Optical spectroscopy system and method possessing spectral selectivity sufficient to distinguish isotopic line of the metal of interest. Each of the light beams, counter-propagating through vial with vapor of the sample, has been originated from the same light output of the laser source and modulated at a corresponding judiciously-determined frequency. The light-output, in turn, possesses a carrier frequency and two side-band frequencies defined with respect to a mean value of excitation frequencies of isotopes in the vapor.