Anal. Chem. 2002, 74, 1241-1248
Electrokinetic Chromatography Utilizing Two Pseudostationary Phases Providing Ion-Exchange and Hydrophobic Interactions Philip Zakaria, Miroslav Macka, and Paul R. Haddad*
Australian Centre for Research on Separation Science, School of Chemistry, University of Tasmania, GPO Box 252-75, Hobart, Tasmania 7001, Australia
The electrokinetic chromatographic separation of a series of inorganic and organic anions was achieved by utilizing an electrolyte system comprising a cationic soluble polymer (poly(diallydimethylammonium chloride, PDDAC) and a neutral β-cyclodextrin (β-CD) as pseudostationary phases. The separation mechanism was a combination of electrophoresis, ion-exchange (IE) interactions with PDDAC, and hydrophobic interactions with β-CD. The extent of each chromatographic interaction was independently variable, allowing for control of the separation selectivity of the system. IE interactions could be varied by changing either the PDDAC concentration or the concentration of a competing ion (e.g., chloride) in the BGE, while the hydrophobic interactions could be varied by changing the concentration of β-CD. The separation system was very robust, with the reproducibility of the migration times being µ(nitrate) µ(nitrate) > µ(iodide) µ(ethylbenzenesulfonate) > µ(tert-butylbenzoate) µ(tert-butylbenzoate) > µ(ethylbenzenesulfonate) µ(toluenesulfonate) > µ(propylbenzoate) µ(propylbenzoate) > µ(toluenesulfonate)
0 0.7 0
0 0 1.5
150 50 150
1.0
0
0
3.6
1.0
0
6b(ii) 6c(i) 6c(ii)
50 150 50
(i.e., high percent PDDAC and low [Cl-]) since the sulfonates have higher IE selectivity coefficients than the carboxylates. The reverse migration order should be achievable when the BGE contains no PDDAC, but with some β-CD since tert-butylbenzate interacts more strongly with the β-CD than ethylbenzenesulfonate. From Table 3, it can be seen that the predicted BGE compositions agree with these points. Similar trends can be seen in Figure 6c for the separation of propylbenzoate and toluenesulfonate. In the case where a particular migration order cannot be achieved, the optimization yields conditions that provide the separation that is closest to the desired order, usually by bringing the two analytes as close together as possible. An example of this is the separation of benzoate and toluate, which can be achieved Analytical Chemistry, Vol. 74, No. 6, March 15, 2002
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only with benzoate migrating faster than toluate. It should also be noted that the process used to attain a desired migration order considered only the analytes in question, and the identified optimal conditions could therefore result in comigration of an analyte with another component of the sample. Again, a straightforward modification of the optimization algorithm could be used to overcome this deficiency. In a similar manner, the process used here could also be adapted to find the conditions needed to elute a particular analyte in the shortest possible time. CONCLUSIONS The MM-EKC system consisting of PDDAC and β-CD combines two complementary chromatographic separation mechanisms with an electrophoretic separation mechanism. The IE and hydrophobic interactions between anionic analytes and both pseudo-SPs can be varied independently and used to control the separation selectivity of the system using the parameters percent PDDAC, [Cl-], and [β-CD]. A mathematical model has been derived that adequately describes the analyte mobilities under changing BGE conditions, and this model can be used to optimize the conditions for separation of an analyte mixture or to obtain conditions where desired peak pairs migrate in a particular order. The described separation system is potentially applicable to any analytes exhibiting different interactions with the PDDAC and
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the cyclodextrin. One particular area of application could be the analysis of pharmaceutically important compounds, especially in the area of enantiomeric separations. The ability to control selectivity via different analyte-SP interactions could potentially allow for both an enantioselective separation and the separation of closely migrating compounds. It should be noted that although the system described here provides control of separation selectivity, it is applicable only to those analytes showing appreciable interactions with one or both of the pseudo-SPs. A further limitation is the lack of electrophoretic mobility of the neutral β-CD, which restricts the degree to which interactions with β-CD can be used to alter analyte mobilities. This could be overcome by using a cationic CD that migrated in the direction opposite to the analyte anions, but this could also possibly introduce further IE interactions, adding to the complexity of the system. ACKNOWLEDGMENT We are grateful to Professor J. S. Fritz of Iowa State University for helpful discussions. Received for review August 10, 2001. Accepted December 13, 2001. AC0109016
