The accuracy of elemental analyses in complex matrices is usually assessed by analysis of a suitable matrix reference material. The reference value is ascribed by consensus mean and by application of primary methods of analysis. However, the quality of this value will be affected by problems such as matrix-induced interferences, moisture corrections and heterogeneity.

Pilot study CCQM-P13 was undertaken to assess the capabilities of National Metrology Institutes to analyse Ca, Cu and Cd in an acidic solution that simulates the digest of a food sample. This study filled the gap between the analysis of a gravimetrically prepared calibration solution and the analysis of an unknown in a complex matrix requiring extensive sample preparation. Having an independent reference value, with a small uncertainty, allowed a more rigorous estimation of the reliability of the institutes' analysis and uncertainty estimates, without including issues around sample digestion.

The reference values were: 1.6617 ± 0.0020 µmol/g for Ca, 7.037 ± 0.012 nmol/g for Cu and 45.57 ± 0.10 pmol/g for Cd (expanded uncertainties are quoted with coverage factor of 2). The other elements in the matrix were: Na (~25 µg/g), K (~90 µg/g), Cl (~120 µg/g), Fe (~100 ng/g), Mg (~5 µg/g), P (~5 µg/g), Sn (~80 ng/g) and Zn (~200 ng/g).

Twelve international laboratories, representing eight countries, determined the amount content of the analytes. A range of techniques that include ID-ICP-MS (high resolution and collision cell), ICP-MS, ICP-OES, AAS, voltametry and potentiometry were used. The results for this pilot study averaged: 1.654 ± 0.058 µmol/g for Ca (n = 10), 7.26 ± 0.53 nmol/g for Cu (n = 12) and 45.2 ± 5.1 pmol/g for Cd (n = 11) where the values associated with the averages are the standard deviations of n participants.

Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/.

The final report has been peer-reviewed and approved for publication by the CCQM Working Group on Inorganic Analysis (IAWG), according to the provisions of the Mutual Recognition Arrangement (MRA).