Anal. Chem. 1997, 69, 628-635
Fundamental Considerations of Packed-Capillary GC, SFC, and LC Using Nonporous Silica Particles Yufeng Shen, Ying J. Yang, and Milton L. Lee*
Department of Chemistry, Brigham Young University, Provo, Utah 84602
In this study, nonporous silica (NPS) particles with diameters from 5 to 25 µm were packed into fused silica capillary columns of 250 µm i.d. using a CO2 slurry packing method. The column efficiency in LC was investigated using least-squares analysis of the experimental data according to the Giddings and van Deemter equations, and the results were compared with those obtained from columns containing porous silica (PS) particles. Less resistance to mass transfer was found for columns packed with NPS particles. For GC and SFC conditions, the van Deemter equation was modified by considering the compressibility of the mobile phase, and the modified equations were used to explain experimental results. The columns packed with NPS particles produced smaller resistance to solute mass transfer and higher optimum mobile phase linear velocities than those containing PS particles under SFC and GC conditions. In GC, it was found that the optimum mobile phase linear velocity increased with increasing particle size. The column permeability was investigated under GC, SFC, and LC conditions by measuring the pressure drop, and it was found that much lower pressure drops were produced along columns packed with NPS particles than with PS particles. Microcolumn separations can be performed using open or packed-capillary columns.1,2 In open-tubular chromatographies, in which the difusivities of solutes in the mobile phase decrease from gases to supercritical fluids to liquids, the inner diameters of the columns must be decreased from 500-100 µm for gas chromatography (GC)3,4 to 100-25 µm for supercritical fluid chromatography (SFC),5,6 and to 10-5 µm for liquid chromatography (LC).7,8 For GC, the open-tubular column is the major column type used for separation of complex samples, and it played a very important role in the development of SFC. However, the use of open-tubular columns becomes difficult in high-density SFC (1) Novotny, M.; Ishii, D. Microcolumn Separations; Elsevier: Amsterdam, 1985; p 3. (2) Yang, F. J. Microbore Column Chromatography: A Unified Approach to Chromatography; Marcel Dekker: New York, 1989; Chapter 1. (3) Lee, M. L.; Yang, F. J.; Bartle, K. D. Open Tubular Column Gas Chromatography; Wiley: New York, 1984; Chapter 1. (4) Poole, C. F.; Poole, S. K. Chromatography Today; Elsevier: Amsterdam, 1991; p 132. (5) Novotny, M.; Springston, S. R.; Peaden, P. A.; Fjeldsted, J. C.; Lee, M. L. Anal. Chem. 1981, 53, 407A. (6) Analytical Supercritical Fluid Chromatography and Extraction, Chromatography Conferences; Lee, M. L., Markides, K. E., Eds.; Provo, UT, 1990; Chapter 2. (7) Tock, P. P. H.; Stegeman, G.; Peerboom, R.; Poppe, H.; Kraak, J. C.; Unger, K. K. Chromatographia 1987, 24, 617. (8) Swart, R.; Kraak, J. C.; Poppe, H. Chromatographia 1995, 40, 587.
628 Analytical Chemistry, Vol. 69, No. 4, February 15, 1997
and LC. The problems include the preparation of extremely narrow-bore open-tubular columns, their small sample and retention capacities, and the serious extracolumn contributions to peak broadening. The packed column is the major column type used in LC, and uniform and mechanically strong microparticulate packings (3-10 µm) with a wide variety of chemical properties provide high performance.9 In SFC, packed columns containing microparticulate packings (5-10 µm) have also been successfully used, although the pressure drop effect on the mobile phase can be a disadvantage of this column type.10,11 In the last 30 years, microparticulate particles with particle sizes of
