Separation of alkaloids and their analogs in HPLC using rosin-based polymer microspheres as stationary phases

Rosin-based polymer microspheres (RPMs) as stationary phases in HPLC were prepared using methyl acrylic acid as the functional monomer, ethylene glycol maleic rosinate acrylate as the cross-linker, and a carboxylic group as the major functional group. The obtained RPMs were characterized by different analytical techniques. The results indicated that the RPMs have a regular spherical shape with porous surfaces, an average particle size of 5.5 mu m, a surface area of 96.73 m(2) g(-1), and an average pore size of 11.68 nm. Moreover, the RPMs displayed good thermal stability. Theophylline, caffeine, tetrahydropalmatine, and berberine hydrochloride were used as probes to study the separation properties of alkaloids and their analogs on an RPM chromatographic column. Theophylline/caffeine and tetrahydropalmatine/berberine hydrochloride were separated by a rosin-based chromatographic column under optimum chromatographic conditions, and their resolution factors were 5.29 and 3.72, respectively. Furthermore, the chromatographic thermodynamics and separation mechanism were also studied. Thermodynamic data obtained by van't Hoff plots showed that their separation on the RPM chromatographic column was an enthalpy-driven exothermic process. Microcalorimetry and zeta potential were employed to investigate the separation mechanism of the binding of alkaloids to the rosin-based polymer surface. The RPM and theophylline (or caffeine) interact through hydrogen bonding interactions, whereas the interactions between the RPM and tetrahydropalmatine (or berberine hydrochloride) arose from hydrogen bonding and electrostatic attraction.