Anal. Chem. 2007, 79, 5462-5467
Open Tubular Anion Exchange Chromatography. Controlled Layered Architecture of Stationary Phase by Successive Condensation Polymerization Petr Kuba´nˇ,*,† Purnendu K. Dasgupta,*,‡ and Christopher A. Pohl§
Department of Chemistry and Biochemistry, Mendel University of Agriculture and Forestry, Zemeˇ deˇ lska´ 1, CZ-61300, Brno, Czech Republic, Department of Chemistry and Biochemistry, University of Texas at Arlington, Arlington, Texas 76019-0065, and Dionex Corporation, P.O. Box 3603, Sunnyvale, California 94088-3603
The preparation and performance of a multilayered stationary phase for open tubular anion exchange chromatography in relatively large bore (75 µm diameter) columns are described. The inner surface of a fused-silica capillary tube is coated with up to 25 successive porous polymeric layers formed by condensation polymerization of a primary amine with a diepoxide. Each layer of the anion exchange stationary phase consists of copolymer of methylamine (MA) and 1,4-butanedioldiglycidyl ether (BDDE). The polymer layers are sufficiently porous or permeable; each successive layer of the stationary phase incrementally increases the observed column capacity and chromatographic performance in the open tubular mode. Even though the column inner diameter is far from optimum for open tubular liquid chromatography, we demonstrate the baseline separation of a suite of inorganic anions (F-, Cl-, NO2-, Br-, NO3-) in a 5 m × 75 µm column coated with 25 layers of the anion exchange polymer using 1 mM KOH eluent and suppressed contactless conductometric detection at a flow rate of 1 µL/ min (operating pressure of ∼1 bar) with a plate count of >30 000. Strategies for construction of microsuppressor devices used in open tubular ion chromatography are discussed. Since its introduction in 1975 by Small et al.1 ion chromatography (IC) has developed into a versatile analytical technique for the determination of ionic species. At the present time, IC is overwhelmingly the analytical tool of choice for ion analysis, notably anion analysis. Since the early years, following the pioneering work of Horvath and co-workers2,3 in liquid chromatography, there has been a continuing interest to carry out IC on a miniature scale. Capillary IC, especially in the open tubular format, however, has not yet emerged beyond research laboratories due both to theoretical and * Corresponding authors. E-mail:
[email protected] (P.K.);
[email protected] (P.K.D.). † Mendel University of Agriculture and Forestry. ‡ University of Texas at Arlington. § Dionex Corporation. (1) Small, H.; Stevens, T. S.; Baumann, W. C. Anal. Chem. 1975, 47, 1801. (2) Horva´th, C. G.; Preiss, B. A.; Lipsky, S. R. Anal. Chem. 1967, 39, 1422. (3) Horva´th, C. G.; Lipsky, S. R. Anal. Chem. 1969, 41, 1227.
5462 Analytical Chemistry, Vol. 79, No. 14, July 15, 2007
practical limitations. In yesteryears, the lack of nanoliter volume injectors, stable low flow rate chromatographic pumps, detectors of suitably small volumes, even the availability of narrow bore capillaries and appropriate chemistry for functionalizing the interior were all the limiting factors. Injection valves capable of injection volumes of 10-20 nL are now commercially available (see, e.g., www.vici.com). Additionally, miniature low-pressure injection valves, capable of reproducibly injecting nanoliter sample volumes, can be easily fabricated.4 Capillaries of i.d.’s ranging up from 2 µm are currently available from several suppliers. Pumping systems have also undergone considerable changes. Dispensing pumps based on microsyringes or a new generation of bidirectional precision pumps5 are commercially available. Electroosmotically driven pumps provide a unique means to achieve stable low flow rates in a pulseless manner with flow rates variable over a very large range.6-13 When properly designed, electroosmotic flow (EOF) pumps can be used to generate very high backpressures and thus be particularly useful for chromatography. However, a true low-pressure system that will nevertheless provide reasonable performance is still alluring. Conductometry and other electrochemical methods (amperometry, potentiometry) would be the preferred detection modes in capillary IC. Conductometry in particular is most generally applicable, and all of the above techniques can be downscaled without penalties. As far back as 1983, Manz and Simon14 demonstrated successful open tubular IC (OTIC) with a potentiometric detector consisting of an end-column microelectrode with an effective detection volume of
