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Abstract
The quantitative analysis of the chromatographic behavior of basic compounds measured with pentyl-, hexenyl-, and octyl-bonded silica gels were analyzed in silico employing model phases. The main retention force was the van der Waals (VW) interaction, and the main desorption force was an electrostatic (ES) interaction. The contribution of hydrogen bonding (HB) was weak compared to that for acidic compounds. The quantitative explanation was achieved utilizing the calculated VW, HB, and ES energy values obtained from a molecular mechanics program. The electron localization was observed at the molecular interaction-site calculated MOPAC program. This fundamental approach was like that of explaining chemical reactions. The difference was electron localization in chromatography or electron transfer in a chemical reaction.
Highlights
Basic compounds were generally purified and analyzed by ion-exchange (IE) liquid chromatography (LC)
The other limitations of basic compounds are the existence of free silanol on the surface which forms hydrogen bonding (HB) with nitrogen-containing compounds, and trace metals, which form chelates with the analytes
Amine modifiers were added to the eluents to conceal the active silanol and improve the peak symmetry of basic compounds
Summary
Basic compounds were generally purified and analyzed by ion-exchange (IE) liquid chromatography (LC). Newly introduced packing materials for HPLC are generally surface-modified silica gels with limitations in strongly acidic and basic eluents. The other limitations of basic compounds are the existence of free silanol on the surface which forms hydrogen bonding (HB) with nitrogen-containing compounds, and trace metals, which form chelates with the analytes These molecular interactions could cause fronting and tailing peaks that deteriorate the quality control. The performance of surface modified-silica gels was evaluated for basic compounds by the modification of the eluent components. Further development of chemically stable bonded silica gels enable the simple handling of the chromatographic separation of basic compounds in reversed-phase [11,12] and ion-exchange modes [13]. Propylsulfonic acid modified silica gel with eluent containing ammonium perchlorate was effectively employed for the chromatography of basic compounds [16]. The detail of the retention mechanisms was investigated by a simple model, and by model phases
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