Abstract: A method and apparatus for determining the concentration of a gas using nondispersive infrared absorption. The method includes measuring the output signal for a plurality of instantaneous time periods, calculating the change of the output signals (CS), calculating the change of the change of the output signal (CCS) which creates a pattern of CCS data and comparing the CCS data with corresponding CCS data for a known reference gas at a known concentration and at a range of known temperatures. The apparatus for determining the concentration of a known gas in a sample gas mixture is a gas chamber, a light source, a detector, a signal converter, and a data acquisition and microprocessor unit which measures the output signal and has a memory with a program to calculate the change of the signal (CS) and the change of the change of the signal (CCS), and compare the CCS data generated from the gas sample mixture to CCS data of known gases with known concentrations and at known temperature ranges.