Immobilized Ionic Liquids as High-Selectivity/ High-Temperature/High-Stability Gas Chromatography Stationary Phases Jared L. Anderson and Daniel W. Armstrong* Department of Chemistry, Iowa State University, Ames, Iowa 50011 Ionic liquids (ILs) are a class of nonmolecular solvents in which the cation/anion combination can be easily tuned to provide desired chemical and physical properties. When used as stationary phases in gas-liquid chroma- tography, ionic liquids exhibit dual nature retention selectivity. That is, they are able to separate polar mol- ecules such as a polar stationary phase and nonpolar molecules such as a nonpolar stationary phase. However, issues such as optimization of the wetting ability of the ionic liquid on fused-silica capillaries, the maximum operating temperatures of the stationary phases, and nonuniform film thickness on the wall of the capillary at high temperatures have limited their use in gas chroma- tography. As described in this paper, these limitations are overcome by cross-linking a new class of ionic liquid monomers by free radical reactions to provide a more durable and robust stationary phase. By lightly cross- linking the ionic liquid stationary phase using a small amount of free radical initiator, high-efficiency capillary columns were produced that are able to endure high temperatures with little column bleed. Two types of cross- linked IL stationary phases are developed. A partially cross-linked stationary phase allows for high-efficiency separations up to temperatures of ∼280 °C. However, by creating a more highly cross-linked stationary phase of geminal dicationic ILs, exclusively, an increase in ef- ficiency is observed at high temperatures allowing for its use over 350 °C. In addition, through the use of solvation thermodynamics and interaction parameters, it was shown that the cross-linking/immobilization of the ionic liquid does not affect the selectivity of the stationary phase thereby preserving its dual nature retention behavior. Room-temperature ionic liquids (RTILs), formerly known as molten salts, are a class of nonmolecular ionic solvents with low melting points. Most common RTILs are composed of unsym- metrically substituted nitrogen-containing cations (e.g., imidazole, pyrrolidine, and pyridine) with inorganic anions (e.g., Cl - , PF 6 - , and BF 4 - ). Ambient-temperature ionic liquids based on the 1-alkyl- 3-methylimidazolium cation were first reported by Wilkes et. al. in 1982. 1 In the last 5-10 years, ionic liquids with widely varying cations and anions have been synthesized to provide specific physical and chemical characteristics for a variety of applications. With these so-called “designer solvents”, the physical properties and solvation interactions of the ionic liquid can be “tuned” by controlling the nature and functionality of the cation or anion. This ability has tremendous advantages, especially when using ionic liquids as solvent systems in organic synthesis 2-10 and in analytical chemistry. 11-18 Others have stated that ionic liquids possess low to negligible vapor pressures when used as solvents in organic chemistry. 2,5,10 Therefore, there has been interest in using ILs as stationary phases in gas-liquid chromatography. 19-27 Although RTILs are said to possess high volatilization temperatures, it is commonly observed that this is controlled by the cation and anion * To whom correspondence should be addressed. Phone: (515) 294-1394. Fax: (515) 294-0838. E-mail: sec4dwa@iastate.edu. (1) Wilkes, J. S.; Levisky, J. A.; Wilson, R. A.; Hussey, C. L. Inorg. Chem. 1982, 21, 1263. (2) Welton, T. Chem. Rev. 1999, 99, 2071. (3) Hussey, C. L. Pure Appl. Chem. 1988, 60, 1763. (4) Seddon, K. R. J. Chem. Technol. Biotechnol. 1997, 68, 351. (5) Chiappe, C.; Pieraccini, D. J. Phys. Org. Chem. 2005, 18, 275-297. (6) Welton, T.; Smith, P. J. Adv. Organomet. Chem. 2004, 51, 251-284. (7) Davis, J. H. Chem. Lett. 2004, 33, 1072-1099. (8) Wilkes, J. S. J. Mol. Catal. A: Chem. 2004, 214, 11-17. (9) Cole, A. C.; Jensen, J. L.; Ntai, I.; Tran, K. T.; Weaver, K. J.; Forbes, D. C.; Davis, J. H. J. Am. Chem. Soc. 2002, 124, 5962-5963. (10) Handy, S. T.; Okello, M. J. Org. Chem. 2005, 70, 2874-2877. (11) Stalcup, A. M.; Cabovska, B. J. Liq. Chromatogr. Related Technol. 2004, 27, 1443-1459. (12) Gutowski, K. E.; Broker, G. A.; Willauer, H. D.; Huddleston, J. G.; Swatloski, R. P.; Holbrey, J. D.; Rogers, R. D. J. Am. Chem. Soc. 2003, 125, 6632- 6633. (13) Liu, J.-F.; Jonsson, J. A.; Jiang, G.-B. TrAC, Trends Anal. Chem. 2005, 24, 20-27. (14) Luo, H.; Dai, S.; Bonnesen, P. V.; Buchanan, A. C.; Holbrey, J. D.; Bridges, N. J.; Rogers, R. D. Anal. Chem. 2004, 76, 3078-3083. (15) Carda-Broch, S.; Berthod, A.; Armstrong, D. W. Anal. Bioanal. Chem. 2003, 375, 191-199. (16) Armstrong, D. W.; Zhang, L. K.; He, L.; Gross, M. L. Anal. Chem. 2001, 73, 3679-3686. (17) Li, Y. L.; Gross, M. L. J. Am. Soc. Mass Spectrom. 2004, 15, 1833-1837. (18) Mank, M.; Stahl, B.; Boehm, G. Anal. Chem. 2004, 76, 2938-2950. (19) Pacholec, F.; Butler, H. T.; Poole, C. F. Anal. Chem. 1982, 54, 1938. (20) Pacholec, F.; Poole, C. F. Chromatographia 1983, 17, 370-374. (21) Armstrong, D. W.; He, L.; Liu, L.-S. Anal. Chem. 1999, 71, 3873-3876. (22) Berthod, A.; He, L.; Armstrong, D. W. Chromatographia 2001, 53, 63. (23) Heintz, A.; Kulikov, D. V.; Verevkin, S. P. J. Chem. Eng. Data 2001, 46, 1526-1529. (24) Heintz, A.; Kulikov, D. V.; Verevkin, S. P. J. Chem. Thermodyn. 2002, 34, 1341-1347. (25) Anderson, J. L.; Armstrong, D. W. Anal. Chem. 2003, 75, 4851-4858. (26) Ding, J.; Welton, T.; Armstrong, D. W. Anal. Chem. 2004, 76, 6819. (27) Mutelet, F.; Butet, V.; Jaubert, J.-N. Ind. Eng. Chem. Res. 2005, ACS ASAP. Anal. Chem. 2005, 77, 6453-6462 10.1021/ac051006f CCC: $30.25 © 2005 American Chemical Society Analytical Chemistry, Vol. 77, No. 19, October 1, 2005 6453 Published on Web 09/01/2005