Article pubs.acs.org/ac
Capillary Ion Chromatography at High Pressure and Temperature Bert Wouters,† Cees Bruggink,‡ Gert Desmet,† Yury Agroskin,§ Christopher A. Pohl,§ and Sebastiaan Eeltink*,† †
Vrije Universiteit Brussel, Department of Chemical Engineering, Pleinlaan 2, B-1050 Brussels, Belgium Thermo Fisher Scientific, Takkebijsters 1, 4817 BL Breda, The Netherlands § Thermo Fisher Scientific, 501 Mercury Drive, Sunnyvale, CA, United States ‡
ABSTRACT: The application of high pressure and temperature in ion chromatography (IC) can significantly improve the efficiency and reduce the analysis time. In this work, the kinetic-performance limits of capillary IC columns with inner diameters of 400 μm packed with 4 and 7 μm macroporous anion-exchange particles were investigated employing a capillary ion-exchange instrument allowing column pressures up to 34 MPa and column temperatures up to 80 °C. Plate heights below 10 μm could be realized using capillary columns packed with 4 μm particles. Compared to conventional IC using 7 μm particles and pressures up to 21 MPa, a 40% improvement in plate number could be achieved when working at the kinetic performance limits at 34 MPa and using columns packed with 4 μm particles. Using coupled columns with a total length of 400 mm, a mixture of seven anions was separated within 7.5 min while yielding 20 000 plates. Increasing the temperature improved the performance limits when operating in the C-term region (for fast IC separation using columns 50% for today’s particles. In addition, recent instrument developments have led to alkaline mobile-phase compatible IC systems of polyetheretherketone (PEEK) allowing a maximum operating pressure of 34 MPa. The separation performance in high-performance chromatography is typically visualized by a van Deemter curve, showing the plate height as a function of mobile-phase
on-exchange chromatography (IC) is widely used for the separation of mixtures of ions and/or ionizable molecules, ranging from small inorganic ions to large proteins.1−3 Retention is mainly governed by the interaction of both charged analytes and competing ions in the mobile phase with the stationary phase. Stationary phases for IC can be subdivided into two main categories, i.e., silica- and polymer-based materials. The polymeric materials are stable over a wide pH range, which is a clear advantage over silica materials, especially when employing alkaline mobile phases.1,4 The most common polymer resins are cross-linked particles prepared from styrene, methacrylate, or vinyl alcohol precursors.5,6 To optimize the separation efficiency and ion-exchange capacity, much effort has been put into the optimization of the surface chemistry and architecture of the stationary phase. The first generation of polymer resins was derivatized to introduce charged groups at the surface. Afterward, the substrate was brought into contact with colloidal latex particles of opposite charge, to form the final surface chemistry via electrostatic interaction.7−10 To further increase the capacity, the same architecture was established using ultrawide-pore resins (1000−3000 Å).11,12 To minimize intraparticle diffusion while maintaining the high ion-exchange capacity, hyperbranched polymer particles containing different layers of chemistries were developed.13 The charged surface of the polymer resin is first shielded via electrostatic interactions with polymer chains containing amine functionalities. A hyper© 2012 American Chemical Society
Received: June 11, 2012 Accepted: July 25, 2012 Published: July 25, 2012 7212
dx.doi.org/10.1021/ac301598j | Anal. Chem. 2012, 84, 7212−7217
Analytical Chemistry
Article
nitrate, 600 μg/L phosphate, and 300 μg/L for sulfate for the seven anion standard. Capillary columns (150 mm × 400 μm I.D. and 250 mm × 400 μm I.D.) packed with 4 and 7 μm anion-exchange AS18 particles were kindly provided by Thermo Fisher Scientific (Sunnyvale, CA, USA). The 4 μm anion-exchange resin are macro-porous particles (1500 Å) coated with 65 nm nanobeads containing quaternary ammonium functionalities. The 7 μm beads contain 2000 Å macropores. Chromatography. Ion chromatography was performed using a capillary ICS-5000 system from ThermoFisher Scientific (Sunnyvale, CA, USA). This setup consisted of an autosampler, a capillary IC dual-piston pump, an eluent-generation module, and a temperature-controlled IC cube module (integrating the capillary EG degasser, an injection valve equipped with a 400 nL loop, the separation column, an anion capillary-electrolytic suppressor, a carbonate removal device, and a conductivity detector). The sample was delivered from the autosampler to a 4-port injection valve for a full-loop injection. Capillary IC separations were performed in isocratic mode using KOH concentrations ranging between 15 and 35 mM. The potassium hydroxide mobile phase was generated electrically online from Milli-Q water (18.5 MΩ) using an eluent generator equipped with a potassium hydroxide cartridge. Post column, the potassium ions were removed from the eluent and replaced with hydronium ions using an ACES 300 electrolytic suppressor prior to conductivity-detection measurements performed using a 20 nL flow cell. The system was controlled by Chromeleon 6.80 (SR11) Chromatography Data System software.
