  We propose a method based on the Electromagnetically Induced Transparency
(EIT) phenomenon for the detection of molecules which exist as a small minority
in the presence of a majority of absorbers. The EIT effect we employ
effectively eliminates the absorption of the majority species in the spectral
region where it overlaps with the absorption of the minority species. The
method can also be used to enhance local-modes transitions which overlap
spectrally with a background of other local-modes transitions of the same
molecule. The general theory is applied to the case of sparse and congested
background spectra within the same molecule and to the recording of the spectra
of isotopomers (of Chlorine and Methanol) that are in minority relative to
other isotopomers which constitute the majority of molecules present.
