The recent development of C-13 NMR to measure absolute C-13/C-12 ratios at natural abundance has made it possible to examine.site-by-site variation at natural abundance. This technique has been applied to We common physical processes distillation and column chromatography. It is shown that the target molecules ethanol and vanillin respectively, bath show isotopic fractionation that differs in size and/or in sense at each carbon atom. The observation of this disparity in behavior is novel and unexpected. C-13 SNIF-NMR appears as a new effective tool to study noncovalent isotope effects (NCIE) an target molecules.