Determination of Morpholine in Linezolid by Ion Chromatography

Importance of the Topic

Morpholine is widely used as a synthetic building block for diverse therapeutic agents and can remain as an impurity in linezolid, a vital antibiotic. Sensitive and selective determination of morpholine is essential in pharmaceutical quality control to ensure drug safety and efficacy.

Objectives and Study Overview

This study aimed to develop an ion chromatography (IC) method using a reagent-free IC system combined with suppressed conductivity detection for quantifying low µg/L levels of morpholine in linezolid. The approach focused on simple sample preparation, matrix elimination, and robust validation.

Used Instrumentation

• Thermo Scientific Dionex ICS-5000+ system with single pump, eluent generator, and suppressed conductivity detector
• Dionex AS-AP Autosampler in Push Concentrate Mode
• Dionex EGC 500 MSA cartridge and CR-CTC 500 cation trap column
• Dionex IonPac TCC-ULP1 ultralow pressure concentrator column
• Dionex IonPac CG19 guard and CS19 analytical columns
• Dionex CSRS 300 self-regenerating suppressor
• Chromeleon 7.1 chromatography data system

Methodology

• Sample preparation: 5 mg linezolid dissolved in 50 mL of 10% methanol/water
• Matrix elimination: In-line concentrator trapping of morpholine and removal of neutral linezolid with 1 mL water via autosampler
• Chromatographic conditions: 7.5 mM methanesulfonic acid eluent at 0.25 mL/min, 100 µL injection, 15 min runtime
• Detection: Suppressed conductivity in recycle mode at 7 mA current
• Calibration: Six levels from 5 to 200 µg/L morpholine with r² > 0.999
• Validation: LOD 0.86 µg/L, LOQ 2.9 µg/L, good precision and accuracy (recoveries 98.8–103%)

Main Results and Discussion

The CS19 column separated morpholine from common cations within 15 minutes with sharp peaks. Matrix elimination reduced retention time drift (< 0.61% vs 2.14% without). The method achieved an LOD of 0.86 µg/L and LOQ of 2.9 µg/L, meeting USP requirements. Recovery studies demonstrated intraday and interday accuracy between 98.8% and 103% and precision below 2% RSD.

Benefits and Practical Applications

• High sensitivity for trace morpholine detection in linezolid
• Automated matrix elimination extends column life and simplifies sample handling
• Simultaneous monitoring of other cations enhances impurity profiling
• Fully automated RFIC system improves reproducibility and throughput

Future Trends and Potential Applications

As IC systems evolve, integration with mass spectrometry, advanced suppressors, and wider application to other APIs and impurities is expected. Growth in reagent-free operation and miniaturized columns will further reduce analysis time and solvent consumption.

Conclusion

The developed RFIC method provides a robust, sensitive, and automated solution for quantifying morpholine in linezolid. In-line matrix elimination and suppressed conductivity detection deliver reliable performance, meeting stringent pharmaceutical criteria.

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