Logit linearization of analytical response curves in optical disposable sensors based on coextraction for monovalent anions

Abstract

The application of a decimal logistic transformation to the sigmoidal calibration curve of ion-selective bulk optodes for the determination of anions based on hydrophobic membranes containing neutral ionophore and chromoionophore is formally established and, consequently, a wide linear calibration function is obtained. The problems derived from the use of a sigmoidal curve in the calibration are therefore solved and the linear dynamic range is increased. The general equation resulting from the logistic transformation is discussed considering the stoichiometric factors for monovalent anions, and the linearization of the theoretical fit to experimental data was checked for two real cases. The strategy was applied to the determination of chloride and nitrate using disposable sensors for different types of waters (tap, well, stream, rain, snow and sea), validating the results against a reference procedure. This new linear calibration proposed for anion determination increases the linear dynamic range up to six orders of magnitude. The main advantage is that it is possible to directly quantify samples with very different analyte contents in a fast and simple way. The methods are easy to use and eliminate the need for prior treatment of the sample.