Continuous mass spectrometric sampling of the transient behavior of various gas species from a dc glow discharge originally filled with CO2, and mixtures of CO2-H2, (pressure range 50–1000 μ, discharge current 50–200 mA) have been studied by means of a specially constructed mass spectrometer (90° magnetic deflection) gas-sampling system. The sampling technique employed differential pumping of the mass spectrometer by a constricted ion-getter pump using a variable entrance aperture (valve) to the sampling system. The response of the system to time-varying concentrations is derived and the realization of optimum conditions discussed. This work is part of a program to investigate the gaseous electronics aspect of sealed CO2 laser operation.