A new test for 'sufficient homogeneity'

Certified reference materials and materials distributed in proficiency test ing need to be 'sufficiently homogeneous', that is, the variance in the mea n composition of the distributed portions of the material must be negligibl y small in relation to the variance of the analytical result produced when the material is in normal use. The requirement for sufficient homogeneity s uggests the use of a formal test. Such tests as have been formulated rely o n the duplicated analysis of the material from a number of portions, follow ed by analysis of variance. However, the outcome is not straightforward. If the analytical method used is very precise, then an undue proportion of th e materials will be found to be significantly heterogeneous. If it is too i mprecise, the test may be unable to detect heterogeneity. Moreover, the Har monised Protocol Procedure (M. Thompson and R. Wood. Pure Appl. Chem., 1993 , 65, 2123) seems to be unduly prone to the rejection of material that is i n fact satisfactory. We present a simple new statistical approach that over comes some of these problems.