Preparation and use of reference materials for quality assurance in inorganic trace analysis.

A general preparation and certification scheme used in the Institute of Nuclear Chemistry and Technology (INCT) for the preparation and use of certified reference materials (CRMs) for inorganic trace analysis is presented and contrasted with approaches used by other CRM producers. 'Definitive' and 'very accurate' methods have a role, based on a combination of neutron activation and quantitative and selective post-irradiation isolation of an indicator radionuclide by column chromatography. Natural matrix CRMs of biological origin including the typical food products are reviewed and the need is identified for new well-characterized materials that would also be suitable for micro-analytical techniques such as PIXE, EDXRF, SS-AAS, etc. There are various approaches to studying homogeneity and its relationship with particle size distribution of a material, and the many problems associated with reliable particle size measurements of CRMs by various techniques are illustrated using real examples. There is an equation relating Ingamells' sampling constant to the mass of single particle of a powder material and this theoretical approach has been compared with experimental results obtained for some CRMs.