I am new to HILIC method development, and have not been able to find a clear answer on how to prepare the necessary mobile phases.

For example:

Mobile phase: 10 mM ammonium formate in ACN/H2O (95:5), pH 3

If for example I need 1 liter, do I first dissolve enough ammonium formate in 50 mL water so that when I add 950 mL ACN it comes to be 10 mM? Or do I add enough so that the 50 mL water has 10 mM ammonium formate (which would then be diluted considerably once I add the 950 mL ACN)?

And do I adjust the pH in the water first once the ammonium formate has been dissolved, and then add the ACN? (that would make most sense since a pH meter doesn't work reliably when organic solvent is present).

If the mobile phase uses ammonium formate what acid do I use to adjust the pH? Formic acid? And if I use ammonium acetate should I use acetic acid?

Thanks for your clarifications!