A quadrupole mass spectrometer equipped with a special sampling tube and positioned on a linear-motion table is shown to be capable of obtaining important information on the variation of the discharge properties with position. We measured signals obtained by post-ionization of species sampled from Ar and C2F6 high-density plasmas using electron ionization energies of 30 and 16 eV. The variation of the ion signal with position of the sampling orifice inside the reactor provides evidence of the spatial nonuniformity of the discharge and the processes that control these variations. For At discharges we show evidence of gas heating which produces a nonuniform radial distribution of the neutral species. The neutral species are depleted in the center of the discharge due to plasma heating. The neutral gas temperature that can be extracted from the variation in density is consistent with temperature measurements using spectroscopic probes. We also monitored the intensity of different fluorocarbon ions as a function of sampling position. We find that the fluorocarbon mass spectrometer signals show qualitatively different dependencies on sampling position. For instance, the CF2+ signal increases towards the center of the discharge and decreases towards the wall of the chamber, whereas the CF3+ signal shows a maximum near the reactor wall. The CF3+ signal also grows as the wall temperature increases, indicating production at the wall. The application of this setup to spatially resolved monitoring of etching and deposition processes is also described. (C) 2003 American Vacuum Society.