Plasma diagnostic studies have been carried out on the discharge source of a commercial glow discharge quadrupole mass spectrometer. Plasma parameters were determined using an electrostatic probe with the objective of determining the dependence (if any) of these parameters on the voltage placed on an auxiliary electrode immersed in the plasma. The biased electrode utilized in this study was the sampling orifice element itself. Our results indicate that, for positive orifice voltages with respect to the grounded anode, variations in the plasma potential and ion energy can be correlated directly to the bias placed on the sampling orifice. The dependence of the electron temperature on this parameter is observed to be more complex in nature, and electron number densities show little significant variation with respect to sampling orifice bias. In addition, increased orifice voltages result in an increase in the ion signal intensity measured with the mass spectrometer. Based on the results obtained here, we feel that this increase is due primarily to an increase in ion transmission to the quadrupole arising from the increased ion energy.(J Am Soc Mass Spectrom 1990, 1, 448-454).