Synthesis and use of quaternized polyethylenimine-coated zirconia for high-performance anion-exchange chromatography

The synthesis of an alkali-stable strong anion-exchange stationary phase by deposition of polyethylenimine (PEI), followed by cross-linking and quaternization, onto porous zirconia particles is described. Physical characterization of quaternized PEI-zirconia and PEI-zirconia shows that 50% and 24% of the amine groups are cross-linked, respectively. A plot of log k' versus log (competing ion concentration) is linear for three homopeptides, suggesting that ion exchange is the primary mechanism of retention on quaternized PEI-zirconia. Column efficiency for two 2,4-dinitrophenyl amino acids increased by 80% upon increasing the temperature from 50 degrees C to 100 degrees C. The hydrophobicity of quaternized PEI-zirconia was studied using a homologous series of p-alkoxybenzoic acids. For quaternized PEI-zirconia and PEI-zirconia, we found that the free energy of transfer of a methylene unit from the mobile phase to the stationary phase was -2.0 and -0.90 kJ/mol, respectively. The free energy of transfer of a methylene unit on quaternized PEI-zirconia is similar to that of a typical ODS phase (-2.4 kJ/mol). A van't Hoff plot for the above two 2,4-dinitrophenyl amino acids showed that the enthalpies of transfer are exothermic and fairly large (approximately -14 kJ/mol). Isocratic separations on quaternized PEI-zirconia of inorganic and organic anions are presented. Quaternized PEI-zirconia, quaternary amine-functionalized silica, and PEI-zirconia are compared chromatographically. Quaternized PEI-zirconia is more efficient than the silica-based phase in the separation of benzoic acid derivatives but slightly less efficient than PEI-zirconia. The major virtue of quaternized PEI-zirconia is that it is chemically stable in the pH range of 1-13 and is also stable at temperatures up to 100 degrees C.