velocity.15 Although different contributions (A-, B-, and Cterm) to the overall dispersion can be distinguished, the plot does not take into account the effect of column permeability on efficiency and time. As an alternative, Giddings proposed a kinetic plot visualizing the time required to generate a specific plate number in HPLC and GC.16 This plot incorporates all possible column-length and particle-size combinations, while operating at the maximum system pressure. This approach has been refined in Hans Poppe’s seminal paper “Some reflections on speed and efficiency of modern chromatographic methods” discussing the performance limits of pressure- and electrodriven LC methods.17 In 2005, Desmet et al. developed a method to establish kinetic plots without the need for recursive computer algorithms and used it to compare performance limits of packed and monolithic columns.18−20 It was demonstrated that each range of desired plate numbers has its own characteristic reference length (such as particle diameter) and external porosity. In ion chromatography, most optimization studies involve research in the use of new stationary phases and matching mobile-phase conditions to achieve the desired selectivity for selected applications. One of the great achievements in IC is the development of migration-behavior models of ions in isocratic and (multistep) gradient mode.21−24 These models in combination with electrolytic eluent generation25 make the technique widely applicable and method development much easier. Recently, the group of Hilder et al. showed the feasibility of the kinetic-plot approach in isocratic IC mode employing standard analytical scale (3 and 4 mm I.D.) columns.26 Although the minimum plate height of the column packed with 5 μm particles was higher than 20 μm, it provided the best results for fast separation yielding plate numbers of 0.99) was observed. Moreover, the slope corresponds with the ion charge.30 The effect of temperature on retention is described by the Van ’t Hoff equation:
described in ref 34. Interestingly, the slope for PO43− at 80 °C does not correspond to its valence, which possibly may be explained by the different degree of ionization.
