Chaotropic salts, how do they work?

[gonzjr]

My first post! Great forum guys!

When you use a chaotropic salt like NaCLO4 or KPF6 in a mobile phase like 0.2% Phosphoric Acid and 50mM NaCLO4 in a C18 column to retain an acidic ionic substance? Is the mechanism of retention similar to Ion Pair Chromatography or is still Reversed Phase Chromatography?

I dont know how chatropic salts work? Any help?

[michaldousa]

The salts - NaClO4 or KPF6 - are used as not very specific ion-pairs. I would not use above-mentioned salts. I would try the other ion-pairs - trifluoracetic acid or heptafluorbutyric acid (excellent!).

[gonzjr]

I am not trying to use them as an ion-pair additive, I do get better retention when using the combination 0.2% H3PO4-100 mM NaCLO4 than just using the phosphoric alone. I am still wondering why though?

[Rafael Chust]

How does your mobile phase pH changes when you add your NaClO4?

Maybe there is the answer...

[gonzjr]

How does your mobile phase pH changes when you add your NaClO4?

Maybe there is the answer...

The pH increases when I add the salt, so by RPC if I increase the pH of an acidic ion the retention will be less because the form is now ionized. I see the opossite effect, the acidic anion is more retained and my peak shape improved. So I think that it does have an ion pairing effect but am not sure of the exact mechanism it employs.

[Rafael Chust]

Have you played with different % of H3PO4?

You can also check some posts in the forum concerning pH and ionic strenght.

The mechanisms might be influenced also by column manufacturing processes (carbon load, endcapping, etc.)

[HW Mueller]

Chaotropy has been discussed many times, especially the use of ClO4-.
Generally, chaotropy refers to substances which lower the organization (structure) of water, they have a salting in effect, thus they also have a tendency to have rather low (relatively) hydrophilicity.