Thermoresponsive poly(N-vinylcaprolactam) as stationary phase for aqueous and green liquid chromatography

Poly(N-vinylcaprolactam) (PVCL) connected to aminopropyl silica is a new stationary phase for temperature responsive liquid chromatography (TR-LC). PVCL shows a transition from hydrophilic to hydrophobic interaction between 30 and 40°C. The synthesis is described in detail. The temperature responsive characteristic of the phase is illustrated with a mixture of steroids using pure water as mobile phase. An increase in retention is observed when raising the temperature. Hu-plots at different temperatures were constructed. Below the lower critical solution temperature (LCST), no optimal velocity could be measured because of substantial resistance to mass transfer indicating an adsorption mechanism. Above the LCST, partitioning controls the separation resulting in higher efficiencies and an uopt of ca. 0.3 mm s -1 . Reduced plate heights decreased from 4 at 45°C to 3 at 65°C. The temperature responsive nature of the polymer is lost in green chromatography with ethanol as modifier in concentrations above 5%.