A Simple Conversion of the USP Method for Diphenhydramine HCl Impurities to the Agilent InfinityLab Poroshell 120 EC-C8 Column

Importance of the Topic

Pharmaceutical analysis of diphenhydramine HCl impurities is essential to ensure product safety and efficacy at all stages of development and manufacturing. Modernizing compendial USP methods using superficially porous columns can improve analytical throughput, reduce solvent usage, and maintain consistent data across laboratories without requiring extensive revalidation.

Objectives and Overview of the Study

This work demonstrates the transfer of the USP diphenhydramine HCl impurity method from a conventional 5 µm, 4.6 × 250 mm C8 column to a shorter, 2.7 µm superficially porous InfinityLab Poroshell 120 EC-C8 column. The aim was to shorten analysis time while adhering to USP37-NF32S1 and Stage 4 Harmonization guidelines without compromising system suitability or method performance.

Methodology

The original USP method employed a binary mixture of acetonitrile and phosphate buffer (35:65 v/v) at pH 3.0, 1.2 mL/min flow rate, and 10 µL injection. To transfer the method, injection volume was scaled proportionally, and flow rate increased to 1.5 mL/min. Method adjustments followed the USP N-rule for column length and particle size, ensuring plate count remained within ±25–50% of the reference. System suitability required a minimum run time of seven times the diphenhydramine retention and resolution ≥2.0 between the API and primary impurity.

Used Instrumentation

• Agilent 1260 Infinity II LC System with binary pump, multisampler, column thermostat, and diode array detector
• ZORBAX Eclipse Plus C8 column, 4.6 × 250 mm, 5 µm
• InfinityLab Poroshell 120 EC-C8 column, 4.6 × 100 mm, 2.7 µm
• OpenLAB CDS software (version C.2.6)
• HPLC-grade acetonitrile, USP-grade phosphate buffer components, and water (Millipore Milli-Q)

Key Results and Discussion

When transferred, the superficially porous column delivered a plate count of 12,873 for diphenhydramine versus 10,334 on the reference column. Retention time dropped from 5.302 to 1.761 minutes, reducing total run time from 37.1 to 12.3 minutes (≈66% time saving). Resolution improved marginally from 2.85 to 3.02, and operating pressure remained below 400 bar, ensuring compatibility with existing HPLC hardware.

Benefits and Practical Applications

• Two-thirds reduction in analysis time
• Lower solvent consumption and cost savings
• Enhanced laboratory throughput without revalidation
• Seamless integration with current instrumentation

Future Trends and Opportunities

Adoption of superficially porous particle columns is expected to expand across other USP compendial methods, fostering further harmonization and efficiency. Growth in UPLC technology will continue to support high-throughput QC environments and drive innovation in method development.

Conclusion

The transfer of the USP diphenhydramine HCl impurity method to an InfinityLab Poroshell 120 EC-C8 column effectively accelerates analysis and conserves resources without compromising performance, fully aligning with current USP guidelines.

References

1. United States Pharmacopeia. Diphenhydramine HCl Impurity Method. USP 43(4).
2. United States Pharmacopeia. General Chapter <621>, USP 37-NF32, First Supplement.
3. United States Pharmacopeia. Stage 4 Harmonization, Official Publication, December 1, 2022.