Often, the largest source of error in a study that depends on the natural abundance of carbon is the slight variation in natural C abundance itself. Such variations arise because the starting materials used in the reaction are themselves products of some other reactions that have kinetic isotope effects and corresponding isotopic enrichments in the products. To compensate for this error when NMR spectroscopy is used to determine the kinetic isotope effect, the following guidelines have been proposed:
* Choose a carbon that is remote from the reaction center that will serve as a reference and assume it does not have a kinetic isotope effect in the reaction.
* In the starting material that has not undergone any reaction, determine the ratios of the other carbon NMR peak integrals to that of the reference carbon.
* Obtain the same ratios for the carbons in a sample of the starting material after it has undergone some reaction.
* The ratios of the latter ratios to the former ratios yields R/R.
If these as well as some other precautions listed by Jankowski are followed, kinetic isotope effects with precisions of three decimal places can be achieved.