pseudostationary

Pseudostationary refers to a dynamic phase that behaves like a stationary phase in a separation system but is not immobilized. In analytical chemistry, the term is most often used in micellar electrokinetic chromatography (MEKC) and related capillary electrophoresis methods, where surfactant micelles act as a pseudostationary phase within a bulk, mobile solvent.

Mechanism: Surfactant molecules aggregate into micelles above the critical micelle concentration. These micelles form a quasi-stationary, partitioning medium that continually exchanges with the surrounding aqueous phase. Analytes distribute between the micellar pseudophase and the aqueous phase according to their hydrophobic and ionic properties, producing differential migration under the influence of electrophoretic and electroosmotic flows.

Applications and behavior: Pseudostationary phases enable the separation of neutral and weakly ionic compounds that are poorly resolved by standard capillary electrophoresis. Common surfactants include sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTAB). Retention depends on the partition coefficient between micelles and water, as well as factors such as temperature, pH, and ionic strength, rather than adsorption to a solid surface.

Limitations and considerations: Because the pseudostationary phase is dynamic, its composition can drift with time, affecting reproducibility. Micelle size, critical micelle concentration, surfactant purity, and interactions with the detection method can limit stability and resolution. Compared with a true stationary phase, pseudostationary separations may have lower capacity and can require careful method development.