2124 zyxwvutsr B. zyxwvutsrqponml Maichel zyxwvutsrqpon ef a/. zyxwvutsrqponml Electrophoresis 1998, 19, 2124-2128 Beate Maichel' Blahoslav Potocek' Bohuslav Gas* Marcella Chiari3 Ernst Kenndler' 'Institute for Analytical Chemistry, University of Vienna, Vienna, Austria *Institute for Physical and Macromolecular Chemistry, Charles University, Prague, Czech Republic 'Istituto di Chimica degli Ormoni, Consiglio Nazionale delle Ricerche, Milan, Italy Separation of neutral compounds by capillary electrokinetic chromatography using polyethyleneimine as replaceable cationic pseudostationary phase Polyethyleneimine (PEI, molecular weight 6 zyxw X lo5 - 1 X lo6) is applied as a positively Gharged pseudostationary phase for electrokinetic chromatography (EKC) of uncharged mono- and oligophenols. EKC is carried out in PEI- coated fused-silica capillaries (with electroosmotic flow directed towards the anode) in 2-(N-morpholino)ethanesulfonic acid (MES) buffer (pH 7.0,.20 mM) with PEI added to the solution in concentrations up to 0.70% wlv. The pseudo- stationary phase leads to a retardation of the solutes mainly according to the number (and the position) of the OH-groups of the separands, and is not influenced significantly by methyl groups. For 0.70% w/v PEI solution, for instance, the relative retention, e, has values between 0.33 and 0.53. For the systems with the highest resolution of the separands (0.25-0.30 O/o PEI) 190 000 plates per meter are observed. The results indicate that the separation selectivity is mainly caused by ion-dipole interactions between the OH-groups of the solutes and zyxwv 1 Introduction Instead of bonding the stationary phase on silica parti- cles [l, 21 or fixing it at the capillary wall [3-71, electroki- netic chromatrography (EKC) can be carried out by applying replaceable pseudostationary phases as reported by Bachmann et zyxwvutsrqpo al. [S, 91 and Fujimoto et al. [lo]. Separa- tion of neutral compounds was also achieved by dif- ferent distribution between the buffer solution and,a sus- pension of dynamically coated [S], chemically modified [9] or colloidal silica particles [lo]. Because of the oppo- site migration direction of the negatively charged silica particles and the analytes, which move with the EOF towards the cathode, interaction with the pseudosta- tionary phase is possible. Another alternative to packed stationary phases are replaceable polymer solutions used as pseudostationary phases. Electrically neutral polymers are not suitable for this purpose, because they move through the capillary with the velocity of the EOF like the neutral separands. In contrast, in untreated fused- silica capillaries with negative zeta potential, a negatively charged polymer migrates against the EOF and against the sample zone towards the anode, and thus forms a kind of counterflow. We have already applied this meth- od using linear polyacrylamide (PAA), that was partly hydrolyzed to polyacrylate, as the pseudostationary phase. Separation of neutral compounds was possible, but the polymer solution was not very stable and the applied voltage was limited due to high ion concentra- tion of the background electrolyte. Correspondence: Prof. Ernst Kenndler, Institute for Analytical Chem- istry, Universtiy of Vienna, Wahringerstr. 38, 1090 Vienna, Austria (E-mail: Ernst.Kenndler@univie.ac.at) Abbreviations: CZE, capillary zone electrophoresis; EKC, electroki- netic chromatography; MES, 2-morpholinoethanesulfonic acid; PEI, polyethyleneimine Keywords: Electrokinetic chromatography / Electrochromatography / Pseudostationary phase / Phenols / Replaceable / Polymer the pseudostationary phase. In the course of further development in micellar electro- kinetic chromatography (MEKC), unimolecular pseudo- stationary phases consisting of new types of separation additives have already been investigated. Contrary to conventional micelle forming surfactants, the application of dendrimers [ 11-15] as the pseudostationary phase allows the use of relatively high amounts of organic modifier, which is advantageous for the separation of more hydrophobic analytes. Charged cyclodextrin deriva- tives [16-221, calixarenes [23, 241 and resorcarenes [25] were successfully used as buffer additives, zyx e.g., for enan- tiomer separations. The range of compounds that was studied to develop different pseudophases further com- prised charge transfer interacting additives [26], tetraal- kylammonium ions [27-291 and polymerized additives, such as mucopolysaccharides [30, 311, dipeptide surfac- tants [32] and high molecular mass surfactants [33, 341. The use of cationic polymers to change ion selectivity in capillary electrophoresis (ion-exchange electrokinetic chromatography) was reported by Terabe and Isemura [35, 361 and Stathakis and Cassidy [37, 381. The separa- tion of ionic compounds with similar electrophoretic mobilities using tetraalkylammonium ions [39, 401 fol- lowed the same principle. Cifuentes el al. [41] applied high molecular mass polyethyleneimine (PEI) as buffer additive for selectivity control of the separation of pro- teins and peptides. PEI is a positively charged and branched polymer that strongly interacts with the negati- vely charged silanolate groups on the capillary wall. Because it creates a positively charged, hydrophilic layer on the silica surface and reverses the EOF towards the anode, PEI is used as a coating material. Towns and Reg- nier [42] developed a cross-linked PEI coating, whereas Erim et al. [41, 431 applied a physically adsorbed PEI layer to reduce adsorption of proteins on the capillary wall. Recently Erim [44] reported the use of PEI as buffer additive for the separation of partially charged nitro- and chlorophenols. The proposed mechanism was capillary zone electrophoresis (CZE) of the ions supe- rimposed by a kind of chromatography due to the solute- polymer interaction. 0 WILEY-VCH Verlag GmbH, 69451 Weinheim, 1998 0173-0835/98/1212-2124 $17.50+.50/0