Insights into the separation, enantioseparation and recognition mechanisms of amphetamine and methamphetamine isotopologues on achiral and polysaccharide-based chiral columns in high-performance liquid chromatography. First Baseline Separation of Isotopomers.

The different behavior of enantiomers of chiral compounds in non-isotropic environments (among them in living organism) is well known. On the other hand, the importance of a kinetic isotope effect in the biomedical field has become evident during past few decades. Thus, separation of both, enantiomers and isotopologues is now critical. Only very few published studies have attempted the simultaneous separation of enantioisotopologues. In this study we report baseline separation of partially deuterated isotopologues of a few amphetamine derivatives in high-performance liquid chromatography (HPLC) using achiral columns. In addition, the simultaneous separations of enantiomers and isotopologues (i.e. enantioisotopologues) were attempted on polysaccharide-based chiral columns. For several compounds the isotope effect was tunable and could be switched from a “normal” to “inverse” by making changes to the mobile-phase composition. A stronger isotope effect was observed in acetonitrile-containing mobile phases compared to methanol-containing ones with both chiral and achiral columns. In a separation system where both “normal” and “inverse” isotope effects were observed the “normal” isotope effect was favored in polar organic solvents while increasing content of the aqueous component in the reversed-phase (RP) mobile phase favored an “inverse” isotope effect. This observation indicates that polar, hydrogen bonding-type noncovalent interactions are involved in the “normal” isotope effect, while apolar hydrophobic-type interactions are mostly responsible for the “inverse” isotope effect. In this study, the feasibility of very challenging separation of isotopomers by high-performance liquid chromatography in a common setup is successfully demonstrated with the examples of deuterated amphetamine and methamphetamine. Understanding the effect of various factors, such as analyte structure, location of heavy atoms within the structure, separation mode (i.e.,surface chemistry of the stationary phase), mobile phase components and their additives, pH of the mobile phase, and separation temperature, that enabled baseline separation of Isotopomers with high selectivity is discussed. The results reported here are significant for both the analytical separation of isotopomers and their preparative-scale separation. The understanding and fine-tuning of the mechanisms underpinning isotopomer retention and discrimination enabled the reversal of isotopomer elution order on specific chiral columns. The simultaneous separation of isotopomers and their enantiomers was observed on polysaccharide-based chiral columns. Molecular modeling was used to provide a reasonable explanation of the separation mechanism of the isotopomers.