Spec trophotometric flow injection analysis for the determination of trace amount of ammonium ion in seawater coupled with on-line gas diffusion/ion exchange concentration technique

A highly sensitive spectrophotometric semi-automated method for ammonium ion determination was proposed using flow injection analysis coupled with an on-line gas diffusion separation/ion exchange concentration technique, which was especially effective for applying to analyses of open sea or fresh seawater. Ammonium ion at the single and/or sub-ppb level in seawater samples was introduced into a gas diffusion system at first, and then subsequently concentrated on a cation exchange resign column. The collected ammonium ion was eluted by 0.5 M hydrochloric acid. After derivatization based on indophenol blue with salicylate, the color development was monitored at 660 nm. The detection limit (3 sigma) for N-NH4+ was 0.5 ppb at 10 minutes concentration and 0.2 ppb at 30 minutes concentration, respectively. The relative standard deviation (n = 5) was found to be 0.68% for 1 ppb of N-NH4+ at 10 minutes concentration. The proposed method could be applicable to the analysis of ammonium ion in seawater samples.