how drug's pKa help in RP-HPLC method development

[Uwe Neue]

You want to stay 2 pH untits away from the pKa of your analyte to get the most reproducible chromatography. Trouble is that pH and pKa shift each by as much as 2 units when you add organic solvent.

The best approach is to make a buffer (in water) that has the highest buffering capacity and does the intended separation. The rest is simply method control.

With respect to the development of a method, the knowledge of the pKa of the analyte(s) is only of secondar importance.

[HW Mueller]

Danko,
please let the original poster decide whether it is important for him to know that at pH above ~3 it could be possible to get unwanted ion exchange, or, what I didn´t mention yet: unwanted ion exclusion if he has appreciable SiO- and negatively charged analyte.
Furthermore, it should be known that one does not get broad peaks in this pH context, because one separates acid from its conjugate base, but because one didn´t keep the equilibrium constant.

[danko]

Hi Hans,

What’s the difference?

but because one didn´t keep the equilibrium constant

That’s exactly what you needed to explain in a more detailed manner.

Best Regards

[parveenqa]

thank you