Ion Pairing

[JA]

Chris, Kostas,
Thanks for your replies. Just going over some of the articles referenced.

[HW Mueller]

In a few minutes I will try to confirm yesterdays experiments with a Waters Atalantis HILIC which separates four amino acids that need to be separated: cis and trans-F-proline and cis and trans-hydroxyproline (attempts with Aqu. or polar embedded columns failed). However, due to some restrictions the injected concentrations are very high which possibly causes peak shapes that are somewhat strange. Also, this "HILIC" application is much more prone to equilibrium disturbances than RP. Thus, it would be very instructive to see further opinions ("educated") on HILIC, polar embedded, etc. versus ion pair, especially in regard to such disturbances.

[JA]

Chris,
Can you please elaborate on the conditions required to form an ion pair in regards to a reversed phase system and the statement that ammonium ions cannot do so.

[Chris Pohl]

JA,

Regarding conditions necessary to form ion pairs, while this is subject some debate (see for example this thread in title: "Doubts about Ion Pair Chromatography" http://www.sepsci.com/chromforum/viewto ... on+pair%2A ), articles cited in that thread indicate that the mobile phase solvent composition must have a dielectric constant less than 44 to support formation of ion pairs in the case of tetrabutylammonium nitrate. This corresponds to a mobile phase composition containing a minimum of 76% methanol or 86% acetonitrile. The exact dialectic constant required is dependent upon both anion and cation so this would vary depending upon your analyte but suffice it to say in general you would need at least 70% organic solvent and probably more than that to have even minimal evidence of ion pairs in solution.

Ammonium ion might be capable of forming ion pairs in systems with significantly lower dielectric constant but because ammonium ion isn't very disruptive of the water structure around it, its ability to form ion pairs in solvent systems containing water is virtually nonexistent. If you're interested in more background on the physical chemistry of ion pairs, I suggest you check out the references I cited in that thread from the thirties.

The gist of the more generally accepted explanation for retention in so-called "ion pair" chromatography is that adsorption of the ion pair reagent onto the stationary phase surface leads to the creation of an electrical double layer at the solid-liquid interface. Retention of counterions occurs through electrostatic interaction with the surface, the counterions occupying the electrical double layer immediately above the surface.