Electroosmotic flow variations caused by the volatility of buffer components: diagnosis and therapy

Area of Science:

• Analytical Chemistry
• Separation Science
• Chromatography

Background:

• Micellar electrokinetic chromatography (MEKC) is crucial for separating polar amine pharmaceuticals and impurities.
• High acetonitrile concentrations are vital for selectivity in MEKC but lead to evaporation and unstable electroosmotic flow (EOF).
• Existing rinsing procedures failed to improve EOF reproducibility.

Purpose of the Study:

• To develop a stable MEKC method for separating polar amine pharmaceuticals and impurities.
• To identify and mitigate the causes of electroosmotic flow (EOF) instability.
• To establish a rapid method for evaluating buffer systems containing organic solvents.

Main Methods:

• Developed a micellar electrokinetic chromatography (MEKC) method using Tris buffer, sodium dodecylsulfate (SDS), and acetonitrile.
• Investigated buffer evaporation by testing protective coverings (paraffin, silicon oils) and measuring electrical conductivity.
• Validated a modified buffer system (10% acetonitrile, 10% isopropanol) for stable EOF.

Main Results:

• Acetonitrile evaporation was identified as the primary cause of EOF instability, not surface interactions.
• Protective coverings significantly reduced acetonitrile evaporation, allowing up to 10% (v/v) acetonitrile with stable EOF.
• Electrical conductivity measurements proved effective for rapidly assessing buffer evaporation potential.

Conclusions:

• Protective coverings are essential for maintaining buffer stability and reliable quantification in MEKC with high acetonitrile content.
• A modified buffer system containing 10% acetonitrile and 10% isopropanol provides a stable EOF and acceptable evaporation.
• Electrical conductivity measurements offer a general and rapid approach for pre-screening buffer systems in capillary electrophoresis.