ANSWER-
formic acid and phosphoric acid have two different contexts.
density of phosphoric acid is 1.88 where formic acid is 1.23.
pka value for formic acid is 3.75 where as for phosphoric acid is 2.12,7.2 and 12.3. so that you can use any of the pH for phosphoric acid, when adjusting to higher side.
But for formic acid its not possible.
Generally as above said formic acid is the best choice for MS/MS.
formic acid and phosphoric acid will behave differently in aqueous solutions and on stationary phase and with your compound. In case if your compound behaves same in both the acids, then its better to choose formic acid because of its volatility, if you don't have any peak shape issues
My experience says some compounds will give very good peak shape in phosphoric acid rather than formic acid because of its buffer strength.
If you have to run HPLC and LC-MSMS Simultaneously then formic acid is the best choice
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