It is found that for heavy elements, in the absence of self-absorption, a simple proportionality exists between the relative intensity of spectral lines belonging to different isotopes and the relative concentration of the isotopes. In the case of the lighter elements a simple proportionality is distorted by the different Doppler broadening of the measured lines and as a result of differences between physical properties of the isotopes. In order to establish the character of the relation between line intensity and concentration, the analytical curves were constructed using standard isotopic mixtures of uraniom and hydrogen. Some of the mixtures (U<sup>235</sup> + U<sup>238</sup>) consisted of isotopes whose physical properties were approximately equal, the others (H<sub>2</sub> + D<sub>2</sub>) of isotopes with notably different properties. When consideration was given to background in the spectrum and superposition of isotopic lines, the experimental results were found to be consistent with the derived equation. Thus, in the case of isotopic spectral analysis for obtaining the desired results, it is necessary to standardize the method with the purpose of establishing the character of the dependence between the quantities I<sub>1</sub>/I<sub>2</sub> and C<sub>1</sub>/C<sub>2</sub>. The true dependence between these quantities, even in the absence of self-absorption, may be strongly distorted by the mutual superposition of comparative lines and by the background of the spectrum.
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