Separation efficiency porous silica columns

These have now been superseded by capillary columns, which offer greatly improved separation efficiency. Fused silica capillary tubes are used which have internal diameters ranging from 0.1 mm (small bore) to 0.53 mm (large bore) with typical lengths in excess of 20 m. The wall-coated open tubular (WCOT) columns have the internal surface of the tube coated with the liquid (stationary) phase and no particulate supporting medium is required. An alternative form of column is the porous-layer open tubular (PLOT) column, which has an internal coating of an adsorbent such as alumina (aluminium oxide) and various coatings. Microlitre sample volumes are used with these capillary columns and the injection port usually incorporates a stream splitter. 

Nimura, N., Itoh, H., Kinoshita, T., Nagae, N., Nomura, M. (1991). Fast protein separation by reversed-phase high-performance liquid-chromatography on octadecylsilyl-bonded non-porous silica-gel — effect of particle-size of column packing on column efficiency. J. Chromatogr. 585(2), 207-211. 

In contrast to IEX, RPLC protein separations show good efficiency using either nonporous (Wall et al., 2000) or porous (Liu et al., 2002 Millea et al., 2005) silica columns, with peak capacities of approximately 100. Although not equivalent to small molecule separations, including peptides, this performance is not the main... 

As a further test of the etched open tubular approach for the analysis of optical isomers, another column was fabricated based on the selector naphthylethylamine that had been attached to porous silica by the silanization/hydrosilation method for use in HPLC [70]. As in the HPLC experiments, this column was best suited for the resolution of the optical isomers of dinitrobenzoyl methyl esters of amino acids. The best separation (a = 1.14) was obtained for the alanine derivative. In addition, the peak symmetry and efficiency for the naphthylethylamine column was significantly better than that obtained on the cyclodextrin column. However, as shown in HPLC experiments, changes in the amino acid moiety (replacing alanine with valine, etc.) often results in a loss of chiral resolution. In the case of optical isomers, the separation mechanism in HPLC and CEC modes is identical since only interaction between the solute and the bonded phase can result in resolution of the enantiomers. 

Venema et al. studied SEEC with porous silica particles [14,15]. They separated narrow polystyrene standards on columns packed with particles of different pore size, and observed a significant improvement of the efficiency in SEEC over that in pressure-driven size-exclusion chromatography. Also, they observed that the efficiency improvement was more significant for large-pore particles and related this to a higher pore flow. 

As an alternative to wholly porous sub-2-pm particles, 1.7-pm fused-core particles surrounded by a 0.5-pm porous silica layer with 90 A pores, have emerged. In a comparative study, substantially lower back pressure was reported when the fused-core particles were used. This allowed for columns to be coupled in series, which increased the peak efficiency up to 92,750 plates (164). Wider-pore fused-core particles have an average pore diameter of 160 A. The wider-pore particles are particularly useful for increased sample loading and the rapid separation of peptides using volatile mobile phases (165). 

Because of unfavorable mass transfer properties in liquids, highly efficient separations and short separation times potentially available for open tubular columns can be realized only in columns of small internal diameter (< 25 xm) [309]. These columns have very low phase ratios and serious detection problems arise. Several methods have been proposed to Increase the surface area, and hence the stationary phase capacity, by chemical etching of the interior wall [335] or by adhesion of a thin porous silica or polymer layer to the wall [336-338]. The sol-gel process allows an increase in surface area and formation of a retentive chemically bonded phase in a single step. None of these processes, however, adequately address the problems of low retention, low sample capacity, poor sample detectability, and unfavorable handling characteristics that prevent wider use of open tubular columns in capillary electrochromatography. 

It should be mentioned that there is another type of relatively new column that is made from the 2.7-pm fused-core silica particles, bonded with C18 alkyl chains, by fusing a 0.5-pm porous silica layer onto 1.7-pm non-porous silica cores. The selectivity of the fused-core particle columns is very similar to that of certain <2-pm C18 columns and has the advantage of a substantially lower back-pressure at much higher flow rates, which allows rapid separations to be performed even routinely on a conventional LC system without significant loss in efficiency or resolution. The fused-core columns are new to antibiotic analysis and may serve as good alternatives to <2-pm columns in the field. 

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