The reference standards of dextromethorphan and impurity C were provided by Unither Pharmaceuticals. Impurity A was acquired from Council of Europe (Strasbourg, France).Impurity B and formic acid (99%) were purchased from Sigma–Aldrich (St.Louis, MO, USA). Ammonium formate was obtained from Acros Organics (Geel, Belgium). Ethanol 96% HPLC grade was bought from Atlantic laboICS (Bruges, France).