External Quality Control Program on Analysis of Sodium in Drinking Water

Abstract

The Tokyo Metropolitan Government has conducted an external quality control program for drinking water laboratories since 2003, in order to evaluate and improve their analytical performances. In 2006, 30 laboratories participated in the program, and the three analytical methods were applied using sodium as a standard sample; namely, atomic absorption (AA), inductively coupled plasma spectrometry (ICP), and ion chromatography. There were no significant differences among the outcome of these 3 methods. As a result, 3 laboratories were revealed to perform the analysis with poor accuracy, because their absolute values of Z-scores were more than 3, while coefficients of variation were more than 10%. On the other hand, in the case of the axial-viewing ICP, luminous intensities of sodium were increased in laboratories using standard calibration solutions containing alkaline metals such as lithium, which gave results lower than reference concentrations. Although some other unsatisfactory performances of laboratories were also observed, they were due to simple errors such as the use of inadequate calibration curves and the unstable settings of instruments.