By mc on Friday, July 23, 2004 - 01:22 am:

Hi everyone

I have a formulation that has two actives in it. One of the actives has a max absorbance at 245nm and the other at 222nm. When analysing this sample which wavelength should I use? At the moment I'm using a wavelength of 220nm for both actives. A colleague told me that you should always work at the wavelength max. I see no reason why I can't use 220nm because this method will be validated at 220nm. Should I use the "isosbetic point" instead. Many methods in the USP are not run at the wavelength max.

Thanks in at advance for you imput

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By GJD on Friday, July 23, 2004 - 03:30 am:

To MC:
Can you not change wavelength in between the peaks? Most detectors allow you to.

GJD

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By Mark on Friday, July 23, 2004 - 05:08 am:

To MC,
There is nothing wrong with working "off-max" on an assay as long as you can validate its ruggedness and there is enough sensitivity for your needs. We do it successfully all the time. Wavelength switching is OK also but there are still plenty of older detectors out there (especially in control labs) that are not capable of this.

Regards,
Mark

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By MC on Sunday, July 25, 2004 - 10:49 pm:

Hi

The sensitivity for both peaks are ± 0.2 AUFS,therefore I don't foresee any problems when I validate the method. If the method fails ruggedness I will either consider wavelength switching or using a detector that is able to extract more than one wavelength (eg waters 2487 or 2996 detector).
Thanks for your advice.
MC

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By popeye on Thursday, August 5, 2004 - 04:31 am:

Take the analysis on a PDA, determine the wavelength which gives optimum response for both the peaks, choose that wavelength and validate the method.