Characterization of a novel impurity in bulk drug eprosartan by ESI/MS and NMR

A simple and sensitive liquid chromatography tandem multi-stage mass spectrometry (HPLC/MSⁿ) method suitable for eprosartan analysis was developed, by which an unknown impurity in bulk drug eprosartan was detected. The fragmentation behavior of eprosartan and the impurity in negative mode was investigated. Two molecules of CO₂ lost from eprosartan precursor ion were observed, while four molecules of CO₂ were extruded from the deprotonated molecular ion to the MS³ product ions of the impurity. Furthermore, a characteristic fragmentation ion at m/z 335 was observed in both eprosartan and the impurity indicated that the impurity might have two eprosartan units. The unknown impurity was initially proposed to be eprosartan dimer connected via methylene unit at the thiophene moiety on the basis of the multi-stage mass spectrometric and exact mass evidences, and it was finally elucidated as 4,4′-(5,5′-(1E,1′E)-3,3′-(4,4′-methylenebis(thiophene-4,2-diyl))bis(2-carboxyprop-1-ene-3,1-diyl)bis(2-butyl-1H-imidazole-5,1-diyl))bis(methylene) dibenzoic acid by NMR experiments including 1D (¹H NMR, ¹³C NMR, DEPT135⁰) and 2D (¹H-¹HCOSY, HMQC and HMBC) data.