Liquid Chromatography - Interactions Between 'Brush' and 'Bulk'' Reverse Phases and Aqueous Solvents 1

Interactions Between 'Brush' and 'Bulk' Reverse Phases and Aqueous Solvents

The interactions between aqueous solvents and brush reverse phases differ very significantly from those with a bulk reverse phase at very low concentrations of solvent. This difference has been investigated by a number of workers (25-27) and the basic difference between the two types of phase are shown in the curves relating retention volume of methanol to the concentration of methanol in the mobile phase in figure 35. The phases shown are the RP-18 brush, reverse phase manufactured by E. M. Laboratories, which had a C18 (dimethyloctadecyl) chain and ODS-3 a bulk reverse phase which had a C18 (octadecyl) chain and was manufactured by Whatman Inc. The curves relating retention volume with solvent composition for the two phases show very different behavior patterns.

The ODS-3 bulk reverse phase behaves in the expected manner, as the concentration of methanol increases the retention volume of the ethanol decreases smoothly and continuously up to a concentration of 10%w/v of methanol. The brush phase, however, behaves in a very unexpected fashion. The retention volume of ethanol at first increases as the solvent concentration increases, to a maximum at a concentration of about 3%w/v of methanol.

Figure 35. A Graph of the Corrected Retention Volume of Ethanol against the Concentration of Methanol in the Mobile Phase for a Bulk and Brush Reverse Phase