The effect of various sample diluents and the injection volume on the peak shapes of small polar compounds (benzoic acid, melamine, nicotine and nicotinic acid) on the two stationary phases (bare silica and amide-bonded silica) was evaluated under hydrophilic interaction chromatography conditions. Seven solvents tested include water, acetonitrile, methanol, ethanol, isopropanol, dimethyl sulfoxide and ethyl acetate. Using the amide phase acceptable peak shapes were obtained in acetonitrile, ethyl acetate, ethanol and isopropanol. In the case of the bare silica phase only acetonitrile and ethyl acetate provided adequate peak efficiencies for all analytes. Higher resistance to a sample diluent of the amide phase may be attributed to its less hydrophilic character. The maximal sample volume that does not cause peak broadening depends on the nature of the sample diluent. When the content of water in acetonitrile increased, peaks were broadened or even distorted. Acceptable peak widths of the most strongly retained nicotinic acid were obtained when the injected sample contained ≤40% or ≤60% water for the bare silica and the amide phase, respectively. For the three less retained analytes, depending on a particular analyte and stationary phase, the critical amount of water in acetonitrile varied between 20 and 30%.

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