pH gradient high-performance liquid chromatography: theory and applications

Area of Science:

• Analytical Chemistry
• Chromatography

Background:

• Reversed-phase high-performance liquid chromatography (RP HPLC) is a common technique for separating chemical compounds.
• Analyzing ionogenic substances, particularly their acidity (pKa), often requires specialized methods.
• Conventional RP HPLC typically uses organic modifier gradients to alter eluent strength.

Purpose of the Study:

• To introduce and validate a pH gradient method for reversed-phase high-performance liquid chromatography (RP HPLC).
• To demonstrate the suitability of pH gradient RP HPLC for determining pKa values of ionogenic analytes.
• To establish a theoretical framework and mathematical model for predicting chromatographic behavior under pH gradient conditions.

Main Methods:

• Developed a pH gradient elution strategy in RP HPLC, keeping organic modifier concentration constant.
• Programmed the mobile phase pH to increase eluting strength for acids or decrease it for bases.
• Elaborated a manageable mathematical model to predict analyte retention and separation based on chromatographic conditions.

Main Results:

• The pH gradient RP HPLC method was shown to effectively predict changes in analyte retention and separation.
• A specific equation was presented for calculating pKa values directly from retention data obtained via this method.
• Chromatographically determined pKa values correlated with reference values but differed due to analyte-stationary phase interactions.

Conclusions:

• pH gradient RP HPLC is a uniquely suitable method for determining pKa values of ionogenic substances.
• The method provides a fast and convenient way to obtain comparable acidity parameters for large compound series, including drug candidates.
• This approach is effective even when analytes are available in minute quantities or as complex mixtures.