March 04, 2021, 02:14:46 AM
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Topic: Possible to keep a certain pH in mixed mobile phase?  (Read 190 times)

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Offline daaniiell14

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Possible to keep a certain pH in mixed mobile phase?
« on: February 09, 2021, 11:00:26 AM »
I'm quite new to HPLC so this might be a dumb question.

I'm working with separating very polar compounds, and pH is important. I want to test certain pH values that are away from analyte pKa values as this could give better method robustness however there is one big problem. I have thought about it for days and googled every way I could but i still don't understand anything about checking pH in any type of LC. They only say "prepared an aqueous phase with x,y ect.." but never anything about the final solution and its pH. Furthermore it seems i cant use a pH electrode in anything that has substantial organic phase, which my HILIC separation definitely will have.

I'm using 10mM ammonium formate in my aqueous phase and i will use formic acid to adjust its pH down to 2.7. But what do i do with my MeOH or Acetonitrile phases to make sure that pH of mixed solution stays constant even as i change aq/org ratio? Do i simply put in the same concentrations ammonium formate and formic acid? Wont pH change as the ratio of aq/org changes? In other words, if i want to make sure my analytes elute out with a mobile phase of a certain pH, how do i know what to do with my organic phase when i cant even measure the pH of it?

Offline MOTOBALL

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Re: Possible to keep a certain pH in mixed mobile phase?
« Reply #1 on: February 17, 2021, 07:37:13 PM »
There was an article addressing the issue of measuring pH values in mixed mode solvents sometime since 2020, in LC Magazine or similar.

I think that you should search the literature again.

Good Luck!

edit: I could not find the article..

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