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Capillary Electrokinetic Chromatography with Charged Anal. Chem. 2000, 72, 74-80 Capillary Electrokinetic Chromatography with Charged Linear Polymers as a Nonmicellar PseudoStationary Phase: Determination of Capacity Factors and Characterization by Solvation Parameters Blahoslav PotocÏ ek,²,§ Emil Chmela,²,§ Beate Maichel,³,§ Eva TesarÏovaÂ,²,§ Ernst Kenndler,³,§ and Bohuslav GasÏ*,² Faculty of Science, Charles University, Albertov 2030, CZ-128 40 Prague 2, Czech Republic, and Institute of Analytical Chemistry, Vienna University, Wa¨hringerstrasse 38, A-1090 Vienna, Austria A permanent polycation, polydiallyldimethylammonium display of stationary and mobile phases allows an optimal choice (PDADMA), is applied as a linear, polymeric, replaceable, for the system. While capillary zone electrophoresis (CZE) in its and nonmicellar pseudostationary phase for the separa- basic mode has some abilities to influence the mobility of the tion of neutral analytes by capillary electrokinetic chro- charged compounds, the presence of additional separation media matography. It is shown that this polymer used in the is necessary when separating neutral analytes. In addition to well- background electrolyte is able to separate the analytes established micellar electrokinetic chromatography (MEKC),1,2 even if it does not form micelles under the given condi- capillary electrochromatography (CEC)3,4,5,6,7 also appears to tions. The most favorable aspect for practical use lays in become attractive. The separation process in CEC is based on the simple replacement of the separation media after each partitioning between two phases similar to HPLC. In contrast to run, thus generating highly reproducible conditions. To the latter technique, analytes are transported in CEC by elec- determine the capacity factors of the analytes, a new troosmosis with an almost flat radial velocity profile. Nevertheless, method, based on an isotachophoretic regime, has been some technical problems arise during reproducible fabrication of introduced for the measurement of the electrophoretic packed capillaries. mobility of the polymeric pseudo-stationary phase. The Polymers have been successfully used as an alternative to a capacity factors in the separation system, derived from packed bed. Fujimoto8,9,10,11 and HjerteÂn12 utilized a cross-linked, the mobilities of the polymer, the electroosmotic flow, and charged, acrylamide-based gel attached to the capillary wall. The the mobilities of 15 individual aromatic analytes, range charged groups of the polymer generate the electroosmotic flow between 0.3 and 1.2 for the given separation media while its hydrophobic parts are supposed to interact with the (aqueous solution of acetate buffer, pH 5.2, with 4% w/w analytes to influence their retardation. Though the efficiency of PDADMA). The type of interaction in the pseudochro- separation is good in these continuous-bed columns, the poly- matographic system was clarified from solvation param- merization of the gel inside the capillary might be problematic. eters based on the linear free energy relationship model. Recently Svec et al.13,14,15 applied charged polymeric monoliths It was found that π and n electron interactions and as separation media in CEC. Monolithic columns show very good hydrogen-bond basicity of the polymer, as compared with separation properties, and their preparation is easier and more the aqueous bulk phase, are the main cause of retention reproducible than those packed with small particles. Polymeric of the analytes. monoliths can be prepared with both well-controlled porous properties and different surface performances. Although electromigration separation methods are generally (1) Terabe, S.; Otsuka, K.; Ichikawa, K.; Tsuchiya, A.; Ando, T. Anal. Chem. considered to exhibit better separation efficiency than high- 1984, 56, 111-113. performance liquid chromatography (HPLC), there is still a (2) Li, S. F. Y. Capillary Electrophoresis; Elsevier: New York, 1992. - substantial advantage of the latter method: HPLC has many (3) Pretorius, V.; Hopkins, B. J.; Schieke, J. D. J. Chromatogr. 1974, 99,23 30. means for varying the separation selectivity, because a wide (4) Jorgenson, J. W.; Lukacs, K. D. J. Chromatogr. 1981, 218, 209-216. (5) Knox, J. H.; Grant, I. H. Chromatographia 1987, 24, 135-143. * Corresponding author: e-mail [email protected] fax +420 2 291958 tel. +420 (6) Knox, J. H.; Grant, I. H. Chromatographia 1991, 32, 317-328. 2 2195 2437. (7) Van den Bosch, S. E.; Heemstra, S.; Kraak, J. C.; Poppe, H. J. Chromatogr., ² Charles University. A 1996, 755, 165-177. ³ Vienna University. (8) Fujimoto, C.; Sawada, H.; Jinno, K. J. High Resolut. Chromatogr. 