WCPS: Analysis of Metal Impurities in Pharmaceutical Ingredients in Preparation for the New USP Methods

Significance of the topic

The control of trace metal contaminants in active pharmaceutical ingredients and excipients is critical for patient safety and regulatory compliance. Legacy colorimetric methods defined in USP<231> rely on hazardous reagents and high‐temperature sample ashing, leading to inconsistent recoveries and safety concerns. Modern spectrometric techniques promise enhanced specificity, sensitivity and throughput, aligning with updated USP<232>/<233> protocols.

Study objectives and overview

This work evaluates the performance of Agilent 7700 and 7500 ICP-MS systems under proposed USP<232>/<233> conditions for quantifying 16 metal impurities, including Pb, Cd, As, Hg and transition elements. Key aims were to demonstrate method accuracy, precision and ease of implementation on pharmaceutical samples such as gelatin capsules and APIs (e.g., amoxicillin).

Methodology

Samples underwent microwave-assisted digestion in nitric and hydrochloric acid matrices. Calibration curves were prepared in 1% HNO₃/0.5% HCl, covering expected PDE-based concentration ranges. Analyses used both no-gas and helium collision modes to address potential polyatomic interferences. Spike-recovery experiments at 0.5×, 1× and 1.5× the in-sample limit (J) assessed accuracy and precision across all target analytes.

Instrumentation

• Agilent 7700 and 7500 ICP-MS equipped with third-generation octopole reaction system (ORS3)
• Helium collision gas for kinetic energy discrimination (KED) to remove polyatomic interferences
• High Matrix Interface (HMI) with aerosol dilution to manage high total dissolved solids and maintain oxide formation below 0.5%

Results and discussion

Recovery studies in gelatin capsules and amoxicillin API showed mean recoveries between 80% and 110% with RSDs generally below 5%, meeting USP<233> precision criteria. Major findings included:

• Effective interference removal via He mode KED, yielding clear quantification of As, Se and other elements even in complex matrices
• Low oxide ratios (CeO/Ce ≈ 0.5%) ensuring robust plasma performance for samples with up to 25% TDS
• Consistent spike recoveries for the “Big 4” toxic elements (As, Cd, Pb, Hg) and transition metals across all spiking levels
• Pass criteria achieved for daily exposure limits in both capsule and API matrices

Benefits and practical applications

The ICP-MS approach under USP<232>/<233> offers:

• Higher specificity and lower detection limits compared to colorimetric sulfide precipitation
• Reduced sample preparation hazards by eliminating high-temperature ashing and toxic reagents
• Capability to handle diverse pharmaceutical matrices through HMI-enabled high solid analysis
• Streamlined workflows with single-mode helium gas operation for most analytes

Future trends and applications

Advancements may include coupling ICP-MS with chromatographic separation for arsenic and mercury speciation when PDE thresholds are exceeded. Reactive collision gases (e.g., H₂) can further lower detection limits into the sub-ppt range for elements such as Se and Fe. Integrated automated sample introduction and hyphenated systems will support comprehensive impurity profiling in complex formulations.

Conclusion

The transition to ICP-MS-based methods as specified in USP<232>/<233> provides a robust, accurate and compliant solution for monitoring inorganic impurities in pharmaceuticals. Agilent 7700/7500 instruments with ORS3 and HMI technologies meet stringent regulatory criteria and improve laboratory safety and efficiency.

Reference

Amir Liba and Steve Wall, Agilent Technologies; Samina Hussain, Exova CA. Analysis of Metal Impurities in Pharmaceutical Ingredients in Preparation for the New USP Methods. Agilent Technologies, January 2011.