Question of the month - October 2018
What are reference standards for impurities, and what are research materials?
Information on what constitutes an impurity reference standard (IRS) is scarce. The European Pharmacopoeia states in text 5.12. that “reference standards are established using suitable procedures and their continued suitability for use is monitored” and further that “a CRS (Chemical Reference Substance) corresponding to an impurity is characterised for identity and purity.” Meanwhile, the ICH simply requires in impurity guidelines Q3A/Q3B that “reference standards used in the analytical procedures for control of impurities should be evaluated and characterised according to their intended uses.” There is not much further information available, so it is not surprising that approaches to impurity standards are highly variable, from both manufacturers of such standards and end users.
In our opinion, assuring “continued suitability for use” in reference standards, as mentioned above in Ph.Eur. 5.12., requires an ongoing stability or “fit for purpose” monitoring programme – an important characteristic which distinguishes reference standards from research materials. We have such a programme in place for our Mikromol™ impurity reference standards.
Research materials normally lack this stability-monitoring programme. Also, it is often unclear how their identity was checked. Often NMR and/or mass spectra are provided, but there is no further explanation about whether and how these data were confirmed. There are even materials on the market calling themselves “reference standards” which are missing this kind of information entirely.
A real impurity reference standard should provide identity evidence, i.e. spectra and information on how they were used for proving identity, at least comprising a short formal sentence that on request can be elaborated into a full identity interpretation report.
All our Mikromol reference standards more than fulfil this requirement, being accompanied by a comprehensive Certificate of Analysis providing a full description of the pharmaceutical standard, and summarising the analyses undertaken during the characterisation process.