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

Abstract

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.

Figure 1

(a) Miniaturized ISEs based on glassy carbon (GC). The working electrode (WE) was prepared with MWCNTs and the ISM. The reference electrode (RE) was prepared with Ag/AgCl ink and the RM on top. (b) Multielectrode flow cell with three ISEs and the RE, the inlet, and the outlet. (c) Tandem potentiometry–IC: the sample is injected through the valve and carried by the mobile phase through the chromatographic column.

Figure 2

(a) Dynamic response of one NH₄⁺-selective electrode in batch mode at increasing NH₄⁺ activity and using the commercial Ag/AgCl reference electrode. Inset: Corresponding calibration plot and that obtained with the handmade reference electrode. (b) Response of one NH₄⁺-selective electrode in the flow cell (0.5 mL min^–1). Inset: Corresponding calibration graph.

Figure 3

(a) Conductivity chromatograms at increasing NH₄⁺ activities and 1 × 10^–3 activity of the other cations (10 μL volume, 0.9 mL min^–1). (b) Potentiometric chromatograms at increasing NH₄⁺ activity and 1 × 10^–3 activity of the rest of the cations (10 μL volume, 0.9 mL min^–1). (c) Potentiometric chromatograms with 10 and 20 μL injected volume of 1 × 10^–3 NH₄⁺ activity (0.9 mL min^–1). (d) Averaged calibration graphs (n = 3). (e) Potentiometric chromatograms with 1 × 10^–3 NH₄⁺ activity at 0.5, 0.7, and 0.9 mL min^–1 (10 μL volume). (f) Averaged calibration graphs (n = 3). Background: 2.5 × 10^–3 mol L^–1 nitric acid.

Figure 4

(a) Chromatograms at increasing NH₄⁺ activity. (b) Corresponding averaged calibration graph (n = 3). (c) Linearization of the averaged calibration graph (2.5 × 10^–3 mol L^–1 nitric acid, 10 μL sample volume, flow rate of 0.9 mL min^–1).

Figure 5

Chromatograms observed for two water samples: freshwater (river) and seawater.

Figure 6

Chromatograms at increasing NH₄⁺, Na⁺, and K⁺ activity provided by the (a) ammonium-selective electrode, (b) sodium-selective electrode, and (c) potassium-selective electrode. Insets: Linearized calibration graphs (2.5 × 10^–3 mol L^–1 nitric acid, 10 μL sample volume, flow rate of 0.9 mL min^–1).