Chromatography Forum

mross,

I didn't say there was no retention of perchlorate, just very little. If you do a comparable experiment with TFA which I would classify as a borderline ion pair reagent you'll see that there is considerably more retention of TFA than perchlorate under the similar eluent conditions. If you then compare that to a real ion pair reagent such as heptanesulfonate, you'll see that neither has very much retention compared to such a reagent. For that reason neither perchlorate nor TFA significantly affects the Zeta potential of the surface compared to a true ion pair reagent such as heptanesulfonate. They are interaction is primarily through lowering the solubility of the analyte in the mobile phase.

Thank you for the replies and explanations.
I think I will try to use TFA to substitute perchlorate as mobile phase component (it is a familiar substance to me than others those recommended by Mr. Pohl). Hope it will give a good result.

It is very difficult to purchase perchlorate in Indonesia. It needs too long and time consuming procedures in Police Department.

I have tried TFA as substitution for perchloric acid. It has good result, the system is suitable.

Thanks for giving us feedback on what worked! This helps us giving better advice in the future...

I'm glad this problem is resolved. However, I am confused by the arguments presented below.


HWM said- It is usually forgotten that near or below an ionic strength of roughly 0.1 just about all salts have properties which one attributes to chaotropy, even the strongly lyotropic salt Na2SO4. So one can use just about any salt at low concentrations to "deorganize" water (increase entropy.....).


Chris Pohl said- You're quite right that any salt that at high enough concentrations is "chaotropic".


HWM is saying that at low concentrations below an ionic strength of 0.1M, all salts are chaotropic. This means that they have a "salting in" effect and should decrease the retention times of the analytes in HPLC. I interpret this as meaning that even things like sodium sulfate, which is an excellent protein precipitant, will decrease retention times, providing the concentration is not too high. In HPLC, I would say that the concentration of additives or buffers is never rarely greater than 0.1M. In this case we could predict that there is rarely a salting out effect in HPLC which increase the retention times. Chris Pohl's statement above seems to entirely contradict this idea-is this a misprint -did you mean that any salt at low enough concentration is chaotropic?

My feelings are on the side of ion pair effects with ClO4- as outlined by MROSS.

I'm glad this problem is resolved. However, I am confused by the arguments presented below.


HWM said- It is usually forgotten that near or below an ionic strength of roughly 0.1 just about all salts have properties which one attributes to chaotropy, even the strongly lyotropic salt Na2SO4. So one can use just about any salt at low concentrations to "deorganize" water (increase entropy.....).


Chris Pohl said- You're quite right that any salt that at high enough concentrations is "chaotropic".


HWM is saying that at low concentrations below an ionic strength of 0.1M, all salts are chaotropic. This means that they have a "salting in" effect and should decrease the retention times of the analytes in HPLC. I interpret this as meaning that even things like sodium sulfate, which is an excellent protein precipitant, will decrease retention times, providing the concentration is not too high. In HPLC, I would say that the concentration of additives or buffers is never rarely greater than 0.1M. In this case we could predict that there is rarely a salting out effect in HPLC which increase the retention times. Chris Pohl's statement above seems to entirely contradict this idea-is this a misprint -did you mean that any salt at low enough concentration is chaotropic?

My feelings are on the side of ion pair effects with ClO4- as outlined by MROSS.

The evidence at my disposal is that at the low Na2SO4 concentrations mentioned, proteins may be solvated better than in, lets say, pure water or especially in high con. Na sulfate. If the pH is wrong than your protein can precipitate regardless of salt concentration, if there are no other reactions, (oxidation, etc.) then a protein will generally feel happier in the buffers you mention than in water.