Theory and practice of rapid flow-through analysis based on ion-selective optode detection and its application to sodium ion determination Original Research Article

One simple example of a rapid flow-through analysis (RFA) system based on ion-selective optode detection was demonstrated, and its performance was discussed. The RFA is a flow analytical method based on a reagentless analyte flow injection system combined with an analyte selective monitor/sensor such as an optical chemical sensor (optode) involving mathematical response simulation using a semi-empirical theoretical response equation. The response of RFA with optode detection can basically be expressed by the equation derived from Fick’s law of diffusion and Kubelka–Munk’s law, and two other variable parameters in relation to the experimental response. Mathematically simulated results using the proposed equation indicated that the equation can be useful for the prediction of the flow analytical response. In the present demonstration of RFA, highly selective ionophores based on 16-crown-5 derivatives were used for the ion-sensing component of the optode for sodium ion determination. A sodium ion-selective optode based on C14-DD16C5 and KD-A3 exhibited high selectivity for sodium ion over potassium ion, with a selectivity coefficient (log Ki,jopt) of −2.6. This optode responded to sodium ion in the concentration range from 10−5 to 10−1 M at pH 8.0 by measuring the absorbance change at 510 nm in the sensing phase of the optode. The RFA system equipped with a sodium ion-selective optode offers a reagent-less advanced ion-determination methodology for conventional flow injection analysis.