AbstractThe [C7H10NO3]+ion in the normal mass spectrum of 2‐ethoxycarbonyl‐5‐oxo‐2‐pyrrolidinepropanoic acid was shown to correspond to a mixture of two isomeric structures. By decreasing the ionizing electron energy, the one containing a CH2CH2COOH group becomes dominant. The proportion of the concentrations of the two isomeric ions was calculated by comparing their daughter ion spectra (obtained by linked scan at constantB/E) with the analogous spectra of a derivative partially labelled by deuterium at the carboxyl and ami