Computer Simulation of Sulfated Cyclodextrin-Based Enantioselective Separation of Weak Bases With Partial, High-Concentration Filling of the Chiral Selector and Analyte Detection on the Cathodic Side.

Abstract

Computer simulation was utilized to characterize the electrophoretic processes occurring during the enantioselective capillary electrophoresis-mass spectrometry (CE-MS) analysis of ketamine, norketamine, and hydroxynorketamine in a system with partial filling of the capillary with 19 mM (equals 5%) of highly sulfated γ-cyclodextrin (HS-γ-CD) and analyte detection on the cathodic side. Provided that the sample is applied without or with a small amount of the chiral selector, analytes become quickly focused and separated in the thereby formed HS-γ-CD gradient at the cathodic end of the sample compartment. This gradient broadens with time, remains stationary, and gradually reduces its span from the lower side due to diffusion such that analytes with high affinity to the anionic selector become released onto the other side of the focusing gradient where anionic migration and defocusing occur concomitantly. The analytes that remain focused until the migrating HS-γ-CD concentration boundary arrives at the cathodic end of the sample compartment become gradually released into the cathodic part and migrate in the absence of HS-γ-CD toward the detector. This behavior is dependent on the length of the HS-γ-CD zone in the cathodic part of the electrophoretic column, the initial sample zone length, and the sample matrix. The data presented reveal the possibility that only one of the enantiomers of an analyte migrates toward the detector, whereas the other is lost for the analysis, or that both enantiomers migrate toward the cathode but do not separate. Enantiomer separation followed by migration toward the cathode can only be achieved for analytes with rather low complexation constants, such as hydroxynorketamine assessed in this work, and is dependent on the slope of the HS-γ-CD focusing gradient. The gained insights illustrate that dynamic simulation is an indispensable tool to investigate electrophoretic processes of complex systems.