Mass balance in forced degradation

[gnikol21]

Hello to all,
Can you help me with the calculation of mass balance? I have one degradation product 16% (this result came usin its correction factor) with MW 300 and the MW of API is around 610. The assay in stressed sample is around 73%.
Thnx in advance

[tom jupille]

If I read your post correctly, you are seeing that 73% of your stressed sample is the API and 16% is the known degradation product; the quantities of both compounds were determined using their own response factors.

That would imply that 11% of your original sample has degraded to something which you are not seeing:
- your detector may not be responding to it (e.g., no absorbance at the wavelength you are monitoring)
- it may be permanently retained on your column.

[gnikol21]

For the mass balance calculation i must consider the MWs of degradant and the API? Or the MWs have already included usin response factor to determine the % of the known impurity?

[tom jupille]

When you prepared your calibrator solutions, did you use mass or moles? If you used mass (e.g., something like mg/mL, then you can ignore the molecular weights. If you used moles, then you will have to multiply by the relevant molecular weights to get the masses.

[prepcolumn]

For the mass balance calculation i must consider the MWs of degradant and the API? Or the MWs have already included usin response factor to determine the % of the known impurity?

I assume that you use concentration as mg/ml. In that case, you have to include the MW of the degradant if you are sure that the rest of the degradant molecule does not give any response in your chromatogram.

[upamaniah]

to the author,
1. i hope you are doing forced degradation study for your RS method of the API (610 MW).
2. you no need to calculate the mass balance of the impurities.
3. your assay value of API {{calculated against the same API std, using the same method unless otherwise the linearity is established}}, is the mass balance of the molecule (610 MW)

[tom jupille]

There is a lot of confusion here between "mass balance" and "mole balance".

If you started with 100g of your API, with a molecular weight of 610, that works out to 0.164 moles
If you ended up with 73g of API in the stressed sample, that works out to 0.120 moles
and if you had 16 g of the known degradant, with a molecular weight of 300, in the stressed sample that works out to 0.053 moles

Those numbers obviously don't add up (0.120 + 0.053 = 0.173 moles), so either there is an error in your methodology or (more likely), I'm misinterpreting the original post.

If you know the molecular weight of the major degradant, you presumably know its structure and should therefore be able to make an educated guess as to the degradation mechanism and what the rest of the molecule is doing.