Analysis of the Degree of Coloration of Liquids Compliant to European Pharmacopeia

Significance of the Topic

The degree of coloration of liquid pharmaceutical samples is a critical quality control parameter defined by the European Pharmacopeia. Traditional visual comparison of samples against defined color standards is subjective and labor intensive. Transitioning to spectrophotometric measurement with automated data analysis enhances reproducibility, traceability and compliance with regulatory requirements.

Study Objectives and Overview

This application note demonstrates a semi-automated workflow for determining the coloration of liquids according to EP chapter 2.2.2. The method employs the Shimadzu UV-1900i Plus UV-Vis spectrophotometer, LabSolutions UV-Vis Color software and the Multi Data Report module to assign EP color numbers with audit trail and electronic signature support.

Methodology and Instrumentation

Primary color reference stock solutions (brown, brownish yellow, yellow, greenish yellow and red) were prepared as specified in EP 11.5. Dilution series B1–B9, BY1–BY7, Y1–Y7, GY1–GY7 and R1–R7 were generated in 10 g/L HCl solvent. Measurements were performed in 10 mm pathlength plastic cuvettes while selected standards were remeasured in quartz cuvettes to simulate unknown samples. Spectral acquisition parameters were:

• Wavelength range: 380 to 800 nm with 1 nm interval
• Scan speed: Medium, dual beam layout, slit width 1 nm
• Light source switch at 340 nm

Color evaluation followed the CIELAB model. L*, a* and b* coordinates were calculated using ASTM E308 standard illuminant C and 2° observer settings. Chroma and hue (C*, h degree) were derived for each spectrum. The Multi Data Report template automated calculation of color differences DeltaEab and DeltaECh for all sample–reference combinations and assigned the color number corresponding to the minimal color difference.

Instrumentation Used

• Shimadzu UV-1900i Plus UV-Vis Spectrophotometer with fixed 1 nm spectral bandwidth
• LabSolutions DB UV-Vis data management software
• LabSolutions Multi Data Report module for automated reporting
• Disposable plastic and quartz 10 mm pathlength cuvettes
• Primary reference solutions and 10 g/L hydrochloric acid diluent

Key Results and Discussion

Spectral profiles showed distinct absorbance peaks for high concentration standards whereas lower dilutions approached the blank spectrum. Calculated L* values for all series exceeded 88 for concentrated samples and approached 100 for weakly colored solutions. Coordinate plots illustrated convergence of a* and b* toward zero with dilution. Test samples measured in quartz cuvettes were correctly matched to their EP color numbers with DeltaE differences below 2 for all cases, confirming method accuracy. Analysis highlighted the need for consistent cuvette material to avoid systematic shifts in color coordinates.

Benefits and Practical Applications

• Minimization of manual steps and subjective errors through automated data processing
• Full compliance with EP and 21 CFR Part 11 via audit trails and electronic signatures
• High throughput potential including autosampler integration for up to 360 samples
• Customizable report templates aligned with corporate identity and SOPs
• Enhanced traceability and data integrity for pharmaceutical QC workflows

Future Trends and Potential Applications

Building on this framework, future developments may include full integration with autosamplers and laboratory information management systems for end-to-end automation. Expansion to additional pharmacopeial color tests or other colorimetric assays can further streamline QC. Incorporation of machine learning algorithms for trend analysis and predictive maintenance of instruments may enhance reliability and efficiency.

Conclusion

The described UV-Vis method coupled with LabSolutions software delivers a robust, reproducible and compliant approach to determining the degree of coloration in liquid pharmaceuticals. Automated DeltaE calculations and report generation reduce labor and increase data integrity, meeting EP requirements and improving laboratory throughput.

References

1. European Directorate for the Quality of Medicines HealthCare EDQM. European Pharmacopoeia 11.5 Chapter 2.2.2 Degree of coloration of liquids Strasbourg 2021
2. ASTM E308 Standard Practice for Computing the Colors of Objects by Using the CIE System ASTM International 2018
3. CIE Technical Report 15 2018 Colorimetry 4th Edition