Objectives: In the current study, an analytical technique has been optimized for the analysis of disinfection by-products ( perchlorate (ClO 4 À ), bromate (BrO 3 À ), nitrate (NO 3 À ), nitrite and sulfite (SO 3 2-) in drinking water obtained from spring, well and tap water (desalinated) sources. Methods: The ultra-performance liquid chromatography-electrospray ionization mass spectrometry (UPLC-ESI/MS) conditions have been optimized for oxyhalides ClO 4 À (isotopes, 37 ClO 4 À and 35 ClO 4 À

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... and BrO 3 À (isotopes, 81 BrO 3 À and 79 BrO 3 À ), and inorganic anions NO 3 À , NO 2 À and SO 3 2- . Separation was achieved by BEH C 18 column with methanol (75%) and water (24.99%, HCOOH 0.01%) mobile phase at flow rate 0.2 mL/min. Results: The analysis was carried out in <1 min with excellent coefficient of determination, (R 2 > 0.995-0. 999), limit of detection (0.016-0.043 mg/L), limit of quantification (0.051-0.104 mg/L) and precision (repeatability, 1.54-2.28% and reproducibility, 3.13-4.08%) in terms of RSD%. The method was successfully applied, and the amounts of 35 ClO 4 À (5.33-22.64 mg/L), 81 BrO 3 À (7.52-16.73 mg/L), NO 3 À (5.42-12.14 mg/L), NO 2 À (1.08-4.37 mg/L) and SO 3 2-(6.84-32.45 mg/L) were identified in spring water, whereas well and tap water contained 35 ClO 4 À (4.20-21.33 mg/L), 81 BrO 3 À (8.05-15.13 mg/L), NO 3 À (1.27-16.11 mg/L), NO 2 À (0.43-10.77 mg/L) and SO 3 2-(7.14-36.10 mg/L), and 35 ClO 4 À (0.89-7.37 mg/L), 81 BrO 3 À (5.15-14.68 mg/L), NO 3 À (1.12-6.33 mg/L), NO 2 À (0.67-3.95 mg/L) and SO 3 2-(5.40-23.26 mg/L), respectively. Conclusions: The levels of 35 ClO 4 À , 81 BrO 3 À , NO 3 À, NO 2 À and SO 3 2were found beyond the maximum contaminant levels and drinking water equivalent levels regulated by the Environmental Protection Agency, respectively. The outcomes also revealed that comparatively tap water produced lower levels of these contaminants especially in the cities with low population densities. In addition, the geographical site would be helpful to categorize the tap water samples.