Ion pairing

[shadow]

Can ion pair reagents like OSA be used at pH 5-6? Thanks

[tom jupille]

Yes.

[shadow]

Thanks Tom. What would be the max concentration recommended for UPLC systems?

[Gerhard Kratz]

I would say 0,1%, but please keep in mind that with ion pairing agents you do a modification of the stationary phase, irreversible.

[tom jupille]

It's complicated! I wouldn't go much above 0.1%, but it depends on the % organic in the mobile phase as well. You certainly want to make sure you stay below the critical micelle concentration (CMC).

In our Advanced Method Development course (shameless plug: http://lcresources.com/training/tramd.html), we describe a systematic approach that basically looks like this:
Assuming a mixture of neutral and ionized analytes, first ignore the ionics and adjust the organic to give reasonably high retention of the neutrals (k' values between about 5 and 15 if possible). The ions will probably be unretained under these conditions.
Then do a series of runs with increasing ion pair concentration, going up in geometric progression (e.g., 1mM, 2 mM, 4 mM, 8mM, . . . or 1mM, 3mM, 10mM, . . . ) to "walk" the ions into the same retention range.

[edit addition] If you have a copy of the Snyder/Kirkland/Dolan "Introduction to Modern Liquid Chromatography" book, there's a table of recommended maximum concentration of different ion-pair reagents as a function of % organic on page 341.

[shadow]

Thanks to all for the answers. I have been running at 50% ACN or 65% MeOH isocratic and would like to transition to the same mobile phase containing ion-pair reagent, could the organic content be a problem? Thank you.

[Consumer Products Guy]

Thanks to all for the answers. I have been running at 50% ACN or 65% MeOH isocratic and would like to transition to the same mobile phase containing ion-pair reagent, could the organic content be a problem? Thank you.

Make up at that concentration, see if anything precipitates.