Identification of a new impurity in lisinopril

LC–UV scan of lisinopril revealed the presence of an unknown impurity (∼0.14%) at a relative retention time of 3.26 employing phosphate buffer-acetonitrile as binary gradient system while LC–MS analysis with binary gradient system comprising of a ammonia–ammonium acetate buffer (pH 5.0) and acetonitrile indicated it to be C31H41N3O7. The impurity was isolated by preparative HPLC utilizing a linear gradient of water and acetonitrile. The structural analysis of the isolated product by 1H, 13C NMR, mass spectroscopy and FT-IR revealed it to be a 4-phenyl butanoic acid derivative (CL) of lisinopril.