The medical mass spectrometry as a monitoring instrument in a respiratory intensive care unit (RICU) is described. Its uses, both routine and as a tool for innovative techniques in respiratory care, are many. As an adjunct to traditional and intermittent mandatory ventilation (IMV) weaning techniques, monitoring of expired respiratory gases can hasten the safe removal of patients from mechanical ventilators. A specialized technique of dealing with ventilating the lungs differentially when their mechanical properties differ is described to illustrate the versatility of the instrument. In critical care areas, monitoring of patients with respiratory problems should include continuous monitoring of end-tidal or expired CO2.