Thomas H. Walter

Universal procedure for the assessment of the reproducibility and the classification of silica-based reversed-phase packings: II. Classification of reversed-phase packings

Abstract The tools developed for the testing of the reproducibility of several commercial packings have been used to study the differences and similarities of over 50 different commercial reversed-phase packings. The tests employed allow a characterization of the hydrophobicity of packings, the silanol activity at neutral and acidic pH, a differentiation between classical packings and packings with an embedded polar functional group, and a differentiation between C18 and C8 packings.

Universal procedure for the assessment of the reproducibility and the classification of silica-based reversed-phase packings : I. Assessment of the reproducibility of reversed-phase packings

A method is described that has been used since 1985 to assess the quality and reproducibility of several popular reversed-phase packings. Both the precision of the method and the reproducibility of the packing materials are described. The reproducibility of the newer packings surpasses that of the older packings. In addition, improved results are achieved today for the packings that existed in 1985.

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 C(8)H(17) to C(18)H(37) 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 KH(2)PO(4)/K(2)HPO(4), 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 weakly dependent on surface concentration. By comparison, tailing factors decreased significantly as surface concentration increased for a set of conventional C(18) alkyl packings.

Dependence of cyano bonded phase hydrolytic stability on ligand structure and solution pH.

As part of our program to develop more stable cyano (CN) high-performance liquid chromatography (HPLC) column packings, we have evaluated hydrolytic stability as a function of ligand connectivity, chain length, and side group steric protection and the pH of the mobile phase. Three accelerated tests were used to evaluate stability: (1) A non-HPLC screening test measuring carbon loss in refluxing MeOH-100 mM KH2PO4 pH 4.5 (1:1, v/v) solution; (2) a continuous flow HPLC test measuring capacity factor maintenance in 1% trifluoroacetic acid in water (pH 1.02) at 80 degrees C; and (3) a continuous flow HPLC test measuring column efficiency maintenance in 50 mM triethylamine in water (pH 10.00) at 50 degrees C. The stability of the CN phases was found to be dependent on both ligand chemical structure and the pH of the test conditions. The starting screen test of intermediate pH was least able to differentiate the CN phases based on structure, because two different degradation mechanisms appear to offset each other (acid induced siloxane bond cleavage vs. base induced silica dissolution). A trifunctional and a sterically protected CN phase were notably stable under the acidic test conditions, but had poor stability under basic conditions. Conversely, chain extension afforded poor stability under acidic conditions, but did afford improved stability at higher pH. In total, the data indicate that good CN column stability can be achieved by using a trifunctional or a sterically protected phase in acidic mobile phases. However, as mobile phases of intermediate or higher pH are employed, shorter column lifetimes can be expected due to an accelerated dissolution of the underlying silica substrate. Materials were also compared chromatographically using a mixture of non-polar, polar, and basic analytes under reversed-phase conditions.