Eluotropic Values

[JBush]

All,
I hate to use this forum for background knowledge, but can anyone give me a quick description of what Eluotropic Values mean for a solvent on a particular type of column (C18). The question arises because I am trying to figure out which property of my mobile phases is providing the selectivity necessary to separate two positional isomers. The solvents are 0.025% phosphoric acid in water for A and 30:70 THF:MeOH for B. I have tried increasing the polarity of the system with HFBA and this does not seem to help.
Thanks

[Uwe Neue]

I believe that you are asking about isoeluotropic conditions. This means water-organic mixtures that give you about the same retention time for different organic solvents. These conditions are used to maximize solvent selectivity effects, but keeping a similar retention time.

Here are two rules (from my book on page 309)
MeCN = 0.32 MeOH^2 + 0.57 MeOH
THF = 0.66 * MeOH

MeCN = 0.9 MeOH^2 - 0.21 MeOH + 0.126
THF = 0.66 MeOH - 0.15

All values to be used in volume fractions (i.e. the values are smaller than 1).

The two sets of rules give you different results. First of all, they are rules, second, they depend on the column that you are using. However, they are still useful.

(PS: if the formula would work perfectly, there would be no solvent selectivity...)

[Uwe Neue]

Just another comment: in your system, I would use the isoleuotropic elution rules to figure out, which composition in methanol and THF give me about the same retention rune them, and see how the selectivity is changing. You can do the same with acetonitrile to see if the results are different.

The only issue is that most of the time, selectivity is not a straightforward additive property of solvent mixtures.

[jitender]

Separation is not always dependent on mobile phase polarity only. Separation in most cases is interplay of various thermodynamic properties of analyte, column and mobile phase.
If mobile phase polarity is only determining factor, why would one need a range of solvents. Simply because only two solvents (in appropriate ratio) should be sufficient to give some desired polarity value.

Polarity largely governs the reverse-phase separation, but not absolutely.

Rules are rules, useful sometime.