Chromatography improving mobile-phase selectivity

New Techniques for Improving Mobile-Phase Selectivity in Reversed-Phase Chromatography... 

Efficiency the organic modifier can be used to adjust solvent selectivity as normally practiced in reversed-phase chromatography. Lowers mobile-phase viscosity and improves solute mass-transfer kinetics. 

Silica has often been modified with silver for argentation chromatography because of the additional selectivity conferred by the interactions between silver and Jt-bonds of unsaturated hydrocarbons. In a recent example, methyl linoleate was separated from methyl linolenate on silver-modified silica in a dioxane-hexane mixture.23 Bonded phases using amino or cyano groups have proved to be of great utility. In a recent application on a 250 x 1-mm Deltabond (Keystone Scientific Belief onte, PA) Cyano cyanopropyl column, carbon dioxide was dissolved under pressure into the hexane mobile phase, serving to reduce the viscosity from 6.2 to 1 MPa and improve efficiency and peak symmetry.24 It was proposed that the carbon dioxide served to suppress the effect of residual surface silanols on retention. 

When compared to the batchwise preparative chromatography, Simulated moving bed (SMB) units exhibit a number of advantages. These advantages are primarily because of the continuous nature of the operation and the efficient use of the stationary and mobile phases, which allows a decrease in desorbent requirement and an improvement of the productivity per unit time and per unit mass of stationary phase. In addition, high performances can be achieved even at rather low values of selectivity and with a relatively small number of theoretical plates. Due to these positive features, SMB is particularly attractive in the case of enantiomer separations, since it is difTicult to separate enantiomers by conventional techniques. More recent applications related to chiral technology were reported [1-3]. 

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