Role of sodium sulphate as additive in mobile phase

[Ricardinio]

Sodium sulphate may be added as additive to a mobile phase. I would like to understand how it works.
Thank u before hand for any comment

[Chris Pohl]

Sodium sulfate is highly hydrated and ties up a lot of water molecules. This lowers the solubility of other molecules less able to compete for with sodium sulfate for water molecules. The result is increased retention when employed in conjunction with a hydrophobic surface (I assume your question is in regards to reversed phase or HIC retention modes). Of course, sodium sulfate has entirely different effects in ion exchange chromatography

[Bill Tindall]

To put what Chris said in PChem terms, at high concentration it can have a big effect on the activity coefficient of neutral species (salting out), as well as ionic species.

In a low dielectric solvent it is a fairly strong base, for example it will neutralize strong acids.

[HW Mueller]

The properties mentioned by Chris and Bill also have led to apply the term lyotropic to Na2SO4. The opposite is chaotropic, substances used to solvate, for instance, proteins better (as well as lower retention, or "salting in"). However, it should not be forgotten that at low concentration (~0.1 M) Na2SO4 is rather chaotropic, you need considerably more to get "salting out" or the lyotropic effect.

[Ricardinio]

Thanks Chris, Bill and HW.
It is well RP chromatography.
Just before reading the comment of HW, I wanted to share my experience, where I oberved an increase of analysis time from about 60 min to 300 min, while attempting to adapt a mobile phase where Na2SO4 (60 g/L of mobile phase) was used, by lowering this concentration down to 30 g/L.
My question now is how to explain the salting in effect of Na2So4 at low concentrations?

[HW Mueller]

I am not sure that your results are understood... lowering the sulfate increased RT? What else is in your mobile phase, what type of compounds do you have? Does the xg/L refer to the aqueous part only?
The salting in effect is explained as that for any chaotropic salt: Chaos of the liquid water is increased, surface tension is reduced, etc.

[Ricardinio]

HW,
The mobile phase initialy comprises Na2SO4, Sodium octane sulfonate, Tetrahydrofuran, Water and is used for the analysis of aminoglycosides. x g/L Na2SO4 is the overall conc in the mobile phase, not the conc of the aqueous part only. In the experiements I mentioned, only the amount of Na2SO4 is changed. In both cases, the necessary amount of this salt is weighed, dissoleved in water, then other components of mobile phase added.
The reason I asked about the chaotropic effect is because, the salting out effect due to the hydratation capability of Na2SO4 seems clear and logical for me, and therefore, I can't explain the chaotropic effect through the same mechanism. There must be another mechanism that I would like to understand

[Chris Pohl]

Richardinio,

The effect of sulfate you are seeing is due to the effect of sodium on retention. It has nothing to do with the salting out or "chaotropic" effect. The more the sodium the less the retention of a cationic solute in this mode of ion pair.

[HW Mueller]

Having the correct info sure helps....I suspected that this huge RT effect was not due to ionic strength (chaotropy, etc.) unless maybe proteins were involved, thus my questions.
Ricardino, maybe you can think of this as a restructuring of liquid water: The structure of pure water is disrupted by low concentrations of salts, at higher concentrations some salts impart a much more restricted (Na2SO4) structure on H2O, while others (LiSCN) don´t. I am keeping this on the "picturesque" side as I havn´t seen any free energy, surface tension.... diagrams on this, only protein, etc., dissolution charts (not theoretical, but rather practical discussions).

[Ricardinio]

Richardinio,

The effect of sulfate you are seeing is due to the effect of sodium on retention. It has nothing to do with the salting out or "chaotropic" effect. The more the sodium the less the retention of a cationic solute in this mode of ion pair.

Does this mean that any sodium salt will have similar effect?
On the other hand, what could have been the intention, in the initial method, of increasing retention (using octane sulfonate) and at the same time decreasing retention (using sodium)?

[Chris Pohl]

Richardinio,

To a first approximation, the answer to your question is yes any salt will have a similar effect (although to the extent that the aforementioned "chaotropic effect" is relevant, the anion may have some effect on the separation). And as to why the method was developed in the manner you describe, I can only guess. I know that without the ion pair reagent retention of aminoglycosides on reverse phase columns is inadequate. Addition of sufficient ion pair reagent concentration to allow the addition of an electrolyte (which would moderate retention) would be expected to make the method more robust in terms of sample ionic strength tolerance. On the other hand, if the method you are using is intended for electrochemical detection then the real answer is that this method just plain wasn't very well-designed. If you're free to do so, I would replace the THF with acetonitrile. It will work equally well although you'll need to use a bit more. THF isn't really very compatible with electrochemical detection and shouldn't have been included in the method.

[Ricardinio]

Richardinio,

On the other hand, if the method you are using is intended for electrochemical detection then the real answer is that this method just plain wasn't very well-designed. If you're free to do so, I would replace the THF with acetonitrile. It will work equally well although you'll need to use a bit more. THF isn't really very compatible with electrochemical detection and shouldn't have been included in the method.

Yes, the method is well intended for electrochemical detection.
Suprisingly, I was instructed that THF would be the best organic modifier, since acetonitrile would result in a film on the working electrode surface and methanol would react on the electrode surface.

[Chris Pohl]

Richardinio,

I'm not sure what electrode material and instrument you are using but in our hands we get much better performance with acetonitrile. The problem with THF is that the stabilized version contains at an electroactive stabilizer (BHT) while the unstabilized version is usually contaminated with electroactive peroxides. Both cause high backgrounds, wandering baselines and poor sensitivity.

[AA]

Chris

Having worked with the above mentioned assay many times is the past, I can assure you that methanol, acetonitrile, and un-stabilized THF give much poorer results than stabilized THF. At least they did when I did the experiments. Sometimes theory and reality are a little different.

AA

[Chris Pohl]

AA,

I wasn't refering to theory but rather practice. I can assure you that acetonitrile works much better than THF with the right waveform. If you would like the conditions I can post them. FYI: one researcher in Germany has demonstrated the freshly distilled THF is much better than stabilized THF but I'm not sure if most people would be up to that. There will be a couple of papers at Pittcon detailing improved methods without THF.