Chromatography Forum

I want to ask if anyone has had success using mobile phases with salt as the additive instead of acids or buffers.

For a current application there is a reason that I want to avoid anything with acid/base characteristics. I know that cations like K+ can be an alternative way to mediate silanol interactions. I have read publications on this type of thing over the years. However, I cannot currently get my hands on any actual examples.

So I am wondering: has anyone tried anything like this. For example, using of 20 mM KCl as the mobile phase additive (just as one might use 20 mM ammonium acetate).

(by the way: for this application we do not need a volatile mobile phase and we do not need very high organic - so no concern with salt solubility)

Thanks very much.

It's actually quite common when doing GFC of proteins (usually with NaCl). The general idea is that there is enough loose charge floating around to swamp out any residual charge from surface silanols. Typical concentrations are > 100 mM .

So you're buffering the ionic strength rather than the pH?

Johnny: We are increasing the ionic strength to swamp out charges from the silanols.

Tom: Why do you suppose they go to such high concentrations. After all when we use something like ammonium acetate it is typically used at around 25 mM (at least for standard reversed phase applications).

Thanks!
Mark

"Ya gotta do what ya gotta do".

The ionic strength is adjusted empirically to give the best recovery and peak shape. Here's an example taken from an old Zorbax applications note:

I apologize for the lack of timeliness of my two cents' worth on this topic:

Another option: the so-called "chaotropic salts," such as perchlorate, trifluoroacetate, hexafluorophosphate (PF6-) or tetrafluoroborate (BF4-). These chaotropic salts work best in eluent compositions containing higher amounts of water in reversed-phase separations under acidic pH to help in the retention selectivity of protonated bases.

Here is a link to a nice article in J. Chrom. A written by some of the first investigators of the chaotropic effect:

http://www.sciencedirect.com/science/ar ... 7304021867

Reversed-phase high-performance liquid chromatography behavior of chaotropic counteranions
Y.V. Kazakevich, R. LoBrutto, R. Vivilecchia, Journal of Chromatography A, Volume 1064, Issue 1, 28 January 2005, Pages 9–18.

Abstract

The retention behavior of inorganic liophilic anions in reversed-phase HPLC columns was studied. Usually, the addition of these ions to the mobile phase influences the retention of protonated basic analytes similar to the effect of amphiphilic ions (ion-pairing agents). The nature of this influence is the subject of this paper. HPLC retention of perchlorate (ClO4−), tetrafluoroborate (BF4−), and hexafluorophosphate (PF6−) ions was studied on six columns with different bonded phases including alkyl, phenyl and perfluorophenyl phases. The effect of the mobile phase ionic strength on the retention of liophilic ions was investigated. The influence of the type of organic modifier, acetonitrile and methanol, on the retention of inorganic ions was also studied and interpreted on the basis of adsorption from solutions. Semi-empirical expression is suggested for the description of the retention profile of studied liophilic ions versus the eluent composition. Significant retention of these ions is observed in acetonitrile–water eluents. Multilayer-type adsorption of the acetonitrile on the reversed-phase surface and its strong dispersive (or π–π) interactions with liophilic ions are responsible for significant retention of these ions. This accumulation of liophilic ions in the adsorbed layer on the surface of reversed-phase material introduces an electrostatic component in the retention of protonated basic analytes.