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