NDIR is used to determine a concentration of a chosen molecule M in a liquid sample which contains one or more interfering molecules MJ which absorb radiation at the signal wavelength used in the NDIR process by addition of an interference source. M is calculated by electronics which use Rave(t) from a pulsed signal and reference channel output and a calibration curve which is validated by use of RJave(t2) from a pulsed interference and reference channel output. Signal, interference and reference sources are pulsed at a frequency which is sufficiently fast so that a given molecule of M or MJ will not pass in and out of the liquid sampling matrix within the pulsing frequency.