Determination of trace levels of ammonia in marine waters using a simple environmentally-friendly ammonia (SEA) analyser

Abstract

A simple environmentally-friendly ammonia (SEA) analyser based on programmable flow was developed for the determination of trace levels of ammonia nitrogen in marine waters. The proposed SEA analyser was based on a gas-diffusion spectrophotometric method. High sensitivity was achieved through the use of large sample volumes and a programmable-flow approach. The degree of pre-concentration of ammonia in the acceptor stream was determined by the controlled and programmable delivery of the sample to the gas-diffusion unit, and the sensitivity of the method was tailored to the concentration range of interest by simple modification of the sample volume used for analysis. Three different working concentration ranges were defined by varying the volume of the sample, thus making the SEA analyser versatile and applicable for use in estuarine or coastal seawaters: 0.028–5.6 μM (2.0 mL), 0.28–13.9 μM (1.0 mL), and 1.4–55.6 μM (0.25 mL). Typical limits of detection of 15, 88 and 440 nM, and repeatability of 0.71, 1.2 and 0.97% RSD (n = 10), were attained for each range, respectively. Good sample throughput was achieved with approximately 20 samples per hour for the lower concentration range, 30 samples per hour for the middle concentration range, and 40 samples per hour for the upper concentration range. Method validation involved the analysis of a certified reference material in artificial seawater, and spike-and-recovery analysis for seawater and estuarine samples collected from various locations around Port Phillip Bay, Victoria, Australia. Small volumes of waste (i.e. alkaline seawater feedstock and 1.5 mL indicator solution) were generated per sample. The reagents used were non-toxic, suitable for long-term storage, and cheap, costing as little as $2.40 AUD per 1000 determinations.