Thermal Field-Flow Fractionation of Colloidal Particles

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Thermal field-flow fractionation (thermal FFF) was used to retain and separate colloidal particles with various surface compositions using acetonitrile (ACN) and water as carrier liquids. The particles included several sizes of unmodified polystyrene, as well as carboxylated and aminated polystyrene. In aqueous carriers, the effects of pH, ionic strength, and surfactant concentration were examined. Compared to ACN, differences in retention time with particle size are greater in aqueous carrier liquids containing sodium azide, and in general retention increases monotonically with azide concentration until a plateau value is reached. A notable exception in this trend is carboxylated polystyrene, whose retention passes through a minimum with increasing sodium azide concentration before increasing to a plateau value. Retention also decreases in sodium azide as the pH is decreased or the concentration of FL-70 surfactant is increased. Yet FL-70 is beneficial because it prevents flocculation of the particles and erratic results at high levels of retention. Without sodium azide, only the carboxylated particle is retained, and its retention increases with FL-70 concentration.