High-accuracy IDMS analysis of trace elements in wheat flour for the provision of reference values to a proficiency testing scheme

Isotope dilution techniques were developed for the analysis of ten trace elements (Cd, Cu, Cr, Hg, Mo, Ni, Pb, Se, Sn and Zn) in a fortified wheat flour candidate sample for a proficiency testing scheme coordinated by NMI. The technique used was exact-matching double isotope dilution mass spectrometry (IDMS) by ICP-MS. A metrological approach was used to examine potential biases and ensure that this potentially primary method of analysis provided appropriate accuracy for the provision of reference values to the proficiency testing scheme. The exact-matching IDMS technique used in this study has been compared with the IDMS methods of other national metrology institutes in the CCQM-K24 and -P29 intercomparisons and the results produced were in very good agreement. Expanded relative uncertainties at the 95% confidence level for the ten elements in the wheat flour varied from 3.1%–14%. The results submitted from the participant laboratories for the proficiency testing scheme generally compared reasonably well with the NMI reference values.