NOVELTY - Measuring the concentration of marker gas in a gas mixture involves obtaining a gas mixture sample (10) initially containing marker gas and calibration gas having independently known concentration and present in the gas mixture for a gas mixture sampling interval and using a spectrometer.    USE - Used for measuring the concentration of a marker gas in a gas mixture.    ADVANTAGE - The method accurately determines the concentration of a specific gas without introduction of any additional reference gas. The method maximizes patient comfort and provides real time results that can enable a health care provider to diagnose and treat a patient in a single clinic visit.    DETAILED DESCRIPTION - Measuring the concentration of marker gas in a gas mixture comprises:    (1) obtaining a gas mixture sample initially containing a marker gas and calibration gas having independently known concentration and present in the gas mixture for a gas mixture sampling interval;    (2) providing a spectrometer having a spectrometer light source, spectrometer gas sample cell, spectrometer detector (24) and spectrometer computer;    (3) placing the gas mixture sample in the spectrometer gas sample cell;    (4) illuminating the gas mixture sample with the spectrometer light source;    (5) detecting a transmitted light beam (20) that has passed through the gas mixture sample with the spectrometer detector to produce a detector electronic signal sequence;    (6) transmitting the detector electronic sequence to a spectrometer computer including an executable program;    (7) acquiring the detector electronic signal sequence using the spectrometer computer for a signal acquisition interval to obtain a gas mixture spectrum;    (8) storing the gas mixture spectrum;    (9) analyzing the gas mixture spectrum using the spectrometer computer which comprises:    (1) identifying a marker gas absorption feature and a calibration gas absorption feature;    (2) determining a marker gas absorption feature intensity and calibration gas absorption feature intensity, and    (3) storing the intensities as a function of time over the gas mixture sampling interval, and    (10) repeating the steps of acquiring and analyzing for multiple signal acquisition intervals to identify a single signal acquisition interval, and    (11) calculating a ratio of simultaneous marker gas absorption feature intensity to a known concentration calibration gas absorption feature intensity and multiplying ration by a proportionality constant to determine the concentration of marker gas.    An INDEPENDENT CLAIM is also included for an apparatus for measuring the concentration of a marker gas in a gas mixture sample which comprises a spectrometer light source for illuminating a gas mixture sample, a spectrometer gas sample, a spectrometer detector for detecting light transmitted through gas mixture sample, and a spectrometer computer.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic drawing of a gas mixture analysis system.    Gas mixture sample (10)    IV-VI diode laser (18)    Light beam (20)    Spectrometer detector (24)    Personal computer (30)