Confidence in Chromatography: Demonstrating Highly Reproducible HPLCCAD Performance with Waters Charged Aerosol Detectors

Significance of the Topic

Charged aerosol detection (CAD) coupled with HPLC provides near-universal, sensitive detection for non-volatile and semi-volatile analytes that lack strong chromophores and are therefore poorly detected by UV. CAD is widely applied in pharmaceutical and biopharmaceutical contexts for analysis of excipients, lipids, surfactants, carbohydrates, impurities and degradation products. Achieving reproducible CAD response across instruments and over time is critical for reliable quantitative results, method transfer, and efficient routine operation because CAD signal generation is inherently dependent on nebulization and aerosol formation, which are sensitive to many practical variables.

Study Objectives and Overview

The application note evaluates the reproducibility and robustness of Waters Charged Aerosol Detectors across multiple modules and over time using a USP monograph method for deoxycholic acid assay and organic impurities. Key aims were to demonstrate: consistent quantitative results across different CAD modules, low inter- and intra-module variability of absolute signal, and sustained detector performance under practical laboratory conditions that include use across different LC systems, operators, and days.

Methodology

Samples and mobile phases were prepared according to the USP monograph for deoxycholic acid (scaled from a 3 µm to 3.5 µm column). Reference standards and sample stocks were made at 1 mg/mL in 80:20 methanol:water, with working standards at 0.010 mg/mL and a sensitivity solution at 5 µg/mL. The method evaluated both assay and impurity tests with system suitability metrics (area precision, signal-to-noise for impurities, and relative impurity limits).

Used Instrumentation

• LC systems: ACQUITY UPLC H-Class (QSM) System with Waters CAD; ACQUITY Arc System with Waters CAD
• Detector settings: evaporator temperature 35 °C; data acquisition 2 Hz
• Column: XBridge BEH C18, 4.6 × 150 mm, 3.5 µm
• Column temperature: 30 °C (active preheating); sample temperature: 12 °C
• Injection volume: 25 µL; flow rate: 0.85 mL/min
• Mobile phases: A = 0.1% formic acid in water; B = 0.1% formic acid in acetonitrile
• Vials: certified 12 × 32 mm glass screw neck with PTFE/Silicone septa
• Chromatography data system: Empower 3 (SR3 and 3.10)

Key Results and Discussion

The study ran the USP deoxycholic acid assay and impurity method on six different Waters CAD modules and also evaluated repeatability on a single CAD across multiple days and LC systems. Main quantitative findings include:

• Intra-CAD relative standard deviations (RSDs): 0.3–2.0% for repeat injections on individual CAD modules
• Inter-CAD RSDs (multiple injections across six CADs): 3.6–4.9%
• Single-CAD inter-day RSD (42 injections over 17 days): 3.2%; intra-day RSDs ranged 0.4–2.6%
• Measured signal intensity for the evaluated low-concentration solutions was approximately 17–24 pA on a detector range of 0–500 pA

Chromatographic profiles and absolute signals across the six CAD modules were reported as nearly identical, and all system suitability criteria for the USP method were met on every unit. The results demonstrate that when instrument components and method conditions are controlled, CAD can deliver highly reproducible quantitative data across modules and over time despite the intrinsic sensitivity of nebulization-based detection to operational variables.

Two features that supported this reproducibility were emphasized:

• Optimized nebulizer design specific to Waters CAD to reduce unit-to-unit variability
• Calibration/tuning of each CAD nebulizer to an optimized gas pressure target to standardize aerosol generation and signal response

Authors also note the importance of best laboratory practices—high-quality solvents, careful mobile phase preparation, cleanliness of the system and column, stable detector gas supply, and routine CAD maintenance—to limit variability from non-instrumental sources.

Practical Benefits and Applications

The demonstrated performance translates into practical advantages for regulated and routine laboratories:

• Confidence in method transfer between instruments and sites when using Waters CAD modules
• Reduction of time lost to troubleshooting inconsistent detector response or to unnecessary reanalysis
• Robust detection of analytes without chromophores, enabling broader impurity profiling and excipient/component quantitation in pharmaceutical workflows

Future Trends and Potential Applications

Expected directions and opportunities related to CAD performance and adoption include:

• Further optimization of nebulizer geometry, gas control and aerosol conditioning to push reproducibility and sensitivity limits at very low signal levels
• Broader adoption of CAD as a complementary detector in QC and R&D for complex matrices where UV fails to detect relevant components
• Integration with automated system suitability verification and predictive maintenance to ensure long-term stability across modules and sites
• Method standardization and harmonization across platforms to simplify regulatory submissions and multi-site studies

Conclusion

This application note demonstrates that Waters Charged Aerosol Detectors can provide highly reproducible HPLC-CAD performance both across multiple detector modules and over extended operational periods when combined with an optimized nebulizer design and calibrated nebulizer tuning. Achieving this level of reproducibility requires adherence to best practices in solvent and system cleanliness, consistent mobile phase preparation, and routine detector maintenance. These attributes make CAD a reliable option for routine pharmaceutical and biopharmaceutical analyses, especially for analytes not amenable to optical detection.

References

1. Barnhart, W. W.; Farooq, M. Q.; Ahmad, I. A. H. A Simplified Tutorial on Charge Aerosol Detection: Understanding the Basics, Optimization, and Troubleshooting. Journal of Chromatography Open, 2024, 100181.
2. Schilling, K.; Holzgrabe, U. Recent applications of the Charged Aerosol Detector for liquid chromatography in drug quality control. Journal of Chromatography A, 1619 (2020) 460911.
3. Vehovec, T.; Obreza, A. Review of Operating Principle and Applications of Charged Aerosol Detector. Journal of Chromatography A, 1217 (2010) 1549–1556.
4. United States Pharmacopeia. Deoxycholic acid. In USP–NF, 2020.