Identification of NISTmAb Charge Variants by Fully Automated Capillary Isoelectric Focusing and Mass Spectrometry

Importance of the topic

The charge heterogeneity of monoclonal antibodies is a critical quality attribute influencing efficacy, stability and immunogenicity. Rapid, high-resolution separation and precise mass identification of charge variants streamline method development and quality control for biotherapeutics.

Goals and Study Overview

This study demonstrates a fully automated online capillary isoelectric focusing–mass spectrometry workflow for characterizing charge variants of the NIST monoclonal antibody reference material (NISTmAb). The objectives include optimizing separation conditions, evaluating reproducibility and confidently assigning molecular mass shifts to specific variants.

Methodology and Instrumentation

Reagents and ampholytes

• Pharmalyte mixtures covering pH ranges 3.0–10.0 and 8.0–10.5
• LC/MS-grade solvents and CIEF/MS reagent kits

Instrumentation

• Agilent 7100 capillary electrophoresis system
• Agilent 6230B time-of-flight LC/MS
• CMP Scientific EMASS-II CE/MS ion source with nanospray emitter
• Neutral-coated PS1 capillaries (60 and 75 cm)

Separation parameters

• Sample and catholyte injection under 5 mbar for hydrodynamic flow
• Separation voltage 15–18.8 kV to achieve 250 V/cm field strength
• Positive electrospray at 2.4 kV and drying gas at 6 L/min, 365 °C

Main Findings and Discussion

Ampholyte optimization revealed a 1:4 ratio of broad-range to basic ampholyte provided best resolution for acidic and basic variants. Extending capillary length from 60 cm to 75 cm improved resolution and minimized disturbance under constant pressure. Five sequential overnight injections showed migration time reproducibility (RSD 3.7%) and mass measurement precision dependent on variant abundance (standard deviation 0.67–1.60 Da). Deconvolution assigned:

• Two basic peaks: +128 Da (C-terminal lysine) and +254 Da (di-lysine)
• Main isoform at 148,041.7 Da
• Acidic peak: +162 Da corresponding to lysine glycation

Benefits and Practical Applications

• Direct, high-resolution variant separation with precise mass assignment
• Automated workflow reduces method development time
• Applicable to comparability studies, stability monitoring and QC release testing

Future Trends and Potential Applications

Integration of high-throughput CIEF/MS with laboratory information systems and automated data analysis will enhance screening of biotherapeutics. Expanding workflows to include glycoform profiling, post-translational modifications and other complex biologics will further support regulatory and product development efforts.

Conclusion

The fully automated CIEF/MS workflow on Agilent and CMP Scientific platforms enables robust separation and confident mass identification of NISTmAb charge variants. This approach offers a streamlined solution for advanced characterization and QC of biotherapeutics.

References

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