Description: Data reported by laboratories contributing to the Geo PT proficiency testing programme for geochemical laboratories over the period 2001 to 2011 have been assessed to identify the elements and concentration ranges over which analytical performance can be considered satisfactory. Criteria developed in the paper indicate that performance in the determination of the elements/constituents SiO 2 , Al 2 O 3 , MnO, Cs, Dy, Er, Eu, Ga, Hf, Ho, Lu, Nd, Pr, Sm, Sr, Tb, Tl, Tm, U, Y, Yb, Zn is satisfactory over the full concentration range assessed. The elements/constituents TiO 2 , Fe 2 O 3 T, MgO, CaO, Na 2 O, K 2 O, P 2 O 5 , Ba, Be, Cd, Ce, Co, Gd, La, Li, Nb, Rb, Sb, Sc, Sn, Ta, Th, V, Zr showed some degradation in performance at lower concentration levels (approaching the detection limit of some techniques). Performance in determining LOI, As, Bi, Cr, Cu, Ge, Mo, Ni, Pb, W was in general unsatisfactory over the full concentration range assessed. Other elements (especially Fe(II)O, H 2 O + , CO 2 , Ag, Au, B, Br, Cl, F, Hg, I, In, Ir, N, Os, Pd, Pt, Re, Rh, S, Se, Te) could not be evaluated as they are not routinely reported by laboratories participating in the Geo PT programme, often because they are present in silicate rocks at sufficiently low concentrations to require a preconcentration stage. Some suggestions are made for the causes of unsatisfactory performance, but further progress will require a detailed assessment of the methods used by participating laboratories, which will form the subject of a further paper. © 2013 The Authors. Geostandards and Geoanalytical Research © 2013 International Association of Geoanalysts

Description: A properly developed consensus from a proficiency test is, for all practical purposes, interchangeable with a certified value derived from an interlaboratory comparison. But some scientists have doubts about the metrological status of such materials. In particular, the materials are not regarded as equivalent to certified reference materials (CRM) because of a lack of traceability of the consensus value. However, the detailed considerations in this paper show that such a distinction is without foundation. The following issues are addressed. (a) Misgivings about the traceability of an interlaboratory consensus are found to be insubstantial because chemical measurements are hardly ever fully traceable. (b) Reproducibility (that is, interlaboratory) conditions account for all sources of variation in results other than long-term stability, so lead to a valid estimate of uncertainty. (c) Certification of a reference material according to ISO procedures is homologous with a proficiency test and no more secure against bias in the characterised value and its standard error. In particular, residual (common) bias cannot be fully addressed in either approach. (d) A consensus in itself attests to a valid outcome. (e) Proficiency tester providers have access to a powerful method of monitoring long-term stability that is denied to CRM producers. This article is protected by copyright. All rights reserved.

Description: Assigned values derived from the Geo PT proficiency testing programme were compared with certified values for six certified reference materials that have been used as test materials in the Geo PT programme. Statistical analysis showed that there were few significant differences between these sets of data and that these differences had no significant impact on the Geo PT assessment when fitness for purpose criteria were taken into account. This article is protected by copyright. All rights reserved.

Description: The analytical results for the modified river sediment, SdAR-1 circulated in Round 31 of the Geo PT proficiency testing programme revealed unexpected discrepancies for Zr, Y and most higher atomic number rare earth elements (HREE), in determinations made by ICP-MS using acid attack digestion methods. This investigation compares these ICP-MS results with those obtained for SdAR-1 by three other methods: (i) ICP-MS using fusion/sintering for sample digestion, (ii) XRF analysis and (iii) INAA. The distribution of results for the elements Rb, Sr, Ce, Y, Yb and Zr, are examined and compared with those of the test material for Round 25 of Geo PT , Paraná basalt, HTB-1. A substantial proportion (though not all) of Y, Yb and Zr determinations in SdAR-1 by ICP-MS/acid attack were variably low (sometimes very low) compared with other methods. A detailed evaluation of the procedures used to determine these results indicated that successful determinations by ICP-MS/acid attack could be made if digestions were made at 180 °C for 48 hours using at least 2 ml HF per 100 mg of sample. We suggest that the more benign conditions (used by many laboratories) resulted in incomplete digestion of resistant minerals, such as zircon. This article is protected by copyright. All rights reserved.

Description: The Geo PT proficiency testing programme for geochemical laboratories has been operating for nearly 20 years. This review, based on an analysis of accumulated data and extensive experience, examines whether the protocol followed during this period continues to be appropriate. Possible changes that might be considered for future implementation are examined, in particular the procedures for calculating assigned values, target values, z-scores, and the applicability of dual fitness for purpose criteria. We conclude that the existing protocol remains largely appropriate and effective, but propose that a single fitness-for-purpose criterion be adopted, with an option for laboratories to recalculate their z-scores if the criterion is inappropriate for their requirements. This article is protected by copyright. All rights reserved.