Methods for simultaneous analysis of PQ and its metabolites using the UHPLC-MS were reported earlier . This involved the use of a BEH Shield RP18 column (100 mm × 2.1 mm ID, 1.7 mm) equipped with an LC-18 guard column (Vanguard 2.1 × 5 mm, Waters Corp, Milford, MA, USA) on an ACQUITY UHPLC™ system (Waters) to which a conditioned auto-sampler (at 20 °C) was attached. Metabolites earlier identified, based on retention time, high resolution mass and MS–MS fragmentation pattern, were individually searched for and profiled against time in the presence and absence of CQ/QN.