Selecting/equivalent Ion Pair Agents

[tzimara]

Hi everyone,
Is there a way of approximating an equivalent ion pairing agent? I have a method that uses 20 mM trimethyl ammonium hydroxide but we have none in stock. We have other ion pairing agents that we could possibly use. Is it all trial and error?

Thanks

[Bruce Hamilton]

There probably is better information on the Internet, but as mentioned in another thread, the section on ion-pairing ( 7.4 ) in " Practical HPLC Method Development " by Lloyd R.Snyder, Joseph J. Kirkland, Joseph L. Glajch, second edition 1997 ISBN 0-471-00703-X , is a good place to start.

It discusses substituting different IP reagents. Besides, the book is filled with plenty of other good information.

Bruce Hamilton

[tom jupille]

tzimara, to a first approximation, the key parameter in ion-pair chromatography is the concentration of charged groups on the stationary phase surface. This will depend on the hydrophobicity of the IP reagent (more hydrophobic = more strongly retained = higher charge concentration) and on its concentration in the mobile phase (higher concentration in the mobile phase = higher concentration on the stationary phase).

If you substitute a more hydrophobic IP reagent, you should decrease the concentration, and vice versa. Exactly how much is a matter of successive approximation (that sounds more scientific than "trial and error" ). At first, you should probably change concentration in geometric progression (half or double).

Because your original method uses a tertiary amine instead of the more commonly used quat, you should also be careful about pH (you may have to tweak the pH adjustment).

If the method works well as is, you might be better off just buying some of the trimethylamine.

[syx]

where could we get information about the pKa value of tertiary amines? should we control pH of mobile phase based on this pKa value beside the pKa value of analytes?