Characterization and Evaluation of C18 HPLC Stationary Phases Based on Ethyl-Bridged Hybrid Organic/Inorganic Particles

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Abstract
The characterization and evaluation of three novel 5-μm HPLC column packings, prepared using ethyl-bridged hybrid organic/inorganic materials, is described. These highly spherical hybrid particles, which vary in specific surface area (140, 187, and 270 m2/g) and average pore diameter (185, 148, and 108 Å), were characterized by elemental analysis, SEM, and nitrogen sorption analysis and were chemically modified in a two-step process using octadecyltrichlorosilane and trimethylchlorosilane. The resultant bonded materials had an octadecyl surface concentration of 3.17−3.35 μmol/m2, which is comparable to the coverage obtained for an identically bonded silica particle (3.44 μmol/m2) that had a surface area of 344 m2/g. These hybrid materials were shown to have sufficient mechanical strength under conditions normally employed for traditional reversed-phase HPLC applications, using a high-pressure column flow test. The chromatographic properties of the C18 bonded hybrid phases were compared to a C18 bonded silica using a variety of neutral and basic analytes under the same mobile-phase conditions. The hybrid phases exhibited similar selectivity to the silica-based column, yet had improved peak tailing factors for the basic analytes. Column retentivity increased with increasing particle surface area. Elevated pH aging studies of these hybrid materials showed dramatic improvement in chemical stability for both bonded and unbonded hybrid materials compared to the C18 bonded silica phase, as determined by monitoring the loss in column efficiency through 140-h exposure to a pH 10 triethylamine mobile phase at 50 °C.