A tool for determining the amount of a carbon isotope in a gas-phase hydrocarbon up to C 4 relative to a reference amount, comprising: a laser source emitting at least one laser beam; a first volume of the hydrocarbon gas fluid positioned so that at least a first portion of a laser beam passes through the hydrocarbon gas; a first upstream optical detector positioned to detect said first beam portion before said first beam portion passes through said first volume, said first upstream optical detector emitting a first upstream signal corresponding to the strength of said first beam portion before passing through the first volume; a first downstream optical detector positioned to detect said first beam portion after said first beam portion passes through said first volume, said first downstream optical detector emitting a first downstream signal corresponding to the strength of said first beam portion after passing through the first volume; a reference cell containing a reference gas with a concentration of the isotope and positioned so that at least a second portion of the laser beam passes through said concentration; a second upstream optical detector positioned to detect said second beam portion before said second beam portion passes through said reference cell, said second upstream optical detector emitting a second upstream signal corresponding to the strength of said second beam portion before passing through the reference cell; a second downstream optical detector positioned to detect said second beam portion after said second beam portion passes through said reference cell, said second downstream optical detector emitting a second downstream signal corresponding to the strength of said second beam portion after passing through said reference cell; and a microprocessor for receiving said first and second upstream and downstream signals and calculating therefrom a parameter indicative of the presence of the isotope in the hydrocarbon gas, wherein the parameter comprises enrichment or depletion of the isotope relative to the reference gas; wherein said first volume and said reference cell are at substantially the same temperature and pressure, said first and second downstream signals are indicative of the transmittance of said first and second beam portions (24, 22) through said hydrocarbon gas and reference cell (12, 14), respectively, and the concentration of the isotope in the reference cell is unknown. a laser source emitting at least one laser beam; a first volume of the hydrocarbon gas fluid positioned so that at least a first portion of a laser beam passes through the hydrocarbon gas; a first upstream optical detector positioned to detect said first beam portion before said first beam portion passes through said first volume, said first upstream optical detector emitting a first upstream signal corresponding to the strength of said first beam portion before passing through the first volume; a first downstream optical detector positioned to detect said first beam portion after said first beam portion passes through said first volume, said first downstream optical detector emitting a first downstream signal corresponding to the strength of said first beam portion after passing through the first volume; a reference cell containing a reference gas with a concentration of the isotope and positioned so that at least a second portion of the laser beam passes through said concentration; a second upstream optical detector positioned to detect said second beam portion before said second beam portion passes through said reference cell, said second upstream optical detector emitting a second upstream signal corresponding to the strength of said second beam portion before passing through the reference cell; a second downstream optical detector positioned to detect said second beam portion after said second beam portion passes through said reference cell, said second downstream optical detector emitting a second downstream signal corresponding to the strength of said second beam portion after passing through said reference cell; and a microprocessor for receiving said first and second upstream and downstream signals and calculating therefrom a parameter indicative of the presence of the isotope in the hydrocarbon gas, wherein the parameter comprises enrichment or depletion of the isotope relative to the reference gas; wherein said first volume and said reference cell are at substantially the same temperature and pressure, said first and second downstream signals are indicative of the transmittance of said first and second beam portions (24, 22) through said hydrocarbon gas and reference cell (12, 14), respectively, and the concentration of the isotope in the reference cell is unknown.