In order to study the differences in the 13C and 12C isotopes assimilation degree related to the rate of photosynthetic reactions, we have developed a number of procedures of sample gasification and a hardware experimental complex for sample preparation before a mass-spectrometric isotope analysis of carbon involved in a plant life. A setup for concentrating the carbon dioxide located around the plant was designed and made. The setup makes catalytic afterburning of organic microimpurities available for increasing the carbon content more than a hundred times. A reaction procedure for oxidation of leaf glucose by yeast generating carbon dioxide was suggested, reagent concentrations selected. The collected samples were free from impurities (not exceeding 10–5). The developed sample preparation technique was used to study the effect of the light exposure characteristics on the carbon isotope interchange between atmospheric CO2 and the plant carbon pool.