Primesep D mixed-mode column separates organic acids such as succinic acid, malic acid, MPS and butanesulfonate by a mixture of anion exchange and reversed phase mechanisms. Retention times can be changed by adjusting the percentage of acetonitrile in the mobile phase. This can not be done by traditional ion-exchange and ion-exclusion chromatography. The HPLC separation uses a mobile phase of water, acetonitrile (MeCN, ACN) and ammonium formate as a buffer, making the method MS-compatible. Can also use UV detection at 250 nm.