1994, 17, § E-mail addresses. Blahoslav PotocÏek: [email protected]. Emil 107-108. Chmela: [email protected]. Eva TesarÏovaÂ: [email protected]. Beate (9) Fujimoto, C.; Kino, J.; Sawada, H. J. Chromatogr., A 1995, 716, 107-113. Maichel: [email protected]. Ernst Kenndler: [email protected]. (10) Fujimoto, C. Anal. Chem. 1995, 67, 2050-2053. Bohuslav GasÏ: [email protected]. (11) Fujimoto, C.; Fujise, Y. Anal. Chem. 1996, 68, 2753-2757. 74 Analytical Chemistry, Vol. 72, No. 1, January 1, 2000 10.1021/ac990979a CCC: $19.00 © 2000 American Chemical Society Published on Web 11/19/1999 Different amphophilic polymers with both hydrophilic and electrically driven separation of neutral analytes. In addition to hydrophobic regions were used as separation media in CEC16,17,18,19 implementing separation selectivity, the polymer forms a stable as well. These polymers are considered to form micelles from a layer on the capillary wall, thus being the cause of an electroos- single molecule. The polymer micelles are very stable as they have motic flow (EOF) toward the anode. Samples injected at the a size and structure fixed by covalent bonds. The critical micellar cathodic end of the capillary are drifted with this electroosmotic concentration of the polymer surfactants is practically zero. Unlike flow to the anode while the charged polymer moves in the opposite that of conventional low-molecular micelles, the formation of direction, acting as a pseudostationary phase. In this way an polymer micelles is not sensitive to organic modifiers or other electrochromatographic system is established. additives used with the mobile phase.20,21 Yang et al.22 applied a Besides the investigation of the principal ability of PDADMA linear solvatochromic model to characterize the triblock copolymer to separate neutral analytes, there are two other main topics of surfactant Elvacite 2669. Tanaka et al.23 controlled the migration this paper. The first is the introduction of a method for the time window and the separation by utilizing a mixture of two determination of the migration velocity and, therefore, the mobility polyallylamine-based surfactant polymers. of the charged polymer. This method thus enables the determi- Charged polymers can be successfully used to influence the nation of the (chromatographic) capacity factors of the solutes, migration of both charged or neutral analytes even if they are which are most important for the discussion of selectivity in not chemically cross-linked or present in the form of polymer pseudochromatographic systems. The knowledge of these capacity micelles. Several authors reported the use of cationic polymers factors is a prerequisite for the second topicsthe application of to influence selectivity in capillary electrophoresis of ions.24,25,26,27 the linear free-energy relationship model. This model allows for The separation principle is based on ion-pair formation of the ionic the characterization of interactions between the solutes and the analyte with a charged polymer ion. Erim28 demonstrated the pseudostationary phase applied, expressed by the solvation electrophoretic separation of substituted phenols using cationic parameters. polyethyleneimine as a buffer additive. PotocÏeketal.29 and Maichel et al.30 performed the separation of neutral compounds, mostly THEORETICAL phenols, in solutions of both negatively and positively charged Capacity Factor and Mobility in Systems with a Polymeric polymers, namely partially hydrolyzed polyacrylamide and poly- Pseudophase. The system proposed in this paper consists of a ethyleneimine, respectively. Maichel et al.31 investigated the solution of a linear charged polymer in a low molecular electrolyte influence of organic solvents (methanol, acetonitrile) on the buffer. This polymer serves as the pseudo-stationary phase and separation of neutral compounds in systems containing polyeth- the rest of the solution, i.e., water with the low molecular yleneimine. electrolytes, (hereafter termed bulk), acts as the mobile phase. The charged cationic polymers can also serve as dynamic The charged polymer migrates under the influence of the electric noncovalent coatings of the inner capillary wall preventing the field in the direction opposite to the solutes, which are moved by adsorption of large positive analytes such as basic proteins at the the EOF. capillary wall. Among various charged polymers the use of poly- Neutral low-molecular solutes are separated if they have (diallyldimethylammonium) cation, PDADMA, which consists of different distribution constants between the polymer and the bulk a diallyl backbone with strongly basic quaternary ammonium solution. According to the model of chromatography, the distribu- groups was described for this purpose.32,33,34 tion of the analyte can be expressed by the capacity factor ki The present paper deals with solutions of a charged, nonmi-
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