Folic acid is (Folate-) + (H+)

or

Folate- is Folic acid - (H+)

Folate or Folic depend on the pH of your solution. Bellow the pKa (8.26) you have the acid form, above this pH, the folate form is predominant. So, is better water solubilized in basic solutions. In the case of acid form you can use an organic solvent for MS analysis.

Thank you so much Antonio and Facundo.