An apparatus for the study of relative ion concentrations in the positive column of noble gas capillary discharges is described. The technique employs ion sampling through a small orifice in the wall of the discharge tube. Consideration is given to identification and removal of possible systematic errors. The performance of the instrument is studied for discharges in helium and neon for pressures varying from 0.10 to 30 Torr and for discharge currents between 10 and 40 mA.