First Indonesian proficiency testing scheme using reference values for cadmium, copper, and iron in drinking water

Abstract

Proficiency testing (PT) performance evaluation based on a consensus value for the assigned value is commonly used because it is considered cheap and thus easily approachable, and also gives a good confidence to the PT participants, since their results contribute to the establishment of the assigned value. However, the consensus value has a weakness in that it could be significantly different from the true value due to largely inexperienced laboratories participating in the PT scheme. Starting from 2013, the Research Center for Chemistry, Indonesian Institute of Sciences, tried to conduct a PT scheme using reference values for cadmium, copper, and iron in drinking water samples in order to increase the reliability of measurement results from Indonesian testing laboratories. The PT sample is a candidate reference material with reference values which were calculated from the measurement results using two comparable methods of inductively coupled plasma optical emission spectrometry and graphite furnace atomic absorption spectrometry. Data evaluation of the PT results was based on ISO 13528 and calculated with the z-score statistic method using PROLAB software. Sixty-two laboratories from the whole Indonesian region participated, and about 94 % and 71 % showed good performance for copper and iron, respectively, since the values were above the quantification limit, and the mean of the result was close to the reference values. However, there was a big difference between the mean and the reference value for cadmium due to the lack of knowledge and experience of most laboratories in Indonesia to measure the trace concentrations of elements such as cadmium. The results of the participants all together were so poor, that the use of consensus value would have been of less quality than the reference value. Therefore, the only way to evaluate cadmium in this PT scheme was by using the reference value.