Abstract: Apparatus and method for gas analysis in which a sample gas is introduced into a high pressure sample cell through a long narrow holding tube, having a volume greater than the volume of the sample cell; and a second gas which will not interfere with the analysis is introduced into the holding tube behind the sample gas, at a pressure sufficient to force a portion of the sample gas contained in the holding tube into the sample cell so that the pressure of the sample gas in the sample cell is increased. An analysis of the sample gas in the sample cell is then made by a suitable technique, preferably by photometric analysis.

Abstract: Apparatus and method for NO.sub.x analysis in which a sample gas is introduced into a sample cell and mixed with oxygen to induce conversion of NO to NO.sub.2. After a given time, prior to complete conversion of NO to NO.sub.2, the absorbance and the time rate of change of absorbance is measured. The time rate of change of absorbance is indicative of the initial NO concentration in the sample gas; and the sum of the absorbance and the time rate of change of absorbance is indicative of the initial NO.sub.x concentration in the gas. A better indication of the initial NO.sub.x concentration can be obtained from the sum of the absorbance and the square root of the time rate of change of absorbance.