Application of Air Segmented Amplitude Modulated Multiplexed Flow Analysis with Software-Based Phase Recognition to the Determination of Ammonium Ion in Water Samples

Air segmented amplitude modulated multiplexed flow analysis (air segmented AMMFA) has been applied to the simultaneous determination of ammonium ions in water samples by an Indophenol Blue spectrophotometry. The flow rates of sample solutions to be merged are periodically varied at different frequencies. The merged solution, comprised of the sample solutions, coloring reagent solutions and water (diluent), is segmented by air bubbles so as to prevent the axial dispersion and thus to enhance the sensitivity. Both liquid and air segments are introduced to the flow cell of a UV/Vis detector and analytical signals at 660 nm are obtained. The signals corresponding to the air phase are removed by an in-house software. Thus deaerated signals are smoothed by a moving average computation and then analyzed by fast Fourier transform. The concentration of ammonium ion in each sample is determined from the amplitude of the corresponding wave component. Through the introductions of the air segmentation and the software-based phase recognition approaches, the sensitivity is increased maximally by a factor of 5.8 and lower detection limit (< 0.11 mg dm as N) is obtained compared with the previous non-segmented AMMFA. The proposed method has successfully been applied to the simultaneous determination of ammonium ions in real water samples.