B.2.1 Selection of candidate substance When it is intended to establish a secondary reference substance for use as an assay standard for the determination of the content of the drug substance itself or in a drug formulation, a source(s) of pharmaceutical grade substance(s) is (are) identified. Availability of the required quantity is assured. The guidelines given in Section 2 of Part A also apply in this case. If a substance is intended to be used as an impurity standard, the candidate material may be obtained from commercial suppliers, provided that the percentage purity is more than 95% (or 90% if for use in TLC).

B.2.2 Documentation to be supplied with the candidate reference substance The supplier of the candidate reference substance is requested to supply the same documentation as required for a candidate primary reference substance (see Section 2, Part A).

B.2.3 Initial testing for compliance with the requirements of the monograph The coordinating laboratory is responsible for verifying that the candidate reference substance complies with the requirements of the monograph, where applicable. In such a case compliance is a prerequisite to proceeding to the interlaboratory study to assign the content of the secondary standard.