Evaluation of alternative procedures for the provision of consensus and assigned values to sixteen trace elements in natural water used for an ICP-MS proficiency testing exercise

Abstract

The University of Oviedo, in collaboration with the company ISC-Science, has conducted a proficiency testing exercise for Spanish ICP-MS users focused on trace elements determination in drinking waters. A total of 47 laboratories participated in this exercise where 16 elements (Al, Sb, As, B, Cd, Cu, Cr, Fe, Mn, Ni, Pb, Se, Sr, Mo, U, and Zn) were determined in a natural water sample. The analytical procedure used at the University of Oviedo was validated by analysis of the certified reference material SLRS-4 (riverine water) from the National Research Council of Canada. Assigned values were provided for 12 multi-isotopic elements by isotope dilution analysis. For this purpose, isotopically enriched spikes (¹⁰B, ⁵³Cr, ⁵⁷Fe, ⁶¹Ni, ⁶⁵Cu, ⁶⁷Zn, ⁷⁷Se, ⁸⁶Sr, ⁹⁵Mo, ¹¹¹Cd, ¹²³Sb and ²⁰⁷Pb) were characterised by reverse isotope dilution analysis both by multicollector ICP-MS and collision cell ICP-MS (¹⁰B and ⁷⁷Se). Finally, the collision cell instrument was adopted for the provision of the assigned values taking into account the results from the homogeneity and stability studies on the natural water sample. Considering the differences in expertise between the different laboratories, consensus values for all sixteen measured elements were calculated using alternative statistical tests for outlier removal. It was observed that the combination of the Cochran and Hampel tests provided the best agreement between the consensus and the assigned values. Finally, the analytical results from the different laboratories were evaluated by different scoring procedures taking into account the requirements of European Directive 98/83/EC for drinking water testing. As these types of exercises will be used by many laboratories to seek or maintain accreditation, in intercomparison exercises where consensus values are usually applied, the evaluated procedures for outlier elimination and scoring are critically compared.