Is there a way of determining the equivalence of one organic modifier for another (i.e. If a buffer has 20% methanol in it, how much isopropanol would need to be added to a new buffer solution in order for it to have the same elution strength as the original buffer). Is there an equation for solving this type of problem?

Yes, there are rules. On page 309 in my book. Unfortunately only for methanol, acetonitrile, and THF, but not for IPA...

Yes, there is an equation

P= (Vfa x Pa) + (Vfb x Pb)


P (polarity of mobile phase) = (volume fraction of component a x polarity of a) + (volume fraction of b x polarity of b)

So, in your example above your MOBILE PHASE contains 20% methanol in water

P = (0.2 x 6.6) + (0.8 x 9.0)

polarity 20% MeOH in water
0.2 is volume fraction of methanol
6.6 polarity of methanol
0.8 is volume fraction of water
9.0 is polarity of water

this only works (and, in any event, only a guesstimate as polarity as given in eluotropic series would have been determined empirically) if the polarities are determined from one eluotropic table. Different tables may give different values.

It is possible to calculate the amount of another (usually organic modifier) if one remembers that the volume fractions must add up to one

therefore, for example 50% methanol would have nearly the same polarity as 43% acetonitile, and 30% THF (in water)

A plea, please use the term mobile phase instead of buffer, buffer is a chemical term for something quite specific, not a general term for anything liquid!

Cheers
Bryan

Here's another standard reference:
Introduction to Modern Liquid Chromatography. Wiley, New York, 1979. L.R. Snyder and J.J. Kirkland.

Similar tables available in our catalog in the FLASH tutorial section. Solvent strength is consistent for most normal phase silica, HPLC or FLASH.

www.Biotage.com

Happy to answer questions,
Tobi
TWilliams@Biotage.com