The purity requirements for a reference substance depend upon its intended use. A reference substance proposed for an identification test by infrared spectrophotometry does not require meticulous purification, because the presence of a small percentage of impurities in the substance often has no noticeable influence on its infrared spectrum. Similarly, reference substances that are applied in low loadings in thin-layer chromatographic tests need not be highly purified.
On the other hand, reference substances that are to be used in assays should preferably possess a high degree of purity. As a guiding principle, a purity of 99.5% or better is desirable for such reference substances, although in cases where the precision of the analytical procedure for which the reference substance is required is low, such a degree of purity may not be necessary. In making a decision about the suitability of a reference substance, the most important consideration is the influence of the impurity on the attribute measured in the assay. Impurities with physicochemical characteristics similar to those of the main component will not impair the usefulness of a reference substance, whereas even traces of impurities with significantly different properties may render a substance unsuitable as a reference substance.