An on-line electrodialyzer-ICP-MS analytical system for direct determination of trace metal impurities in KOH

Abstract

We have developed an on-line electrodialysis system equipped with a cation-permeable membrane that, when coupled to an ICP mass spectrometer, provides a simple and sensitive hyphenated method for the determination of trace metal impurities in concentrated KOH. Because our proposed on-line electrodialysis method allows the quantitative removal of K⁺ and OH⁻ ions, which may seriously interfere with and potentially damage the ICP-MS system, the safety risks and the analytical errors arising from the presence of extremely high alkalinity KOH are both low. To prevent loss of the analyte cations during the electrodialysis process, a selective chelating agent—ethylenediaminetetraacetic acid (EDTA)—was added to form anionic metal–EDTA complexes. To achieve maximum signal intensities, we optimized the sample flow rate and the amount of EDTA in terms of their effects on the retention and removal efficiencies of the analyte species and K⁺ ions. Based on the optimized operating conditions, near 100% of trace metal ions in 11.25% KOH solutions—added with 10 mg EDTA L⁻¹ in advance—could pass through electrodialyzer and be delivered to the ICP-MS at a flow rate of 0.1 mL min⁻¹. Because over 99.99% of 11.25% KOH was removed prior to ICP-MS measurement and low blank levels were achieved, this hyphenated method is sufficiently robust for the determination, with detection limits ranging from μg L⁻¹ to sub-μg L⁻¹, of trace Cu, Mn, Zn, Co, Ni, Cd, and Pb impurities in high-alkalinity samples. Because no certified values for the analyte content in KOH were available, the overall analytical method was validated for accuracy and precision by analyzing KOH solutions spiked with known amounts of analytic ions. Based on the analytical data, the analytical accuracy, expressed as spike recovery was found to be at 100 ± 10% for all the elements tested. This method is superior to the reported voltammetric methods for the determination of trace impurities in terms of detection limits, ability of simultaneous multi-elements analysis and speed of analysis. The validated method was successfully applied to determine trace levels of metal impurities in concentrated KOH solutions.