Systematic Study of Chromatographic Behavior vs Alkyl Chain Length for HPLC Bonded Phases Containing an Embedded Carbamate Group

A series of HPLC bonded phases containing an internal carbamate group were studied by changing the terminal N-alkyl group from C8H17 to C18H37 in increments of two methylene units, i.e., C8, C10, C12, C14, C16, and C18. Each material was prepared via bonding of silica with the respective 3-(chlorodimethylsilyl)propyl N-alkylcarbamate silane. The materials were compared under reversed-phase conditions using a test mixture of nonpolar, polar, and basic compounds in a 65:35 (v/v) methanol/20 mM KH2PO4/K2HPO4, pH 7, mobile phase. Retention factors were found to generally increase from the C8 length to the C12−C16 lengths but decreased for the C18 length. Retention factors were also measured as a function of three ligand surface concentrations for the C12, C14, and C18 materials. In this study, retention generally decreased with increasing surface concentration, especially for the C18 chain length. Changes in particle surface area and porosity caused by bonding did not fully account for the observed changes in retention factors. Peak shapes for the basic analytes propranolol and amitriptyline were also studied as a function of N-alkylcarbamate chain length and surface concentration. Tailing factors were unaffected by chain length and only weakly dependent on surface concentration. By comparison, tailing factors decreased significantly as surface concentration increased for a set of conventional C18 alkyl packings

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

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