Bubble fractionation of enantiomers from solution using molecularly imprinted polymers as collectors

Adsorptive bubble separation methods have been used to enrich components from both heterogeneous and homogeneous solutions. These methods are particularly effective for processing large solution volumes at low cost. Previous work demonstrated that chiral, surface-active collectors could be used to enrich enantiomers from homogeneous solution in a foam fractionation process. In a significant extension of this work, the use of highly selective molecularly imprinted polymers (MIPs) and heterogeneous solutions for the bubble flotation of enantiomers was evaluated. The high selectivity and ease of recycling of the MIP make this a potentially powerful approach for process-scale separations from large-volume bulk solutions. New MIPs were produced with low swelling properties which allowed them to retain enantioselectivity after numerous recyclings.

Analysis of nicotine and its oxidation products in nicotine chewing gum by a molecularly imprinted solid-phase extraction

Chromatographic stationary phases showing exceptional selectivity for nicotine can be prepared by the technique of molecular imprinting. Such phases were used in the search for a rapid cleanup step for nicotine and some of its oxidation products in chewing gum formulations. Thus, using an organic mobile phase, the nicotine analytes from chewing gums dissolved in nonpolar solvent were retained, whereas the nonpolar matrix eluted close to the void peak. A subsequent switch to an acidic mobile phase resulted in elution of the analytes as one sharp peak. Due to weak binding of the less basic oxidation products, other imprinted polymers were tested, and the solid-phase extraction procedure was optimized. Polymers were prepared using various functional and cross-linking monomers, templates, porogens and thermal treatments. This resulted in phases that, when compared with a nonimprinted or a C18 reversed-phase column, showed significantly higher recoveries of the analytes. Furthermore, no bleeding of template from the phases could be detected. The cleanup step was coupled off-line to reversed-phase HPLC, and the efficiency of the analysis was compared with and without the cleanup step. Three out of four analytes were quantitatively recovered using the imprinted phase, whereas, using the nonimprinted phase, only nicotine was recovered. Without the cleanup step, none of the analytes could be determined using the reversed-phase HPLC method.

Chiral ion-exchange chromatography. Correlation between solute retention and a theoretical ion-exchange model using imprinted polymers

Mobile phase effects were studied in the separation of D- and L-phenylalanine anilide (D,L-PA) on an imprinted chiral stationary phase (CSP). Using an aqueous-organic mobile phase, an improved column performance was seen, reflected in a two-fold decrease in the reduced plate height and an almost doubling of the resolution as compared to when a pure organic mobile phase was used. A strong dependence of retention (k') and enantiomer selectivity (alpha) on mobile phase pH was observed. k' reached a maximum at a pH close to the pKa value of the solute and alpha was high at low pH value but decreased when pH exceeded the solute pKa. Potentiometric titration data allowed estimation of the state of protonation of both the carboxylic acid containing CSP and the amino group containing solutes. The data are analyzed using a simple cation-exchange model to allow simulation of the retention as a function of mobile phase pH. The close agreement between the simulated and experimental curves for retention versus pH suggests that a simple cation-exchange mechanism controls the retention in this system. Moreover, the slightly lower average pKa of the imprinted polymer compared to that of a corresponding blank polymer explains the high selectivity seen at low pH values. Based on these findings, a model describing the events controlling binding and selectivity as a function of pH is proposed.