Mass Balance

[rojitea]

I can get 10 % degradation in the RS method but still can get Assay around 100 % is it possible ???

How can we justify Mass Balance ???

[LCFan]

Hello,

is assay datermined by HPLC too? Often, some drug substance are shown to be sufficiently pure by an LC method (related substances) but due to the absence of a suitable standard, the assay is determined by UV (using a fixed absorption coefficient) or by titration. E.g. if you apply a non-aqueous titration for N-containing drug substance, you might also titrate the N-containing degradants which could lead to a 100% value for assay.

Other possibility:
your degradation leads to two degradants. Degradant 1 is about 10% of the main peak area and well separated from the main peak. Degradant 2 coelutes with the main peak which might lead to a fictive 100% assay. Peak purity check by UV or better MS might help.

Florian

[rojitea]

Assay & RS is determined by same method and peak purity factor is ok as well.

But mass balance cannot be established as related substance degradation is not reflected in Assay.

[aceto_81]

If the peak purity is OK, this implies you have an PDA detector.
Are the spectra of the peaks the same, or is there a difference?
If the spectra are really different, this can be the reason why there is such a mass balance error.

HTH

Ace

[tom jupille]

If the spectra are really different, this can be the reason why there is such a mass balance error.

Actually, I think it's the other way around: if the spectra are *very similar*, then peak purity will pass even with substantial overlap. For that matter, of peaks exactly coelute, the peak purity will pass regardless of spectral similarities/differences. In either case, your hypothesis makes sense.

[rojitea]

I think that if the degradation product shows difference in absorptivity as compared to the analyte then mass balance cannot be proved.

[danko]

Have you calculated the standard’s strength upon correction for potential degradation/impurities of/in it. Integrate and calculate the peaks you see when chromatographing the standard and subtract them from the calibration value used for quantitation of your samples.
Also consider differences in absorption coefficients of the different degradation products as pointed out by rojitea.
Best Regards

[DR]

The combination of unknown absorbtivities and unknown masses of most unknowns in stability indicating assays makes the prospect of ever *really* achieving mass balance somewhere between troublesome and impossible (especially with UV based detection).

At best, it's a capricious demand frequently made by people who fail to see how time consuming and overrated a goal it is.
(in my opinion)