Ageing of silica in HPLC
thesisposted on 23.05.2018 by Hasan Ertas
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This study may be divided into two sections which cover different aspects of the reproducibility problems encountered in HPLC. In the first, the study has involved the separation of basic drugs, on different manufacturer's reversed-phase columns in conjunction with an acid buffered acetonitrile/water gradient. The retention reproducibility of each drug was assessed and compared on the basis of the retention index scale of 1-nitroalkanes. The effect of changing gradient run time on the reproducibility of the retention values of the 1-nitroalkanes was demonstrated on reversed-phases of different makers. The optimisation of initial isocratic composition of organic (acetonitrile) was carried out and its effect on the reproducibility of retention of basic drugs was evaluated. The effect of a premixed eluent on the retention reproducibility of selected basic drugs with time intervals between injections was demonstrated. The same method was further extended with or without using helium gas with small flow. The prediction of dwell volume and its effect on retention reproducibility was evaluated. Determination of retention times changes for selected aqueous basic solutes against eluent with different pH values on Capcell ODS column was studied. Applicability of each reversed-phases (Cl8) for the separation of basic analytes was demonstrated. In the second section, a number of different unbonded (bare) silicas were studied in terms of surface analysis using of solid state cross-polarisation (CP) magic angle spinning (MAS) NMR and Fourier Transform (FT) Diffuse Reflectance Infrared Spectra (DRIFT-IR) data. It is believed that silica material used for HPLC separation with eluent undergoes an ageing process with acidic (at pH:2–3) and basic eluents (higher than pH:8). To examine this process more clearly, some basic analytes were selected to evaluate each of the accelerated ageing process followed by showing the final surface properties by the method most commonly used such as solid-state NMR and FT-IR along with BET surface analyser.