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Chem. Mater. 2007, 19, 2292-2298
Spherical Siliceous Mesocellular Foam Particles for High-Speed Size Exclusion Chromatography Yu Han, Su Seong Lee, and Jackie Y. Ying* Institute of Bioengineering and Nanotechnology, 31 Biopolis Way, The Nanos, Singapore 138669 ReceiVed December 22, 2006. ReVised Manuscript ReceiVed February 27, 2007
Spherical micrometer-sized siliceous mesocellular foam (MCF) particles with narrow window size distributions and ultralarge pore volumes were synthesized and used as packing materials for size exclusion chromatography (SEC). MCF columns exhibited excellent size exclusion properties in separating polystyrenes in organic media. The window size of spherical MCF could be tailored between 10 and 26 nm without affecting the particle morphology, allowing this packing material to be used for analyzing different molecular weight ranges. Compared to a conventional SEC system consisting of two polymerbased columns, a single, smaller MCF column could achieve similarly high resolution with significantly reduced analysis time and solvent consumption. The MCF column also exhibited better separation capability than a commercial silica-based SEC column of the same dimensions. These advantages could be attributed to the ultrahigh porosity of MCF materials. Spherical MCF particles were demonstrated to be an excellent packing material for SEC, especially for high-speed applications.
Introduction Size exclusion chromatography (SEC), or gel permeation chromatography (GPC), is a high-performance liquid chromatography (HPLC) technique commonly used for the separation and molecular weight (MW) determination of polymers, peptides, and proteins.1 The separation mechanism in SEC is based strictly on the molecular size of the analytes with respect to the pore size of the stationary phase. Molecules larger than the pore size of the stationary phase would be excluded from the porosity of the packing material and be eluted quickly. Molecules similar to the pores in dimension would enter the pores and be eluted next. Small molecules would enter the pores easily and be eluted last. SEC requires the column packing material to have narrow pore size distribution and large pore volume to achieve high selectivity and resolution in separations. This is different from interactive HPLC, in which the separation performance is mainly dependent on the surface area and the surface property of the packing material. For high column efficiency, packing materials in SEC should be uniform, fine, and spherical particles (3-20 µm), as in the common interactive HPLC. Polymer and silica can be used as packing materials in SEC columns. In general, silica-based packing materials are more versatile than polymeric packing materials because they can be used with various organic eluents and under a wide temperature range without being affected in pore size and particle morphology. Silica is also mechanically more robust than polymer and can endure a high backpressure. However, conventional silica-based packing materials generally have less porosity than polymer-based packing materials, resulting in lower resolutions. * Corresponding author. E-mail:
[email protected].
(1) Yau, W. W.; Kirkland, J. J.; Bly, D. D. Modern Size Exclusion Chromatography; Wiley: New York, 1979.
SEC is conventionally conducted using 2-4 columns of large dimensions (7.8 mm i.d. × 300 mm) connected in series. The use of these large column banks provides sufficient pore volume to achieve high resolution and accurate MW measurement.2 However, it requires long analysis time and significant solvent consumption. In recent years, there is increasing interest in developing high-speed SEC using a single column of smaller dimensions.3 Highspeed SEC would not only improve sample throughput substantially but also greatly reduce solvent usage. Thus, it would be very useful for combinatorial polymerization studies. Moreover, small SEC columns would allow for rapid analysis even under a low flow rate, which is very important for on-line SEC-mass spectrometry applications.3 Compared to normal SEC packing material, the packing material for high-speed SEC has more stringent requirements for high porosity, narrow pore size distribution, and uniform particle morphology so as to achieve satisfactory resolution with smaller column dimensions. However, it is challenging to produce highly porous materials with controlled pore size as well as uniform particle morphology. Usually, mechanical stability is compromised when porosity is increased, especially in the case of polymer-based packing materials. A series of mesoporous silicas with controlled pore size has been developed by supramolecular templating synthesis in the past decade. These materials are promising candidates as stationary phase in HPLC because of their large surface area and well-defined pores compared to the conventional porous silica used in HPLC. Mesoporous silicas such as MCM-41, MCM-48, FSM-16, APMS, and MSU-1 have been prepared in the form of spherical particles and evaluated in normal-phase or reverse-phase HPLC.4-11 However, there have been relatively few studies on the use of mesoporous (2) Yau, W. W.; Kirkland, J. J.; Bly, D. D.; Stoklosa, H. J. J. Chromatogr. 1976, 125, 219. (3) Barth, H. G. LC-GC Eur. 2003, 16, 46.
10.1021/cm063050x CCC: $37.00 © 2007 American Chemical Society Published on Web 04/05/2007
Spherical Siliceous MCF Particles for SEC
silicas in SEC, and limited separation capacity was reported.12,13 This is because most of the mesoporous silicas are limited in pore size ( 0.99 (Table 1), which was desired for high accuracy in MW determination. Polystyrene standards with MWs of 7.5 × 103 and 1.3 × 104 were used to evaluate the MCF columns. It was found that the MW determination errors in MCF-26, MCF-17, and MCF-10 columns were
