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In many monographs, the assay procedure refers to the LC or GC purity test procedure (TESTS section of monographs) using the phrase ‘as described in the test for related substances with the following modifications’.

In these assays, the system suitability test (SST) is part of the analytical procedure. Therefore, the reference solution prepared for the selectivity test*, as part of the purity test, must be analysed in the assay even if this solution is not expressly mentioned under ‘ASSAY’.

Past experience showed that the use of a cross-reference was not very clear and could be misunderstood by Ph. Eur. users. Thus, for clarity purposes, from the 13th Edition (adoption November 2025 & Implementation January 2027), new and revised monographs will clearly include under ‘ASSAY’ which of the SST criteria described in the purity test (in addition to the SST requirements of general chapter 2.2.46) need to be checked by stating:

  • the name of the reference solution to be injected for the selectivity* test described in the purity test, in addition to the other solutions relevant for the assay;
  • the selectivity* criterion.

Justification of absence of a selectivity* test under ‘ASSAY’:

The absence of a selectivity* test must be justified during the elaboration of a monograph: if such a test is not relevant for assay, this has to be demonstrated (e.g., for sufficiently pure substances and/or highly selective analytical procedures).

* resolution or peak-to-valley ratio

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