Bioequivalence and Bioavailability Forum

Dear VP!

❝ In Project sample analysis, we got ISTD area in predose sample though it

❝ is predose sample without ISTD.

Interesting approach. To my knowledge nobody asks for a predose sample without IS - only one sample without IS in the calibration curve.

❝ In that case we have repeated predose sample analysis. Is it correct?

I think so.

❝ If yes, What reason we should mention for that?

The same reason you would give for any other repeated analysis. If you 'see' no more response at the retention time of the IS, most likely it was an error in the sample preparation.

1. If the response is roughly the same you would expect for a normal sample (with IS) most likely the analyst erroneously added the full amount of IS.
   
2. If the response was much lower, you may suspect carry-over (both in pipetting or in the autosampler).

I would not bother much on case (1) - errors happen.
In case (2) probably you should check your procedures: Watch for a potential contamination in the sample preparation, flush the injector with a solution of more organic modifier than the mobile phase, inject a blank/mobile phase after samples with high concentrations.