Quantitation of N-Nitroso Duloxetine N-Nitrosamine Drug Substance Related Impurities (NDSRI) in API and Formulation by 6475 LC/TQ

Significance of the topic

N-nitrosamine impurities in pharmaceuticals represent a major safety concern because many are probable human carcinogens and can be present at trace levels in active pharmaceutical ingredients (APIs) and finished drug products. Analytical methods capable of selective, sensitive and reproducible quantitation at sub-ppb levels are therefore essential for regulatory compliance and patient safety. The work summarized here demonstrates a targeted LC–MS/MS approach to quantify the N-nitroso duloxetine NDSRI in duloxetine HCl API and pellet formulations, addressing matrix interferences and regulatory sensitivity requirements.

Objectives and study overview

The study aimed to develop and qualify a robust, high-sensitivity LC–MS/MS method, using multiple reaction monitoring (MRM), to detect and quantify N-nitroso duloxetine across the range 0.5–40 ng/mL (ppb). Key goals included achieving chromatographic separation from the bulk API to minimize matrix effects, demonstrating linearity, precision, accuracy (recovery) in API and formulation matrices, and establishing method limits of detection (LOD) and quantitation (LOQ) suitable for routine QC.

Methodology

The method is based on reversed-phase liquid chromatography coupled to triple quadrupole MS operating in MRM mode with atmospheric pressure chemical ionization (APCI). A systematic evaluation of stationary phases and gradient conditions was conducted to ensure adequate resolution between duloxetine HCl and the N-nitroso impurity. To protect the MS and reduce contamination from high API load, an automated divert valve was used to send API-rich fractions away from the detector.

Key analytical performance metrics:

• Calibration range: 0.5–40 ng/mL with seven calibration levels and 1/x weighting;
• Linearity: R² > 0.99 across the calibration range;
• LOD: 0.5 ng/mL; LOQ: 2 ng/mL;
• S/N: acceptable signal-to-noise at LOD and LOQ levels confirming detectability;
• Precision: %RSD < 5% at LOD and specification levels;
• Recovery: API 92–99% at tested levels; pellet formulations 109–116% at LOQ/spec/200% spec levels;
• Chromatographic resolution: approximately 2.5 min retention time separation between duloxetine HCl and the N-nitroso impurity;
• Total run time: 18 min per injection.

Used instrumentation

The study used an Agilent 6475A LC/TQ triple quadrupole mass spectrometer with an APCI source. Key chromatographic and source settings included:

• LC column: Poroshell HPH C18, 4.6 × 150 mm, 2.7 μm;
• Mobile phases: A: 0.1% formic acid in water; B: methanol;
• Gradient: initial high aqueous (approx. 10% MeOH) ramped to high organic (approx. 90% MeOH) by ~8 min, followed by re-equilibration (total 18 min run);
• Flow rate: 0.5 mL/min; injection volume: 8 μL; column temperature: 40 °C; sample cooler: 5 °C;
• APCI/source settings: gas temperature ~300 °C; gas flow ~7 L/min; nebulizer ~45 psi; vaporizer temperature ~350 °C; capillary/corona settings as optimized for sensitivity;
• MRM transition reported for N-nitroso duloxetine: 183.0 → 123.0 (quantifier).

Main results and discussion

The optimized LC–MRM method delivered reliable quantitation of N-nitroso duloxetine at sub-ppb concentrations with excellent linearity (R² > 0.99) and reproducibility (%RSD < 5%). Chromatographic optimization provided ~2.5 min separation between the API peak and the nitrosamine, which reduced ion suppression and matrix interference. Use of an automated diverter valve further protected the MS detector by excluding API-rich eluent from the source during its elution window. Recovery experiments demonstrated acceptable extraction efficiency in API samples (92–99%) and slightly elevated recoveries in pellet formulations (109–116%), consistent across LOQ, specification and 200% specification spike levels. These results indicate the method is fit-for-purpose for trace nitrosamine determination in both neat API and formulation matrices.

APCI ionization, rather than ESI, helped achieve robust sensitivity for the relatively nonpolar nitroso analyte and mitigated some matrix-driven ionization variability. Method optimization of MS/MS transitions and collision energies was important to maximize signal while maintaining selectivity against coeluting background ions.

Benefits and practical applications

• Regulatory suitability: The method meets sensitivity and precision criteria required for nitrosamine control strategies and can support release testing and stability studies;
• Routine QC deployment: Short run time (18 min) combined with an automated diverter and robust chromatography enable routine screening of production batches with reduced instrument contamination risk;
• Matrix versatility: Validated in both API and pellet formulation matrices, demonstrating adaptability for different sample types encountered in pharmaceutical control labs;
• High specificity: MRM detection and chromatographic separation reduce false positives and allow confident quantitation at trace levels.

Future trends and opportunities

Potential directions to enhance nitrosamine control and analytics include:

• Use of isotopically labeled internal standards (isotope dilution) to improve accuracy and correct for residual matrix effects;
• Expansion to multi-nitrosamine panels to enable simultaneous screening of multiple NDSRIs in a single run;
• Application of high-resolution mass spectrometry (HRMS) for suspect screening and non-targeted discovery of unknown nitrosamine species;
• Further lowering of instrument detection limits through optimized sample enrichment (e.g., SPE) and instrument source tuning to meet evolving regulatory expectations;
• Inter-laboratory method harmonization and proficiency testing to strengthen regulatory acceptance and method transferability;
• Integration of automated sample preparation workflows to increase throughput and reduce operator variability.

Conclusion

The presented Agilent 6475A LC/TQ MRM method provides a sensitive, specific and reproducible analytical solution for quantifying N-nitroso duloxetine in both API and pellet formulations down to 0.5 ng/mL (LOD) and reliably at 2 ng/mL (LOQ). Effective chromatographic separation combined with APCI ionization and strategic use of a diverter valve minimized matrix effects and detector contamination. The method's linearity, recovery and precision metrics demonstrate suitability for routine quality control and regulatory testing related to nitrosamine impurities.

Reference

1. U.S. Food and Drug Administration. Safety communications and press announcements relating to angiotensin II receptor blocker (ARB) recalls (valsartan, losartan) due to nitrosamine impurities. FDA drug safety updates.
2. European Medicines Agency. Guidelines on nitrosamine impurities in human medicinal products. EMA/409815/2020 Rev. 23, effective 10 October 2025.
3. U.S. Food and Drug Administration. Guidance document on recommended acceptable intake limits for nitrosamine drug substance-related impurities. Docket Number: FDA-2020-D-1530, August 4, 2023.