Calculating relative response factors for impurities

[mikea]

Hello,
I have an impurity with a lambda max of 234nm but the parent peak has a lambda max of 280nm. I am currently analyzing both peaks @ 280nm. Can anyone point me in the direction of how to properly calculate RRF for the impurity?

Thanks

[tom jupille]

I assume that you have enough of the impurity to develop a calibration plot for it (since you have enough to determine the spectrum). The response factor for any compound is the slope of its calibration plot. The relative response factor would be the ratio of the response factor for the impurity to the response factor for your API (both measured at the same wavelength).

All of that said, my 2 cents' worth is that you would get more reproducible results by detecting each compound at its respective optimum wavelength, with a separate calibration plot for each. It's more work, but better science.

[Uwe Neue]

Tom: I do not see a reason why the relative response factor needs to be determined at the same wavelength. At least for a given instrument and fixed settings, one should be able to use relative response factors for different wavelengths.
I can see reasons why you do not want to do this on different instruments, or even on the same instrument when varying detection parameters. But I do not see a problem, if you fix everything, which one should do anyway.

[tom jupille]

Uwe, I interpreted the original poster as detecting both peaks at the same wavelength, so I focused on that. You're quite right that the same approach works for measurements at different wavelengths, so long as all conditions are identical.

[mikea]

I do indeed want to use just one wavelength to quantitate both peaks, preferably at the API's lambda max.

As long as I correct for this difference in lambda max, this should be adequate, no?

thanks