The energy spectra of fast electrons resulting from pair collisions between metastable atoms and from collisions of the second kind with electrons are observed in the afterglow of a helium-filled microplasma collisional electron spectroscopy (CES) detector at a pressure of 5–40 Torr. It is demonstrated that impurities present in the main inert gas can be detected and their composition can be determined using a planar double-electrode detector in which the cathode simultaneously serves as an analyzer of electrons in the afterglow.