Determination of lanthanides in synthetic standards by reversed-phase high-performance liquid chromatography with the aid of a weighted least-squares regression model estimation of method sensitivities and detection limits.

A reversed-phase liquid chromatography method was used for determining lanthanides in synthetic standards. The separation of lanthanide group was achieved in less than 15 min using a linear gradient program of alpha-hydroxyisobutyricacid eluent from 0.05 to 0.5 M (pH 3.8) with a UV-Vis detection system at 658 nm. A post-column reagent of Arsenazo III was employed for improving the sensitivity and selectivity of the method as well as for lowering the limits of detection (LODs). Linear calibration curves for all lanthanides were constructed using an ordinary least-squares (OLS) regression as well as a weighted least-squares (WLS) regression model for taking into account the heteroscedastic errors. The WLS model was successfully used for a better estimation of the sensitivities and the LODs of the RP-HPLC method than the conventional OLS model. The lanthanide sensitivity obtained from the slope of each calibration curve seems to be better for a lanthanide with an odd-atomic number compared to its neighboring element with an even-atomic number, as if nature is helping us to quantify the concentrations of the less abundant lanthanides. This observation was also confirmed when the LODs computed for all lanthanides were examined. The LODs observed for all lanthanides depicted a clear systematic "zigzag" pattern. This is actually the first time that the lanthanide detection limits determined by a HPLC method are shown to mimic the zigzag patterns for the concentration data in geological and cosmological materials. Such a "zigzag" pattern should be used as a standard criterion for evaluating the quality of detection limit data.