Linear elution adsorption chromatography : VII. gradient elution theory

Abstract

The fundamental correlational equations previously developed for normal, linear elution from alumina, silica, and Florisil have been extended to include the technique of gradient elution. It has been shown that in general the eluent gradient should be of the linear strength form. The theoretical advantages of linear strength gradient elution include: (I) approximately equal spacing of solute peaks according to increasing solute adsorption energy,providing maximum resolution between both weakly and strongly adsorbing solutes with minimum separation time; (2) approximately equal band widths for maximum detection sensitivity; (3) no band splitting with development of spurious double peaks; (4) easily calculable retention volumes for solutes whose adsorption parameters have been measured or can be estimated. For some sample types a mildly convex eluent strength gradient is predicted to give a better separation system, although the advantage over the corresponding linear strength case will normally be small. The use in gradient elution of solvents of widely differing strengths can lead to displacement, rather than elution. with resulting loss of resolution for weakly adsorbing. The expermental condition required for displacement to occor are discussed, as is the avoidance of this phenomenon through various means including the use of ternary solvent mixtures. The theoretical stregnth of such ternary eluent has been derived as a function of the eluent composition and the stregnth of the pure constituent solvent.