The spectral subtraction technique was applied to solid samples of HCOONa and CH3COONa along with small amounts of D-variants. The weak signals due to the D-species heavily overlapped by strong absorption bands from the H-species were separately observed by removing the interfering absorbances of the H-substances from the composite spectra of the isotopic mixtures. The best result was achieved when the homogeneity of the sample thickness and composition was satisfactory.
