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., C-8, C-10, C-12, C-14, C-16, and C-18. 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 C-8 length to the C-12-C-16 lengths but decreased for the C-18 length. Retention factors were also measured as a function of three ligand surface concentrations for the C-12, C-14, and C-18 materials. In this study, retention generally decreased with increasing surface concentration, especially for the C-18 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 wealdy dependent on surface concentration. By comparison, tailing factors decreased significantly as surface concentration increased for a set of conventional C-18 alkyl packings.