Bonded Phases - The Wetting Characteristics of Brush and Bulk Reverse Phases
The ready wettability of the ODS phase, though far from ideal for many chromatographic applications because of its susceptibility to erosion, can, nonetheless, chromatographically, be extremely useful. Because it is wettable by pure water, it is possible to directly add it to a water sample and be able to extract and isolate any organic materials that may be contaminants. The ODS material can then be removed by filtration and the adsorbed material can then be extracted with the aid of an appropriate dispersive solvent. The efficiency of extraction, using this procedure, can be very high, (e.g.>98%), and as a consequence. can be advantageous in analyzing water samples for impurities in environmental studies. Such material can eliminate the need for small extraction tubes or columns that can serve the same purpose but as a result of their poor wettability require the sample liquid to be forced through them under pressure.
The resolution obtained from different types of reverse phase, carrying varied carbon contents and bonded moieties with diverse chain lengths, is demonstrated in the chromatograms given in figure 8. The higher retention capability of the bulk phase ODS2 phase is clearly shown. Recalling that for biotechnology separations, there is a real need for short chain reverse phases to reduce the probability of proteins being denatured, it is clear from the chromatogram obtained from the C2 reverse phase, that there will be a serious price to be paid in retention and resolution loss if, due to the nature of the separation, the use of such material is necessary. The development of an adequately chromatographically retentive material that does not cause the denaturing of proteins may be best solved by the use of polymeric stationary phases.
Figure 8. Chromatograms from Brush Phases Carrying Different Carbon Loads and Different Chain Lengths
Generally, because the brush type bonded phases can be synthesized to provide a more reproducible product, (particularly when carried out in a fluidized bed) the brush phases are usually recommended for the majority of applications. For improved retention and for systems that must be operated with aqueous solvent mixtures with very high water contents, the bulk phases might well be preferred. The partially derivatized, low-carbon content, bulk phase, such as the ODS, will have special areas of application particularly in sample preparation.