Ion Pair Reagent and Its Salts

[syx]

What will happen if we use ion-pair reagent in salt form instead of its base/acid?
For instance, we have found a method using tetrabutylammonium hydroxide solution:
Dilute 25 mL of 0.4 M tetrabutylammonium hydroxide solution with water to obtain 1000 mL of solution, and adjust with 1.5 M phosphoric acid to a pH of 6.5.
Could we substitute it with 0.01 M tetrabutylammonium hydrogen sulphate solution (adjusted as the same way)?

Best regards,
Siswanto Tanuatmojo

[Noser222]

Yes, as long as you end up at the same pH and ionic strength.

[Mark Tracy]

In this particular case, the hydrogensulfate salt will give an initial pH <6.5, so you won't be able to use phosphoric acid to do the adjustment.

[syx]

how we decide when to use the acid/base or the salt form of ion pair?

[Mark Tracy]

The salts of quaternary amines are more stable, have less impurities, and are easier and safer to handle than the hydroxides.

[syx]

Are there any guidances about how to choose salts of ion pair reagents based on the range of pH of mobile phase as in buffer selection?

[Kostas Petritis]

No there are not such guidances. However, there are software tools out there that they will help you decide on the exact amounts that you have to add from your acid/base/salt in order to achieve the desired pH (i.e. Phoebus is one of them). The effects of the counter ion(s) in the ion pairing chromatography can be complex...