using reference fluorescence methods. Compared with the normal indophenol blue (off-line) method, the proposed method was superior due to its lower reagent consumption, greater convenience, higher sample throughput, wider linear range (10 nmol·L− 1 to 30 μmol·L− 1), as well as higher sensitivity. The method was applied in-field in Wuyuan Bay for 24 h on-line monitoring of ammonium concentrations in the surface seawater. In addition, it was also used to analyze surface seawater samples collected from the South China Sea for the study of ammonium distribution.