Determination of methanesulfonic acid in busulfan by ion chromatography

Significance of the topic

Accurate determination of methanesulfonic acid (MSA) in busulfan is critical for assessing drug stability and safety. Busulfan hydrolyzes in aqueous conditions to MSA, which serves as an indicator of product degradation and potential toxicity at elevated levels. Including a reliable test for MSA in the United States Pharmacopeia (USP) monograph ensures robust quality control for tablet and injectable formulations.

Objectives and study overview

The study aimed to:

• Evaluate the USP proposed limit test for MSA in busulfan using ion chromatography (IC).
• Assess a modified sample-preparation approach with cold acetonitrile–water and reduced autosampler temperature.

The comparison focused on sensitivity, stability, reproducibility, and compliance with USP acceptance criteria (MSA ≤0.15%).

Methodology and Instrumentation

Busulfan samples and MSA standards were analyzed by suppressed-conductivity IC with the following setup:

• System: Thermo Scientific™ Dionex™ ICS-6000 HPIC with eluent generation (EGC 500 KOH) and suppressor (ADRS 600).
• Columns: Dionex IonPac AG11-HC guard (2×50 mm) and AS11-HC analytical (2×250 mm).
• Eluent: Potassium hydroxide gradient (1–71 mM over 20 min). Flow rate 0.3 mL/min, column at 35 °C.
• Detection: Suppressed conductivity. Injection volume 10 µL.

Sample preparation:

• USP method: Dilute busulfan in cold water; autosampler at 25 °C.
• Modified method: Use cold 30% acetonitrile–water for dilution; autosampler at 8 °C.

Results and discussion

Separation and detection:

• MSA eluted at ~9.4 min, well resolved from acetate, chloride, and sulfate.

Stability studies:

• Busulfan degrades fastest in water at room temperature; stability improved in cold 30% acetonitrile.
• Standard MSA solutions remained stable for >14 days at ambient temperature.

System suitability and calibration:

• Tailing factor ≤1.13; RSD (area) ≤2.6% for both methods.
• Linearity r² ≥0.9998 from 0.1 to 5 µg/mL.
• LOD/LOQ: USP method 1.25/4.16 µg/L; modified method 3.45/11.52 µg/L.

Robustness:

• Minor changes in flow rate (±10%), column temperature (±10%), and eluent concentration (±10%) produced ≤6% shift in retention time and ≤4% change in peak shape.

Sample analysis:

• Three commercial busulfan samples were tested; one sample exceeded the 0.15% MSA limit under both methods, confirming method specificity.
• Under USP conditions, delay >40 min led to false positives due to ongoing hydrolysis; modified method extended acceptable analysis window to ~30 hours at 2–8 °C.

Spike recovery:

• Modified method recoveries between 90–110% for spiked MSA levels (0.1 and 0.2 µg/mL).

Benefits and practical applications

This IC procedure delivers:

• High selectivity and sensitivity for MSA impurity in busulfan.
• Improved sample stability and flexible analysis time with cold acetonitrile dilution.
• Compliance with USP monograph limits, supporting routine QC in pharmaceutical labs.

Future trends and applications

Potential developments include:

• Advanced column chemistries for faster or higher-capacity separations.
• Automated sample preparation and online dilution for throughput increases.
• Broader impurity profiling using tandem mass detection or multi-dimensional IC.

Conclusion

The proposed USP IC method and its modified version both satisfy monograph requirements for MSA determination in busulfan. Employing cold acetonitrile and refrigerated autosampling enhances stability and extends analysis flexibility, while maintaining accuracy, precision, and robustness. Implementation in quality-control workflows will ensure reliable monitoring of busulfan stability.

Reference

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