In a neon r.f. sputtering glow discharge with a copper target, the Cu+ ion current, measured by glow discharge mass spectrometry, is shown to be proportional to the product of the densities of the neutral Cu atoms and the neon metastable atoms, both determined by atomic absorption spectroscopy. This result supports the premise that Penning ionization is the dominant process for the ionization of trace species in this type of glow discharge and also provides confidence in the reliability of mass spectrometric diagnostics of a glow discharge. A discussion is presented of the relationship between the sputtering rate of the copper target and the neutral Cu atom density in the discharge as the discharge pressure is varied. © 1975.