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CONCLUDING REMARKS Capillary anion-exchange columns yielding reduced plate heights below 2.2 were employed to visualize the performance limits in ion chromatography while operating at the maximum system pressure and temperature settings. Compared to conventional IC systems (ΔPmax = 21 MPa), a significant decrease in analysis time could be obtained when operating at the performance limits of the high-pressure system. Increasing the temperature was only useful when operating short columns high in the C-term region. To achieve a critical-pair IC separation within the shortest analysis time, a selectivity plot, depicting log k versus ion strength, can be employed to select the desired ion strength. Kinetic plots then allow the selection of the optimal column packing, column length, and operating pressure to generate a specific number of plates in the shortest possible time. IC columns are typically operated at a flow rate (high) in the C-term region, whereas the best chromatographic performance is always obtained when working at the flow rate in the van Deemter minimum. To improve the trade-off in time and plate
d ln k dH = (1) dT RT 2 where R is the universal gas constant and H is the enthalpy of exchange. Both exothermic (decrease in k with temperature) and endothermic (increase in k) behavior have been observed in ion-exchange chromatography,31,32 in contrast to the pure exothermic behavior observed in RP-LC.33 When comparing the retention of monovalent anions (Figure 6A), it is apparent that the retention of early eluting anions increases with increasing temperature, whereas for late-eluting anions a decrease in retention factor was observed. Apparently, this effect is mainly governed by ion diameter (in solution). It should be noted that changing the temperature also may affect the degree of ionization and solvation, which may vary for different analytes. However, since the analytes (F−, Cl−, NO3−) are similar in nature and the slope corresponds with the ion charge, we speculate that this effect is small. The retention factor for di- and trivalent anions increased significantly with increasing temperature (see Figure 6B), which confirms results 7216
dx.doi.org/10.1021/ac301598j | Anal. Chem. 2012, 84, 7212−7217
Analytical Chemistry
Article
(20) Eeltink, S.; Desmet, G.; Vivó-Truyols, G.; Rozing, G. P.; Schoenmakers, P. J.; Kok, W. T. J. Chromatogr., A 2006, 1104, 256− 262. (21) Haddad, P. R.; Cowie, C. E. J. Chromatogr. 1984, 303, 321−330. (22) Madden, J. E.; Haddad, P. R. J. Chromatogr., A 1999, 850, 29− 41. (23) Breadmore, M. C.; Hilder, E. F.; Macka, M.; Avdolovic, N; Haddad, P. R. Electrophoresis 2001, 22, 503−510. (24) Shellie, R. A.; Ng, B. K.; Dicinoski, G. W.; Poynter, S. D. H.; O’Reilly, J. W.; Pohl, C. A.; Haddad, P. R. Anal. Chem. 2008, 80, 2474−2482. (25) Sjörgen, A.; Boring, C. B.; Dasgupta, P. K.; Alexander, J. N. Anal. Chem. 1997, 69, 1385−1391. (26) Causon, T. J.; Hilder, E. F.; Shellie, R. A.; Haddad, P. R. J. Chromatogr., A 2010, 1217, 5057−5062. (27) Causon, T. J.; Hilder, E. F.; Shellie, R. A.; Haddad, P. R. J. Chromatogr., A 2010, 1217, 5063−5068. (28) Robinson, C.; Watson, P. J. Dent. Res. 2005, 84, 1086−1087. (29) Bristow, P. A.; Knox, J. H. Chromatographia 1977, 10, 279−289. (30) Pohl, C. A.; Stillian, J. R.; Jackson, P. E. J. Chromatogr., A 1997, 789, 29−41. (31) Yu, H.; Li, R. Chromatographia 2008, 68, 611−616. (32) Kulisa, K. Chem. Anal. 2004, 49, 665−689. (33) Zhu, C.; Goodall, D. M.; Wren, S. A. C. LC-GC 2005, 8, 48−59. (34) Hatsis, P.; Lucy, C. A. J. Chromatogr., A 2001, 920, 3−11.
number, columns should be operated at uopt and the column length should be adjusted such that the maximum operating pressure is applied. This implies that to reach the optimal conditions for IC columns packed with 4 μm particles and a maximum system pressure of 34 MPa, a column length of approximately 100 cm should be used yielding 125 000 plates in a t0 time of 30 min. Therefore, column coupling in IC may be an important aspect that needs to be addressed to make the next performance leap. Furthermore, to speed-up separations, while operating at their kinetic performance limit, there is a need for IC columns packed with smaller (sub-3 μm) particles that exhibit improved diffusional properties. Possibly, this maybe be addressed with development of fused-core IC particle technology.
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +32 (0)2 629 3324. Fax: +32 (0)2 629 3248. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Support of this work by a grant of the Research Foundation Flanders (G.0919.09) is gratefully acknowledged. B.W. acknowledges the Institute for Promotion of Innovation through Science and Technology in Flanders (IWT-Flanders) for a research grant. Dr. Hamed Eghbali (VUB), Dr. Kelly Flook, and Franck van Veen (Thermo Fisher Scientific) are gratefully acknowledged for helpful discussions.
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