For determining concentration of targeted molecules MG in a liquid sample admixed with interfering molecules MJ which overlap their absorption band, a special NDIR sampling and calibration technique is employed. Besides the signal source, a reference and one or more interference sources are added. The selection of the wavelength for the interference sources enables its measured transmittance value to be used for deciding the validity of the calibration curve for molecules MG in the liquid sample. This value can further be used to adjust the calibration curve via a parameter linking the transmittances measured at the signal and interference wavelength channels in order to assure its validity.