On the use of a small synthetic calibration set for the simultaneous spectrophotometric multivariate determination of Ca2+ and Mg2+ in groundwater: chemical and spectral considerations

A critical evaluation of chemical and spectral considerations for the simultaneous spectrophotometric determination of calcium and magnesium in groundwater samples with methylthymol blue (MTB) using a calibration set composed by synthetic standards is presented. The method is based upon the spectral changes observed in MTB spectra in the visible region (400-700 nm) when concentrations of calcium and magnesium are varied. First-order multivariate calibration models were built using net analyte pre-processing followed by classical least-squared regression (NAP/CLS) method. The concentration of MTB, the pH of the solution at which the reaction is carried out and the spectral range were optimised. Based on theoretical concentration distribution plots, discussion of experimental effect of each chemical variable is provided. Through a suitable chemical and spectral optimisation and based on a factorial central composite design, we succeeded in obtaining excellent prediction results using a calibration set composed only by eight synthetic solutions in the analytical concentration range of 0-200 and 0-100 mg L-1 for Ca2+ and Mg2+, respectively. The developed methodology was applied to real groundwater samples from Great Buenos Aires area, with no pre-treatment except filtration of turbid samples, showing a very good agreement with the well-known flame atomic absorption spectrometry technique, which was used as reference method. Critical performance comparison with previous work is provided