tailieunhanh - HPLC for Pharmaceutical Scientists 2007 (Part 4C)

Figure 4-43. Adsorption isotherms of alkylsulfates on Hypersil-ODS from methanol/water (20/80) with M phosphate buffer at pH . (Reprinted from reference 119, with permission.) Figure 4-44. Capacity factor of tyrosinamide versus concentrations of dodecyl sulfate (upper curve), decyl sulfate (middle curve), and octyl sulfate (lower curve). (Reprinted from reference 119, with permission.) | ION-INTERACTION CHROMATOGRAPHY 203 Mobile Phase Concn mmol dm-3 Figure 4-43. Adsorption isotherms of alkylsulfates on Hypersil-ODS from methanol water 20 80 with phosphate buffer at pH . Reprinted from reference 119 with permission. Figure 4-44. Capacity factor of tyrosinamide versus concentrations of dodecyl sulfate upper curve decyl sulfate middle curve and octyl sulfate lower curve . Reprinted from reference 119 with permission. 204 REVERSED-PHASE HPLC Figure 4-45. Dependence of the retention factor of adrenaline on the concentration of amphiphilic ions on the stationary phase surface. Retention factor shown in logarithmic scale. Reprinted from reference 136 with permission. similarly charged analytes as the ion pairing reagent will elute faster. This indeed has been observed experimentally Figure 4-46 . Figure 4-45 shows the similar retention dependencies of adrenaline retention for different amphiphilic ions adsorbed on the surface of the reversed-phase material indicating that at the same surface concentration of any amphiphilic ion adsorbed the retention of basic analyte is the same thus the retention is dependent on the surface charge density of adsorbed ions. Comparison of Figures 4-46 and 4-45 indicates that the retention of a charged analyte in ion-pairing mode is dependent on the adsorption of ion-pairing ions on the surface of the stationary phase and not on its concentration in the mobile phase. Same were also observed by Knox in a salt-controlled methanol-aqueous eluent for the analysis of normetadrenaline as a function of octyl decyl and lauryl sulfates 119 . In the contrast to the irreversible adsorption of amphiphilic ions on the reversed-phase surface the liophilic ions shows relatively weak interactions with the alkyl chains of the bonded phase. Liophilic means oil-loving. These liophilic ions are usually small inorganic ions and they possess an important ability for dispersive type interactions. They are a characterized by significant