Chromatography Forum

Can any body tell me why do use methanol and acetonitrile together in mobile phase.
Since both have almost same polarity index 6.60 & 6.20

Thanks
Raj B

That difference of 0.4 is actually significant enough where the combination of the two solvents is needed to resolve certain peaks. I know I've come across more than a few methods that have such a mobile phase composition.

MeOH and MeCN differ in their solvation properties. Mixtures of them are a powerfull tool to separate very closely related neutral substances, such as steroid hormones. We even had a method that was such sensitive to MeOH / MeCN ratios that solvents had to be weighted bevor mixing.
Recently I had to separate two double bond isomers from the main product. Using pure (i.e. MeCN or MeOH) solvent/water mixtures, separation was impossible on 10 different columns (at one or two configurations you would see some separation - still far away from baseline) , using solvent mixtures with three different columns separations in fairly short times were possible.

Also, acetonitrile is a hydrogen bond acceptor only, whereas methanol in both a donor and acceptor.

ACN, MeOH, and THF are considered to be the three solvents which are most different from one another in their selectivity while maintaining miscibility with water. When intermediate selectivity is required, the solvents can be blended.

No one likes to work with THF (stinks, unstable, high UV cutoff), so ACN and MeOH are the first two sovent choices. If the selectivity is similar in both, then there is little point to blending, and a different way of changing selectivity is explored (column type, temperature, pH, etc.). If the selectivity is different, then the blending ratio can be fine-tuned.