Calculation using Relative response factor and molecular wt.

[abheenaik]

Dear members,
Hope you all are doin good .
I have an elementary doubt in calculating % degradation impurity obtained by HPLC for a particular formulation.
I would like to know in detail when do we use Relative Response factor (RRF) for calculating % impurity and how do we go about it's calculation.Do we need to use molecular weight of impurity and the active peak along with RRF for calculating the % impurity.If yes then why? what is the co-relation of RRF and molecular weight.
Also if one of the impurity is a base of the active or acid form of the active do we need to incorporate RRf and molecular weight in it's % impurity calculation?

I am aware it is an elementary calculation but can be of great help to me if responded soon.

Please do reply with any facts you know.It would of help to me.

Thanks in advance!

[tom jupille]

Since you posted in the LC-MS section, I assume you are using MS for detection. Are you doing ESI or APCI? positive or negative ionization?

[abheenaik]

I am so sorry.I am a new member and was unaware of the this being LCMS section.
Actually the analysis is by HPLC.

[Murugan Saravanan]

Dear abheenaik,
RRF is a concept to calcualte the response of impurities with respect to your parent drug component based on uv absorption.
It is calcualted based on area responses of individual impurity and parent compound due to differences of lamba max.
for eg. prepare a series of solutions containing impurity as well as parent compound with known concentrations like, 10 ppm, 5 ppm, 2 ppm etc.
Inject at particulat wavelenghth and find out a slope for each impurity and parent compound.
Calculate the RRF as slope of parent compound / slope of impurity.
Hope this helps.
Regards
Murugan Saravanan

[abheenaik]

Thanx Murugan,
It is certainnly helpful.
Can you please explain when we include the molecular weight of impurity into calculation?

[Murugan Saravanan]

Mol. wt. will not be required for calculation of impurities. It will be useful to calculate the mole ratio for starting the reaction.
Regards
Murugan Saravanan