Chromatography Forum

Hi,
I am working on ion apir chromato graphy. During this working I have on question that why pH selection is just opposite to the analyte.
Example: Oxalic acid: 10mM Tetrautyl ammonium hydrogen sulphate+ 20mM K2HPO4 , the pH is near about 6.85.
When using hexane sulphonic acid na salt the pH is set near to 2.5
Why this oppositr pH is needed and what is the basic logic for this

Praveen [/list]

The short answer is that your analyte needs to be charged in order to form an ion pair with the ion pairing (IP) agent, which will carry an opposite charge at the pH your mobile phase is buffered to.

It's a very useful technique, but one that has a few more variables than standard RP-HPLC. I'd advise doing some reading before diving in; Snyder's Practical HPLC Method Development has a good explanation of the technique and variables to be considered. The type and concentration of IP agent and buffer used matter, of course. Column equilibration takes significantly longer, column temperature needs to be tightly controlled, and so forth.

There are a few volatile IP agents, which I have found to be a bit more forgiving than "traditional" IP agents such as sulfonates.

Good Luck!

Juddc is correct. In your first example, the ion pair reagent (tetrabutylammonium ion) is opposite in charge to the analyte, (oxalic acid), so you want a pH which ensures full ionization of the analyte.

Your second example, however (using hexane sulfonate) is *not* ion pair, because both the "ion pair reagent" and the analyte have the same charge. The most likely mechanism is ion exclusion: the hexane sulfonate sticks negative charge on the stationary phase which "repels" the negatively charged oxalic acid (I'm over simplifying, but that's the general idea). In that case, you want the pH to be nearer to the pKas of oxalic acid (1.3 and 4.3) so that you can control the degree of exclusion by controlling the charge on the analyte.

Below is oxalic acid retained on Unison UK-C18:
http://www.imtakt.com/TecInfo/TI257E.pdf