Determination of Benzenesulfonic Acid Counterion in Amlodipine Besylate by Ion Chromatography

Significance of the Topic

Pharmaceutical active ingredients are often isolated as salts to enhance their solubility, stability, and bioavailability. Although sulfonate counterions represent a smaller fraction of pharmaceutical salts, their use has grown in response to the low aqueous solubility of many modern drug candidates. Accurate determination of the benzenesulfonate counterion in amlodipine besylate is critical for verifying salt stoichiometry, ensuring drug purity, and confirming complete salt formation.

Objectives and Study Overview

This work aimed to develop and validate a robust ion chromatography (IC) method for quantifying the benzenesulfonate anion in amlodipine besylate. Key goals included achieving high sensitivity, specificity, and reproducibility while minimizing analysis time and manual reagent handling.

Methodology

Benzenesulfonate standards (0.5–20 µg/mL) and amlodipine besylate samples (1–10 µg/mL equivalent) were prepared in deionized water. Calibration followed a quadratic fit over six concentration levels. Limits of detection (LOD) and quantification (LOQ) were determined using signal-to-noise criteria (S/N of 3 for LOD, 10 for LOQ). Method precision was evaluated by replicate injections (n = 6) over three days. Accuracy was assessed through recovery experiments at 50, 100, and 150% spikes relative to the theoretical benzenesulfonate content (27.9%).

Used Instrumentation

• Thermo Scientific Dionex ICS-5000+ HPIC system with SP pump, EG eluent generator, and DC detector compartment
• Dionex EGC 500 KOH eluent generator cartridge with CR-ATC 500 trap column
• Dionex IonPac AG18 2 mm guard (2 × 50 mm) and AS18 2 mm analytical (2 × 250 mm) columns
• Dionex ASRS 300 anion self-regenerating suppressor (2 mm) in recycle mode
• Dionex Chromeleon 7.2 chromatography data system

Results and Discussion

The method achieved excellent specificity, with no interfering peaks at the benzenesulfonate retention time (~12 min). Linearity over 0.5–20 µg/mL produced an r2 of 0.9995. The LOD and LOQ were determined as 0.02 and 0.07 µg/mL, respectively. Intraday and interday precisions for retention time and peak area were below 0.05% and 0.9% RSD. Accuracy studies yielded recoveries between 98.7% and 104.4%, within 2% of the theoretical value. Use of a shorter 4 µm AS18 column (2 × 150 mm) at 0.38 mL/min reduced run time to ~5 min, doubling sample throughput while maintaining performance.

Benefits and Practical Applications

• High sensitivity and selectivity for benzenesulfonate in complex pharmaceutical matrices
• Automated KOH eluent generation reduces manual handling and improves reproducibility
• Rapid analysis option with 4 µm column increases laboratory productivity
• Applicable to quality control and regulatory compliance for API counterion determination

Future Trends and Potential Applications

Advances in high-pressure IC and sub-2 µm particle columns will further accelerate analysis times and enhance separation efficiency. Integration with mass spectrometric detection could broaden the scope to trace-level impurity profiling. Automated sample preparation and on-line dilution techniques may enable high-throughput screening of multiple drug salt forms in industrial environments.

Conclusion

An electrolytically generated KOH-based IC method was successfully validated for quantifying benzenesulfonate in amlodipine besylate. The approach delivers high accuracy, precision, and sensitivity while offering flexibility through rapid separations. This reliable method supports robust counterion analysis in pharmaceutical quality control.

References

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