The Abundance Ratio of the Isotopes in Natural or Isotopically Separated Carbon

The relative intensity of the 1,0 band heads of the ${\mathrm{C}}_{12}$ ${\mathrm{C}}_{12}$ and ${\mathrm{C}}_{12}$ ${\mathrm{C}}_{13}$ molecules in the Swan system of the ${\mathrm{C}}_2$ spectrum has been measured by a photographic spectrophotometric method and found to be 0.0217±0.0009. This yields a value of 92.2±3.7 for the ${\mathrm{C}}_{12}$/${\mathrm{C}}_{13}$ abundance ratio of the carbon isotopes in good agreement with the value reported by Vaughan, Williams and Tate (91.6±2.2), from mass-spectrographic data. This work was done in connection with the development of a simple method suited to the determination of the abundance ratio in the small samples obtained in the separation of natural carbon into isotopes. This method is described.