Use of Sodium Chloride in mobile phase

[AdrianF]

I have come across a mobile phase which uses 0.47% NaCl, 0.16% sodium acetate, 0.2% acetic acid as the buffer.
column is phenyl
mobile phase 50% MeOH/Buffer
Compound bendroflumethiazide

Why the Sodium Chloride?

[tom jupille]

One possibility is that the method was adapted from a previous method in which the NaCl did serve a purpose.

[Bruce Hamilton]

I think that's an old USP method for analysis in formulations, and also for the impurity 2,4-disulfamyl-5-trifluoromethylaniline in the API.

The salt may have some effect on retention, resolution of the two compounds, or solution stability. Why not try it with and without the salt and report back?

Please keep having fun,

Bruce Hamilton

[AdrianF]

I have checked the USP NaCl is still there. I would like to have a logical explanation as to why NaCl should be incoporated into a mobile phase. Is it to change ionic strength and so lessen interactions with silanol groups?

[Slammy1]

Hydrophobic Interaction Chromatography, sometimes it's all you got when NP and RP fail. Could be conformational (several degrees of freedom in the molecule so a very polar aqueous phase may stabilize a single conformation), or even provide stabilization of the drug substance. From some brief reading it seems the action of the API involves adsortion of NaCl, so it seems a simple step to ensure a single conformation of analyzed drug.

[HW Mueller]

HIC is sort of a salting out. As such it would go under the phenomena associated with ionic strength.
So, Slammy1, I have a bit of a conceptual difficulty connecting the favorization of a polar rotamer with salting out. Would appreciate an elaboration.

[AdrianF]

When I asked the question I was hoping someone would have a clear reason with a reference to back it up.

[tom jupille]

All joking aside, it may be there to "swamp out" interactions with ionized silanols on the stationary phase. I'm too lazy to calculate the pH of that sodium acetate/acetic acid mix, but it's probably somewhere in the low to mid 4 range, and many older silica-based columns have silanols with pKa values that low.

That said, I still think there's a good chance that it's simply the vestigial remains of an ancestral method.

[chromatographer1]

This NaCl method does remind me of reversed phase TLC mobile phase. I wonder if it was simply transfered (successfully) to HPLC and never developed further.

Rod

[Slammy1]

HIC is sort of a salting out. As such it would go under the phenomena associated with ionic strength.
So, Slammy1, I have a bit of a conceptual difficulty connecting the favorization of a polar rotamer with salting out. Would appreciate an elaboration.

A better conceptualization would be the addition of salt to a sep funnel to improve phase separation by increasing the polarity of the aqueous phase. I wasn't mentioning HIC as a reason, since no salt gradient was described, just a description of the role salts can play in separation. The action of the API, from my very limited google search, is to prevent chloride readsorption into cells so I would think it's related to that rather than a simple polarity/conformational question (though I have seen where that's exactly why some additives are put into mobile phases). Just dealing with a peak shape issue caused by different forms is my guess, short of further research (such as looking at what happens when the salt is removed from the mobile phase). Probably the easiest and most accurate way to answer the question.