Supercritical Fluid Generated Stationary Phases for Liquid Chromatography and Capillary Electrochromatography

Chromatographic silica-bonded stationary phases have been prepared using supercritical CO₂ as the reaction medium. ²⁹Si solid-state NMR spectra of the generated bonded silica phases show unreacted silica species Q³ and Q⁴, alongside important resonances for surface-bound ligands, T¹, T², and T³. Initially, a fiuorinated fluorinated octyl silica (C₈) phase was produced, by reacting ¹H,¹H,²H, ²H-perfluorooctyltriethoxysilane with silica particles (3 μm) in sc-CO₂ at 60 °C and 450 atm for 3 h. In-house-packed LC columns of this fluorinated sc-C₈ silica phase yielded typical reversed-phase behavior when a standard test mix of benzamide (k′ = 1.03), benzophenone (k′ = 8.11), and biphenyl (k′ = 14.92) was eluted. When packed into fused-silica capillaries for CEC, this fluorinated sc-C ⁸ silica phase gave linear plots of log k′ versus percentage MeOH for benzophenone and biphenyl and, in contrast to octyl or octadecyl silica phases, displayed selectivity for aromatic thioureas when chromatographed among a series of synthetic organic thiourea test solutes. Similarily, an octadecyl silica phase (sc-C₁₈ silica) was prepared by reaction of n-octadecyltriethoxysilane in sc-CO₂, packed at 9500 psi and examined by LC. The sC-C₁₈ silica LC column yielded high column efficiency (up to 141 000 N/m (fluorene)) and excellent asymmetry factors (1.06, fluorene) without resource to end-capping. Following a second silylating or end-capping step using hexamethyldisilazane in sc-CO₂, sc-end-capped sc-C₁₈ silica phases elute N,N-DMA before toluene and the toluidine isomers as a single peak, indicating lowered silanol activity according to the Engelhardt test. A rapid separation of the important pharmaceutical substances, ketoprofen, naproxen, fentoprofen, and ibuprofen, on an sc-end-capped sc-C₁₈ silica phase is also shown.