Quantitation of N-Nitroso N-Desmethyl Diphenhydramine in Diphenhydramine HCl API

Significance of the Topic

A critical concern in pharmaceutical manufacturing is the occurrence of nitrosamine impurities, which are potent genotoxins. N-nitroso N-desmethyl diphenhydramine is an NDSRI that can arise during diphenhydramine HCl synthesis and must be controlled below regulatory thresholds to ensure patient safety and meet global quality standards.

Objectives and Study Overview

This study aimed to develop and validate a robust, highly sensitive, and reproducible LC/MS/MS method to quantify trace levels of N-nitroso N-desmethyl diphenhydramine in diphenhydramine HCl API using an Agilent 6475A triple quadrupole system. Key performance metrics included limit of detection (LOD), limit of quantitation (LOQ), linearity, specificity, accuracy, and precision.

Methodology and Instrumentation

Chromatography was performed on an Agilent InfinityLab Poroshell HPH-C18 column (4.6 × 150 mm, 2.7 µm) with a gradient of 1 mM ammonium trifluoroacetate (plus formic acid) and methanol at 0.5 mL/min. The Agilent 1290 Infinity II LC was coupled to a 6475A triple quadrupole MS operated in positive electrospray ionization with dynamic multiple reaction monitoring (dMRM). Two MRM transitions (m/z 271.3→167 and 271.3→152) were optimized, and source parameters were tuned to maximize sensitivity.

Instrumental Setup

• Agilent 1290 Infinity II high-speed pump (G7120A)
• Agilent 1290 Infinity II multisampler (G7167B)
• Agilent 1290 Infinity II multicolumn thermostat (G7116B)
• Agilent 1290 Infinity II diode array detector (G7117A)
• Agilent 6475A triple quadrupole LC/MS with Jet Stream source (G1958B)

Main Results and Discussion

Chromatographic separation provided >7 min resolution between the API (RT ≈ 3.1 min) and the impurity (RT ≈ 10.96 min). LOD and LOQ were determined at 0.05 ng/mL (0.01 ppm) and 0.1 ng/mL (0.02 ppm), respectively, with S/N ratios >25:1 and >45:1. The calibration curve (0.05–10 ng/mL) was linear (r² > 0.999, 1/x² weighting). Spike recovery at the LOQ level averaged 102.5%. Reproducibility of seven replicates at LOQ showed %RSD of 6.6%. Carryover was <5% of the LOQ response. A triggered MRM (tMRM) confirmatory scan generated product ion spectra to guard against false positives.

Benefits and Practical Applications

• Enables routine QC of diphenhydramine HCl API for nitrosamine control
• Meets stringent regulatory limits for genotoxic impurities
• High sensitivity and specificity reduce risk of false negatives
• Automated diverter valve reduces source contamination by diverting high-concentration API to waste

Future Trends and Opportunities

Advances may include integration of high-resolution MS for broader screening, real-time in-line monitoring, AI-driven data processing for rapid method optimization, and extension of the workflow to other amine-containing APIs. Enhanced library matching and broader tMRM libraries will further improve confirmatory power.

Conclusion

A sensitive, selective, and reproducible dMRM LC/MS/MS method on the Agilent 6475A platform was established for trace quantitation of N-nitroso N-desmethyl diphenhydramine in diphenhydramine HCl API. The method achieves LOQ at 0.1 ng/mL with excellent linearity, recovery, and precision, supporting robust QC compliance and patient safety.

References

• Chidella, S.; Kulkarni, G.; Banerjee, S.; Balakrishnan, K. Highly Sensitive Quantification of Mutagenic NDSRI N-Nitroso Propranolol in Propranolol API and 40 mg Tablets Using LC/MS/MS. Agilent Technologies Application Note 5994-5161EN, 2022.
• Mastovska, K.; Zulkoski, J.; Zweigenbaum, J. Triggered MRM LC/MS/MS Method Development – Practical Considerations for MRM Optimization Using Agilent MassHunter Data Acquisition Software. Agilent Technologies Technical Overview 5991-7195EN, 2017.
• Sosienski, T.; Covert, K. Trace Level Quantification of Potential Mutagenic Impurities in Pharmaceuticals. Agilent Technologies Application Note 5994-1238EN, 2019.
• Joseph, P.; Banerjee, S.; Vyas, S. Determination of Genotoxic Nitrosamine Impurity in Bumetanide API and Tablets Using the Agilent 6470 Triple Quadrupole LC/MS. Agilent Technologies Application Note 5994-2967EN, 2020.