Characterization of several stationary phases prepared by thermal immobilization of poly(methyltetradecylsiloxane) onto silica surfaces

Variations of a thermal immobilization procedure using poly(methyltetradecilsiloxane) and silica produced fourteen stationary phases with carbon contents of 4–18%. The stationary phases were chromatographically evaluated with the Engelhardt, SRM 870 and Tanaka tests. Classifications using USP and Euerby procedures indicate that the new immobilized phases are different from most commercial phases although there was some similarity with phases that have high ion-exchange interactions. The retention mechanism involved in the separation of basic solutes on several of the new stationary phases was studied by varying pH, type of Lewis base and the ionic strength of the eluent. The separations are strongly influenced by the chemistry of the accessible free silanols. The stationary phases present good selectivity at intermediate pH where the basic analytes were protonated, suggesting use of intermediate pH for these separations. Stability tests show that the stationary phases have poor stability at very high pH, even at 23 °C, but good stability in acidic mobile phases, even at 75 °C, as expected for an immobilized polymer stationary phase.