The RRT is calculated from a single chromatogram, thus you would normally confirm the RRT of any standard impurities that the method requires from your standard mixture chromatogram, and the RRT of sample impurities from the sample chromatogram.
There may even be situations where standard impurities are spiked into sample solutions, in which case the impurities RRT are referenced to the sample major API peak in the same chromatographic run.
Exceptions would be detailed in the appropriate monograph.
In general, the retention times are calculated relative to the major API, but that is also usually defined in each monograph.
I'm sure the following will appear offensive, but if you are training in an institute or organisation, your training programs should cover these critical aspects of pharmaceutical testing.
If you are working for a manufacturer, perhaps you should ask to be sent onto some pharmceutical chromatographic analysis training courses, such as those offered by Tom.
Otherwise, your employer could be facing regulatory/client audit issues concerning the requirement for analysts to be formally trained and competent in the tests they are performing...
Please keep having fun,
Bruce Hamilton
