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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.

In proficiency tests the selection of an appropriate assigned value is one of the most critical points. Determination of the assigned value for proficiency test can be done in 2 ways: based on reference value and consensus value. The use of such a consensus value as an assigned value is a commonplace in proficiency testing, because there is seldom at present a cost effective alternative. However, the consensus value has a weakness in that it could be significantly different from the true value due largely inexperienced laboratories. Therefore in this work, the reference values were used as the assigned values for all analytes of interest. The reference values in spiked drinking water samples were produced by the Metrology in Chemistry Laboratory of Research Center for Chemistry - Indonesian Institute of Sciences (RCChem-LIPI) with traceable to SI units through standard solutions from the National Institute Standard and Technology, USA. The reference values of cobalt and manganese were obtained by measurement using two from three comparable methods of Graphite Furnace-Atomic Absorption Spectrometry (GF-AAS), Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES) and Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) while that for cadmium was obtained by a primary method of ICP-Isotope Dilution Mass Spectrometry (ICP-IDMS).The obtained reference values were used to evaluate the laboratory performance result in a proficiency testing scheme of cadmium, cobalt and manganese in drinking water which was provided in 2014. All fifty nine laboratories reported cadmium and cobalt at low concentration levels which were about more than 82% showed satisfactory performance results. Similarly, the results of manganese were also comparable at high level concentrations.

Two significant parameters to evaluate the measurement results are known as precision and accuracy. Both are associated with indeterminate and determinate error, respectively, that normally happen in such spectrometric measurement method as Inductively Coupled Plasma - Mass Spectrometry (ICP-MS). These errors must be eliminated or suppressed to get high precision and accuracy of the method. Decreasing the errors thus increasing the precision and accuracy of the method. In this study, bracketing method using two-point standard calibration was proposed in order to suppress the indeterminate error caused by instrumental drift thus increasing the result precision, and applied for measuring cadmium in drinking water samples. Certified reference material of ERM CA011b-Hard drinking water UK-metals was used to know the determinate error or measurement bias. When bias is obtained, some corrections are needed to get the accurate measurement result. The result was compared to that by external calibration method.