Isotope Effect in Capillary Electrophoresis: First Baseline Separation of Isotopomers

Separation of isotopologues of various weak bases was studied by free solution capillary electrophoresis. In addition, the separation of isotopomers was successfully demonstrated for the first time based on the electrokinetic migration mechanism. It was shown that the difference in the charge density of the isotopologues of amphetamine, metamphetamine, and their analogues was not sufficient for their separation. Therefore, the only way to achieve the separations described in the present study was to exploit the minute differences in the pK_a values of isotopologues, as well as of isotopomers. This study also illustrates the power of capillary electrophoresis to leverage pK_a differences on the scale of 0.01 pH units in a reliable way. Such sensitivity to only marginal differences in pK_a values has also been reported with advanced methods of NMR spectroscopy. However, the amount of sample required for such measurements in NMR spectroscopy is significantly (one to two orders) higher compared to capillary electrophoresis.