The uncertainty of purity of reference materials must be known

Quantitative chromatographic analyses do not belong to the class of primary methods, but the analyst needs to calibrate the obtained signals with well-known solutions made of high-purity reference materials. Every analyte, to be quantified by a non-primary method, needs the chemically identical compound as its reference. It is not necessary that the content of the reference is 100.0%, but it is crucial that its purity is known, together with the uncertainty of this value (e.g., 99.9±0.05%). Any high-quality analytical result should include information about the associated measurement uncertainty, and the purity uncertainty of the reference is a parameter which always appears in the overall measurement uncertainty calculation of the measurand (such as the concentration or content of an analyte). Our general postulation is that the purity and the uncertainty of all reference materials must be known. It concerns all types of reference materials although we focus our discussion on the analysis of drug products. Unfortunately, at present only a few pharmaceutical reference materials are commercially available from regulatory bodies with a certificate confirming their purity with the related uncertainty. In contrast, such well-characterized materials, traceable to the International System of Units (SI), are available from various commercial suppliers.