Separation, Identification and Structural Elucidation of a New Impurity in the Drug Substance of Amlodipine Maleate Using LC-MS/MS, NMR and IR

Amlodipine maleate is a maleate salt of 3-ethyl 5-methyl 2-[(2-aminoethoxy)methyl]-4-(2chlorophenyl)-6-methyl-1,4-dihydropyridine-3,5-dicarboxylate. An unknown impurity at m /z 392.2 for [M+H] ion has been detected during the accelerated stability analysis (40 °C /75 % RH) of amlodipine maleate drug substance by reverse-phase high performance liquid chromatography–mass spectrometry (RP-HPLC-MS). MS and MS/MS spectra of amlodipine maleate and unknown impurity are obtained using HPLC-MS/MS equipped with positive electrospray ionization (ESI). The nuclear magnetic resonance (NMR) and infrared (IR) spectra of the unknown impurity are recorded after isolation of the impurity by preparative HPLC. Based on MS, NMR and IR spectral data, the structure of the unknown impurity was proposed as 5-ethyl-7-methyl-6-(2-chlorophenyl)-8-methyl-3,4,6,7-tetrahydro-2H-1,4-benzoxazine-5,7dicarboxylate.