Abstract
The relationships between retention and mobile-phase pH in gradient elution are studied for acids and bases. The apparent pH shift caused by the increasing amount of acetonitrile and methanol has been determined starting from a wide range of pH values. It is shown that good relationships between the retention of ionizable compounds and the pH of the aqueous buffer can be established if the same type of buffer (ammonium acetate in this work) is used for all pH points. Equations are proposed to fit the gradient retention data to the pH of the aqueous buffer. The proposed equation gives an account of the relative variation of the pKa of the compound in the reference to the variation of the pH of the buffer as both parameters change during gradient elution.
MeSH terms
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Acetic Acid / analysis
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Acetic Acid / isolation & purification
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Amines / chemistry*
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Amines / isolation & purification
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Ammonia / analysis
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Ammonia / isolation & purification
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Aniline Compounds / analysis
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Aniline Compounds / isolation & purification
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Benzoates / chemistry
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Benzoates / isolation & purification
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Benzylamines / analysis
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Benzylamines / isolation & purification
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Carboxylic Acids / chemistry*
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Carboxylic Acids / isolation & purification
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Chromatography, High Pressure Liquid / methods*
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Ephedrine / analysis
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Ephedrine / isolation & purification
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Hydrogen-Ion Concentration
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Kinetics
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Lidocaine / analysis
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Lidocaine / isolation & purification
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Papaverine / analysis
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Papaverine / isolation & purification
Substances
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Amines
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Aniline Compounds
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Benzoates
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Benzylamines
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Carboxylic Acids
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Ammonia
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Lidocaine
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Papaverine
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Ephedrine
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Acetic Acid