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Generic method for isocratic separation of ten common pharmaceutical counterions

Many medicines and dietary supplements are bases that carry a positive charge, and consequently these small molecules are accompanied by negatively charged counterions. Such anionic excipients may influence the properties of the pharmaceutical formulation and can also influence the effect of the active ingredient since different ion pairs can have different dissolution rates or abilities to penetrate cell walls.

The generic method used in this application was developed to enable analysis of the ten most common counterions used in pharmaceuticals and dietary supplements within 20 minutes, using an isocratic carbonate-based eluent. A few coeluting anions were also injected to illustrate potential interferences.

Pharamaceutical counterions with XAMS suppressor and ASUREX-A200

Separation of pharmaceutical counterions on a Shodex IC SI-50 4E column (250×4 mm) using an eluent containing 4 mM Na₂CO₃ and 12 mM NaHCO₃ in water pumped at 0.8 mL/min at 24 °C from an EQAX-B1 eluent bottle equipped with an EQAX-TC1 trap cartridge for carbon dioxide removal. Background reduced to ~25 µS/cm by XAMS suppressor with ASUREX-A200 automatic regenerator. Eluent pumping and conductivity detection by Metrohm 761 Compact IC. Injection of 20 µL of acetate (1), mesylate (2), chloride (3), bromide (4), phosphate (5), nitrate (6), malate (7), sulfate (8), maleate (9), tartrate (10), oxalate (11), fumarate (12), at 5 or 20 mg/L (inorganic ions and organic acids, respectively) in water, giving signals up to 8 µS/cm.

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