Abstract: Dapagliflozin is a sodium-glucose co-transporter-2 (SGLT2) inhibitor for treating type Ⅱ diabetes. In the process of synthesizing dapagliflozin, 5-bromo-2-chloropheny-4’-ethoxyldiphenylmethanone (compound 1) is a key intermediate. According to the literatures, there is always by-product 5-bromo-2-chloropheny-2’-ethoxyldiphenylmethanone (compound 2) in the synthesis procedure of compound 1. Due to the similarities of the two isomers in physical and chemical properties, it’s difficult to separate and differentiate compound 1 and 2. As compound 2 affects the purity of dapagliflozin, it is of great importance to establish a method to rapidly distinguish compound 1 from compound 2. In this article, high performance liquid chromatography-ion trap-time of flight mass spectrometry (HPLC-IT-TOF MS) method was used to analyze compound 1 and 2. The samples were dissolved in methanol for direct injection. Under the optimized chromatographic conditions, good separation for the two isomers was obtained on a Capcell PAK C18 column (250 mm×4.6 mm×5 μm) with a mobile phase of water （0.1% formic acid） and acetonitrile-methanol (3∶1, V/V) at a flow rate of 0.6 mL/min. The target compounds were assayed by IT-TOF MS/MS in positive mode with the spray voltage set at 1.77 kV. The nitrogen flow was set at 1.5 L/min, and the capillary temperature was set at 200 ℃. The collision gas was helium. The ion at m/z 338.97 was chosen as the target to perform MS/MS experiment, and the collision energies were set at 10%, 15%, 20% and 25%, respectively. The MS/MS results under different collision energies were gained simultaneously by setting events in parallel on IT-TOF mass spectrometer. In order to visually distinguish the two isomers, breakdown curves were mapped by M+H⁺ and fragment ions. The breakdown curves of the two isomers show significant differences, which provided strong support in this study. In addition, the element compositions of the three product ions were also confirmed by IT-TOF MS. Loss of C₂H₂ from the precursor ions of M+H⁺ was observed in the MS/MS spectra of both the isomers. For compound 1, the major fragmentation pathway was loss of C₂H₅OC₆H₅ from M+H⁺via a 2-step proton transfer. While, for compound 2,M+H-C₂H₄O⁺ via 1-4 H shift from ortho-substituted ethoxy group to the benzene ring was found to be the major product ion. The difference in the fragmentation pathways of the two isomers may be due to the different substitution sites of the ethoxy groups. When the ethoxy group is ortho-substituted, the 2-step proton transfer is quite difficult. As a result, it goes through loss of C₂H₄O mainly. The method is simple, effective and accurate, which is suitable for rapidly qualitative and semi-quantitative analysis of the isomers, and also can provide a reliable and effective technique for the quality control of dapagliflozin.