A Novel Ultra performance liquid chromatographic method with QDa detection technique was developed for trace level quantification of potential genotoxic impurity “3-Chloro-N-(2-((2-dimethylamino) ethyl) (methyl) amino)-4-methoxy-5-((4-(1-methyl-1H-indol-3-yl) pyrimidin-2-yl) amino) phenyl) propanamide” in Osimertinib mesylate an Antineoplastic agent. The chromatographic separation was achieved on Kinetics 2.6 μm Polar C18 100Å (100 mm x 4.6 mm) column at a flow rate of 0.3 mL min-1 and column temperature 40°C. The mobile phase consisted of 10mM Ammonium acetate at pH 5.5 and acetonitrile under programmed gradient conditions. Mass spectroscopic quantification was carried out in positive ion mode at probe temperature 600°C. The method was validated according to ICH guidelines and achieved LOD and LOQ values as 0.3 ppm and 1.0 ppm respectively. The proposed method was found to be linear (r2; ≥0.99), accurate (% Recovery ≥ 99%) and can be used for routine testing at quality control.