 Reference material stability

Reference material stability concerns whether the specified properties of a reference material remain suitable over a stated time and under stated storage, transport and use conditions. Stability is a requirement relative to an intended measurement use, rather than a promise that every property remains exactly constant.^[ref] Reference material homogeneity asks a different question: whether different portions adequately represent the same material.

A stability investigation must distinguish material change from a changing measuring system. Repeatedly measuring one bottle with an instrument that drifts does not identify which changed. Keep the property definition and temperature conditions consistent, use suitable comparison references, record handling and examine the uncertainty of any fitted trend. A stable comparison reference helps separate the two effects; it also has its own measurement uncertainty.

For an original optical example, suppose a trend model for refractive index at fixed wavelength and temperature iswhere $t$ is measured in months. A stipulated $\beta =2\times 10^{-6}\mathrm{mont}{\mathrm{h}}^{-1}$ predicts a change $6\times 10^{-6}$ after three months. If the intended use permits a change of at most $5\times 10^{-6}$, that model reaches the limit after $2.5$ months. This is a teaching calculation under an assumed trend, not an observed shelf life. A real decision also accounts for uncertainty in $\beta$, changes between transport and storage conditions, and the reliability of the trend model.

The archived NIST SRM1922 certificate gives concrete handling conditions: keep the oil in its original bottle, tightly recapped, at approximately $20$–$25{}^{\circ }\mathrm{C}$, protected from excessive moisture and light. Its certification conditions exclude damaged, contaminated or modified material.^[ref] Those conditions belong alongside the assigned value; an unchanged label does not establish reference material stability.