Phosphoric acid, monodecyl ester, monoammonium salt can be analyzed by this reverse phase (RP) HPLC method with simple conditions. The mobile phase contains an acetonitrile (MeCN), water, and phosphoric acid. For Mass-Spec (MS) compatible applications the phosphoric acid needs to be replaced with formic acid. Smaller 3 µm particles columns available for fast UPLC applications. This liquid chromatography method is scalable and can be used for isolation impurities in preparative separation. It also suitable for pharmacokinetics.