The mass spectrograph separates molecules on the basis of mass to charge ratios. A gas  sample is aspirated into a high vacuum chamber (10-5 mmHg) where an electron beam ionizes and fragments the components of the sample. The ions are accelerated by an electric field into a final chamber, which has a magnetic field, perpendicular to the path of the ionized gas stream. In the magnetic field the particles follow a path wherein the radius of curvature is proportional to the charge:mass ratio. A detector plate allows for determination of the components of the gas and for the concentration of each component. Mass spectrometers are quite expensive and too bulky to use at the bedside and are rarely used presently. They are either “stand alone,” to monitor a single patient continuously, or “shared,” to monitor gas samples sequentially from several patients in different locations (multiplexed). Up to 31 patients may be connected to a multiplexed system and the gas is simultaneously sampled from all locations by a large vacuum pump. A rotary valve (multiplexer) is used to direct the gas samples sequentially to the mass spectrometer. In a typical 16-station system, with an average breathing rate of 10 breaths min-l, each patient will be monitored about every 3.2 min. The user can interrupt the normal sequence of the multiplexer and call the mass spectrometer to his patient for a brief period of time.1,5