Determination of Morpholine in Linezolid by Ion Chromatography

Importance of Topic

The presence of trace amounts of morpholine as an impurity in the synthetic antibiotic linezolid poses potential risks to drug efficacy and patient safety. Accurate quantification of low µg/L levels of morpholine is therefore essential to meet stringent pharmacopeial impurity limits and to ensure consistent pharmaceutical quality.

Study Objectives and Overview

This study aimed to develop and validate a reagent-free ion chromatography (RFIC) method with suppressed conductivity detection for the determination of morpholine at low microgram-per-liter concentrations in linezolid. The approach integrates automated sample matrix elimination to protect the separation column from non-ionic matrix components.

Used Instrumentation

• Thermo Scientific Dionex ICS-5000+ RFIC system: SP Single Pump, EG Eluent Generator, DC Detector/Chromatography compartment
• Dionex AS-AP Autosampler in Push Concentrate mode with 5.0 mL syringe and buffer line
• Dionex EGC 500 MSA Eluent Generator Cartridge and CR-CTC 500 Continuously Regenerated Cation Trap Column
• Dionex IonPac TCC-ULP1 Ultralow Pressure Trace Cation Concentrator Column
• Cation Self-Regenerating Suppressor CSRS 300 (2 mm, recycle mode, 7 mA)
• Dionex Chromeleon Chromatography Data System software, version 7.1

Methodology

Linezolid samples (≈5 mg) were dissolved in 50 mL of 10% methanol/water to accelerate solubility. The AS-AP autosampler performed in-line matrix elimination by loading sample onto the TCC-ULP1 concentrator, followed by a 1 mL DI-water flush to remove uncharged linezolid. The trapped morpholine was then eluted onto a Dionex IonPac CG19/CS19 column set using 7.5 mM methanesulfonic acid at 0.25 mL/min. Detection employed suppressed conductivity.

Main Results and Discussion

Calibration of morpholine over 5–200 µg/L was linear (r2>0.999). The method LOD and LOQ were 0.86 µg/L and 2.9 µg/L, respectively. Intraday and interday recoveries ranged from 98.8 to 103%, and precision (n=7) yielded retention time RSD ≤0.09% and peak area RSD ≤1.6%. Matrix elimination reduced retention time drift from 2.14% to 0.61% over 70 injections. No interference from linezolid or common cations was observed, and morpholine eluted at ~9.0 min.

Benefits and Practical Applications

This RFIC method offers high sensitivity for morpholine at levels well below USP impurity thresholds, combined with automated sample cleanup to extend column life. It enables simultaneous monitoring of morpholine and common cations in a single run without extensive manual pretreatment, supporting quality control in pharmaceutical manufacturing.

Future Trends and Opportunities

Further developments may include coupling RFIC to mass spectrometry for structural confirmation of related amines, miniaturization of concentrator columns for reduced solvent usage, expansion to additional heterocyclic impurities, and integration with in-line sampling in continuous manufacturing workflows.

Conclusion

The validated RFIC method provides a robust, automated approach for trace analysis of morpholine in linezolid, offering excellent sensitivity, precision, and compliance with regulatory impurity limits while minimizing sample preparation complexity.

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