•Instrument for measuring the energy of electrons produced in ion trap.•Calibration with electrons from photodetachment.•Energy distribution of electrons from associative detachment. Design of a novel instrument for the analysis of the kinetic energy of the detached electrons is presented. The electron spectrometer with multipole trap combines a radiofrequency ion trap with a magnetic adiabatic collimation filter and is capable of measuring the energy of electrons produced in the trap. The theory of operation and numerical simulations of the instrument are presented together with a proof of concept measurements using three different mechanisms of electron production – thermionic emission from a tungsten filament; laser photodetachment from O− anions; associative detachment of O− + H2, D2, and CO.