Non-linear calibration functions in ion chromatography with suppressed conductivity detection using hydroxide eluents.

The linearity of calibration curves in ion chromatography with suppressed conductivity detection using hydroxide eluents was investigated. Theoretical calibration curves were derived for strong electrolytes and weak monobasic acids and the results compared with experimental data. At low concentrations up to 1 micromol l(-1) the autoprotolysis of water induces left-curved calibration functions even for strong electrolytes like nitrate. The experimental data are best described by a quadratic function, the differences between linear and quadratic regression being up to 10%. At higher concentrations the calibration curves for strong electrolytes are linear. Due to incomplete dissociation, the calibration curves for weak mono- and dibasic acids show a right curvature. Thus, depending on the analyte and the concentration range of interest, analysts should carefully choose between a linear and a quadratic regression function.