A novel mid-infrared (IR) flow-through sensor system for the determination of metal ions in aqueous solution was developed. The flow-through sensor comprised weak cation exchanger beads (Sephadex with carboxylic acid moieties) placed in the optical path of a 37 μm IR transmission cell. The combination of the flow-through sensor with a sequential injection analysis (SIA) system allowed us to carry out the whole analysis procedure including analyte-sensor interaction and sensor regeneration, as well as data acquisition, under computer control. The first step of an analysis sequence was regeneration of the flow-through sensor by rinsing with diluted (0.012 M) hydrochloric acid followed by distilled water, thus protonating the carboxylic acid moieties. Then, after acquisition of a background spectrum and pumping 500 μL of sample followed by 1000 μL of distilled water through the flow-through sensor, the sample spectrum was recorded. Upon passage of the sample, the metal ions were retained on the cation exchanger, giving rise to spectral changes mainly resulting from the exchange of the protons by metal cations. It was found that the spectral changes were characteristic for the metal cations investigated. This observation was exploited to construct a multivariate calibration model for the simultaneous determination of calcium and magnesium ions in water. The developed method covered the range from 0 to 200 nmol for each cation (root mean square error of cross-validation: 9.2, and 7.6 nmol for Ca<sup>2+</sup> and Mg<sup>2+</sup>, respectively) and was successfully applied to the determination of the water hardness of the tap water in our laboratory.

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