 Reference material homogeneity

Reference material homogeneity concerns whether variation of specified properties within or between usable portions is small enough for the intended measurement purpose. Homogeneity is property-specific and depends on the portion taken: uniform colour need not establish uniform refractive index, composition or another measured property. Sufficient homogeneity is part of the definition of a reference material.^[ref]

An original study takes several bottles from a batch and measures multiple portions from each, with the measurement order varied to avoid confusing bottle order with instrumental drift. A simple measurement model iswhere $b_i$ describes bottle-to-bottle variation and $e_{ij}$ includes variation among sampled portions and measurement noise. For independent zero-mean contributions, the variance of a single observation isRepeatedly reading one portion mainly probes repeat measurement scatter. It does not replace sampling different portions or bottles. Separating material variation from measurement repeatability requires a sampling and replication design appropriate to both.

Suppose an original fitted model gives a between-bottle standard deviation $3\times 10^{-6}$ in refractive index and within-bottle measurement scatter $4\times 10^{-6}$. Their combined standard deviation for one observation is $5\times 10^{-6}$ under the independence assumption. Averaging more readings from the same bottle can reduce the second contribution, but does not average away that bottle's common $b_i$ contribution. These are illustrative model values, not certificate data.

NIST's archived SRM1922 characterization tested multiple oil portions using minimum-deviation measurements and an Abbé-type refractometer.^[ref] Sampling different portions answers the homogeneity question directly; reference material stability separately addresses changes over time. A report should retain the sampled portion size and conditions so that a user can judge whether the evidence applies to their own use.