Chromatography Forum

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

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

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.

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

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

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?

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...