Liquid Chromatography - Macroporous Polymers 1
Short chain reverse phases reduce the extent to which proteins are denatured in the separation of substances of biological origin, it is seen by the chromatogram from the C2 reverse phase, that a serious price must be paid in loss of resolution if the nature of the separation demands the use of such material. However, the development of the polymer packings have, at last, partly solved this problem.
In general, because the brush type phases can be synthesized in a more reproducible manner, particularly if carried out in a fluidized bed, the brush phases are generally recommended for the majority of applications. For high retentive capacity and for systems that will be operated with aqueous solvent mixtures having a very high water content, the bulk phases might be preferred. The partially reacted, low carbon content bulk phase also have special areas of application particularly in sample preparation.
Polymeric ion exchange materials were developed for chromatography in the early sixties resulting in the introduction of macro-porous polymers (29-31). The advantages of this material lay in the macro-porous nature of the resin packing, which consisted of resin particles a few microns in diameter, which, in turn, comprised of a fused mass of polymer micro-spheres a few Angstroms in diameter. The resin polymer micro-spheres play the same part as the silica gel primary particles, and confer on the polymer a relatively high surface area together with a high porosity. The high surface area provided increased solute retention and selectivity together with a superior loading capacity and, consequently, a wide dynamic range of analysis. The material consists of a highly cross-linked polystyrene resin with about a 50Å pore diameter. In the case of the ion exchange materials, inorganic groups of appropriate charge were chemically attached (e.g., by sulfonation). The initial resins manufactured by Rohm and Haas were called Amberlite but were largely employed in production processes and, consequently, had very large particle diameters. Modern macroporous resin-type stationary phases have a range of particle diameters which can be as small as 2.5 or 3.0 microns. The more popular resin based packings are based on the co-polymerization of polystyrene and divinylbenzene. The degree of cross-linking determines the rigidity of the resin and the greater the cross-linking the harder the resin becomes. Ultimately, at extremely high cross-linking, the resin becomes brittle. To produce an ion exchange resin, the surface of the polystyrene-divinylbenzene copolymer is reacted with suitable reagents and covered with the required ionogenic interacting groups.