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 incr ements 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-(ch lorodimethylsilyl)propyl N-alkylcarbamate silane, The materials were compar ed 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, m obile 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. Retent ion factors were also measured as a function of three ligand surface concen trations for the C-12, C-14, and C-18 materials. In this study, retention g enerally decreased with increasing surface concentration, especially for th e C-18 chain length, changes in particle surface area and porosity caused b y bonding did not fully account for the observed changes in retention facto rs. Peak shapes for the basic analytes propranolol and amitriptyline were a lso studied as a function of N-alkylcarbamate chain length and surface conc entration. Tailing factors were unaffected by chain length and only wealdy dependent on surface concentration. By comparison, tailing factors decrease d significantly as surface concentration increased for a set of conventiona l C-18 alkyl packings.