A method which provides continuous background subtraction in the mass spectrometer measurement while continuously sampling gas into the ion source is described. The method employs a pulsed gas sampling system that is capable of continuously admitting gas into the closed ion source from a wide range of sample gas pressures. This gas sampling system, when combined with synchronous detection by a digital filter, allows the mass spectrometer to be operated in ac mode so that only the signal derived from the gas sample entering the ion source is measured. The signals generated from varying gas background in the vacuum chamber, drift due to environmental temperature changes, pump instability, drift in the ion detection circuitry, and line signals are excluded. The stability of the mass spectrometer measurement is therefore greatly improved. The sampling system can be operated in molecular flow at low sampling gas pressures or viscous flow when sampling from a high pressure environment. The stability of the flow regime has been investigated and data showing the overall performance of the system are reported. This method can potentially improve the measurement capability of mass spectrometers used in a variety of applications such as residual gas measurements in industrial process control, medical gas monitoring, and trace gas monitoring in the atmosphere.