Solvent Strength for Reverse Phase Separation

[hajdaei]

Hi

I want to ask what may seem like a basic question today. When we do reversed phase analysis, we all know that solvent strength is as follows:

MeOH < ACN < THF

But I am wondering about solvents that are even stronger than THF.

Text books always give a whole range of solvents for Normal Phase, but for Reversed Phase it only seems to be these three. The reason I ask is that we recent did an analysis where we needed a solvent that is even stronger than THF. We tried ethyl acetate but that didn't seem to work.

Is anyone aware of any reference that gives a more complete picture of solvent strength in reversed phase systems.

Thank You

[prepcolumn]

Hi

I want to ask what may seem like a basic question today. When we do reversed phase analysis, we all know that solvent strength is as follows:

MeOH < ACN < THF

But I am wondering about solvents that are even stronger than THF.

Text books always give a whole range of solvents for Normal Phase, but for Reversed Phase it only seems to be these three. The reason I ask is that we recent did an analysis where we needed a solvent that is even stronger than THF. We tried ethyl acetate but that didn't seem to work.

Is anyone aware of any reference that gives a more complete picture of solvent strength in reversed phase systems.

Thank You

The limiting point of solvent selection in reversed phase is miscibility with water. Ethyl acetate does not work because it does not mix with water. Stronger solvent for reversed phase means more nonpolar solvent and when you go through non-polarity, you with face with miscibility problems..

[HPLCaddict]

Prepcolumn already pointed out the miscibility problem. In theory, you might go to NARP (non-aqueous reversed-phase) using only organics to elute your compounds. In practice, if your compounds are retained that much that they won't elute with THF, I'd rather change the stationary phase than trying to use more powerful eluents. A simple change of the column from C18 to C8, C4 or even down to C1 might work wonders. If your compounds are ionizable, a change of eluent pH might do the trick.

[shfo]

The limiting point of solvent selection in reversed phase is miscibility with water. Ethyl acetate does not work because it does not mix with water. Stronger solvent for reversed phase means more nonpolar solvent and when you go through non-polarity, you with face with miscibility problems..

Ethyl acetate is miscible with all three of the common organic RP solvents though, so wouldn't a small percentage of it in those make the solvent more nonpolar yet it would still be miscible with water because of the high amount of other solvent? I feel like I've seen methods that do this, but perhaps they weren't well thought through?

I agree with the sentiment that you would probably want to change column phases before you go too far down this path though.

[rrm1987]

if you use stronger like chloroform,etc the chances of stationary phase i.e.,stationary functional groups may get disturbed or solubilised.so its better to look for normal phase chromatography columns.

[Vlad Orlovsky]

EA can be miscible with water/ACN and water/THF solutions, problem is that you will have very limited choice of buffer concentrations, since your organic solvents become separated when you start adding buffer. We usually use THF to increase strength of ACN/water mixture. You can also try propanol but keep in mind that UV cut-off will go up with THF and PA.

[rrm1987]

on adding EA to mixture(ACN/water),water will seperate or not?

EA can be miscible with water/ACN and water/THF solutions, problem is that you will have very limited choice of buffer concentrations, since your organic solvents become separated when you start adding buffer. We usually use THF to increase strength of ACN/water mixture. You can also try propanol but keep in mind that UV cut-off will go up with THF and PA.