with the ir detector and the details of the sampling system by which the cooperation of the two detectors is accomplished is the subject of my co pending application ser no 228 119 filed concurrently herewith now u s pat no 4 363 635 the basic elements of the detection portion of the apparatus include a breath inlet line 10 surrounded through a portion of its length by a heater 11 the free end of the breath inlet line 10 is adapted to receive a mouthpiece or tube by which breath is introduced in a conventional manner a breath inlet check valve 12 communicates on its inlet side with the breath inlet line and on its outlet side with a sample inlet conduit 15 the sample inlet conduit 15 communicates by way of an inlet passage 70 with one end of a sampling arm channel 90 which in turn communicates with an orifice chamber 19 with an orifice 20 a thermistor 22 projects into the orifice chamber the orifice chamber 19 communicates through the orifice 20 and an outlet passage 76 with a sample chamber 30 of the ir detector 2 which is vented to the atmosphere through a vent pipe 36 the ir detector 2 is conventional with a reference chamber 31 an ir source 32 at one end.