It is a good idea to use ammonium acetate or ammonium formate as additives in HPLC, however keep in mind that neither are soluble in neat acetonitrile.

Therefore, you will need to add some water to this mobile phase in order to allow for dissolution. 10% aqueous is usually a good choice or starting point, although if you need your compounds to retain longer at the start of an ANP gradient, you may want to use as low a concentration as 5% with a buffer concentration not to exceed 10mM.

Note: Also be sure to always vacuum filter any mobile phase containing dissolved solids in case they are not completely dissolved.