Addressing the Detection of Ammonium Ion in Environmental Water Samples via Tandem Potentiometry-Ion Chromatography

An analytical methodology for detecting ammonium ion (NH₄ ⁺) in environmental water through potentiometry-ion chromatography (IC) in tandem is presented here. A multielectrode flow cell is implemented as a potentiometric detector after chromatographic separation of cations in the sample. The electrodes are fabricated via miniaturized all-solid-state configuration, using a nonactin-based plasticized polymeric membrane as the sensing element. The overall analytical setup is based on an injection valve, column, traditional conductometric detector, and new potentiometric detector (in that order), permitting the characterization of the analytical performance of the potentiometric detector while validating the results. The limit of detection was found to be ca. 3 × 10^-7 M NH₄ ⁺ concentration after linearization of the potentiometric response, and intra- and interelectrode variations of <10% were observed. Importantly, interference from other cations was suppressed in the tandem potentiometry-IC, and thus, the NH₄ ⁺ content in fresh- and seawater samples from different locations was successfully analyzed. This analytical technology demonstrated a great potential for the reliable monitoring of NH₄ ⁺ at micromolar levels, in contrast to the conductivity detector and previously reported NH₄ ⁺ potentiometric sensors functioning in batch mode or even coupled with IC. Additionally, the suitability of the potentiometric cell for selective multi-ion analysis in the same sample, i.e., Na⁺, NH₄ ⁺, and K⁺ in water, has been proven.