Chromatography Forum

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?

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!).

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?

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

Maybe there is the answer...

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.

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.)

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.