Run time related substances

[danor285]

Hello.
In a method for related substances, if in the specificity all the degrades have a retention time less than the principal peak, is necesary to have a run time longer than the retention time of principal peak?
Thanks

[Gerhard Kratz]

No, it is not necessary, but if the peaks are not well base line separated try a different separation mode. For example if you used RP try to separate on HILIC. Mostly the elution order is different. Maybe that will help.

[HPLC chemist]

Your retention time of your related substance can be substantially different than the API because it is based on the molecule's chemistry and structure (among other things). As long in method validation the run time was increased 2-3X and no other peaks eluted later than the API, you are golden! Eventually, you will want to set specifications for the related substances at expiry.

[Peter Apps]

Bear in mind that if you do not elute the main component it will come back to haunt you as a ghost peak in subsequent runs.

Peter

[danor285]

In method validation i have a run time 3x RT of mean peak. No one impurity/degraded eluted after mean peak. So, i want to define run time for stability.

[HPLC chemist]

Continue on! Remember, this does not mean you don't have an additional related substance, but it does mean;

1. This related substance could be fully retained on the HPLC column under those chromatographic conditions and will not interfere in subsequent injections.

2. You will NEVER have 100% 'mass balance' based on your degradation path. Example =, do you include the mass from water from the
de-esterification of an API ester.

3. Other...