Quantification of Inorganic Counterions Using Hydrophilic Liquid Chromatography Coupled with Charged Aerosol Detection (CAD)

Importance of the topic

Accurate identification and quantification of inorganic counterions in pharmaceutical drug substances is essential for confirming salt stoichiometry, ensuring product purity, and controlling properties such as solubility, stability and bioavailability. Counterions are often non-chromophoric and require alternative detection strategies; a robust, single-method approach that handles both cations and anions simplifies development and QC workflows and reduces analysis time compared with separate ion chromatography methods.

Objectives and overview of the study

The application note describes development and optimization of a hydrophilic interaction liquid chromatography (HILIC) method coupled to a charged aerosol detector (CAD) for simultaneous quantitative analysis of common inorganic pharmaceutical counterions: chloride, nitrate, sodium, phosphate, potassium, calcium and magnesium. The goals were to (i) achieve baseline separation of anions and cations on a single stationary phase, (ii) optimize CAD operating parameters to maximize sensitivity and linearity, and (iii) demonstrate method precision, accuracy and applicability to representative drug substances.

Methodology and used instrumentation

The method uses HILIC separation on an Atlantis Premier BEH Z-HILIC Column (4.6 × 150 mm, 5 µm) with gradient mobile phases based on acetonitrile and water, buffered with ammonium formate and acidified with formic acid. Key chromatographic conditions included 45 °C column temperature, 1.3 mL/min flow, 10 µL injection volume and a 50:50 water/acetonitrile diluent for standards and samples. Standards covered 10–160 µg/mL per counterion (calculated from salt stoichiometry).
Used instrumentation (reported):

• Arc HPLC System (Waters) with column heater/cooler and active pre-heater
• Atlantis Premier BEH Z-HILIC Column, 4.6 × 150 mm, 5 µm
• Waters Charged Aerosol Detector (CAD)
• Empower Software v3.6.0 for acquisition and processing
• Milli-Q purified water and LC/MS-grade acetonitrile, ammonium formate, formic acid

CAD-specific settings optimized and used: power function value (PFV) = 1.30, evaporator temperature = 45 °C, ion trap voltage = 40 V, sampling rate = 10 pts/s, filter time constant = 1.00 s.

Main results and discussion

• Separation: HILIC separation on the BEH Z-HILIC column produced baseline separation of the seven inorganic counterions, enabling single-run analysis of both anionic and cationic species.
• CAD optimization: Systematic evaluation of CAD parameters showed trade-offs: raising evaporator temperature reduced baseline noise but decreased analyte signal; 45 °C was chosen as the best compromise. An ion trap voltage of 40 V minimized noise while preserving analyte signal. Adjusting the PFV to 1.30 markedly improved detector linearity across the calibration range compared with the default PFV of 1.00.
• Linearity and calibration: Calibration curves were prepared from 10 to 160 µg/mL using a linear fit with 1/x weighting. Correlation coefficients were ≥ 0.998 for all counterions when using the optimized PFV of 1.30, indicating excellent linearity after PFV compensation.
• Precision and reproducibility: Inter-day performance (10 replicate injections of a 60 µg/mL standard mixture across three days) produced peak area RSDs ≤ 1.15% and consistent retention times and USP resolution values, demonstrating robust reproducibility.
• Accuracy (recovery) and sample application: Analysis of four drug substances (memantine HCl, metformin HCl, losartan potassium and ranitidine HCl) yielded counterion recoveries between 99% and 103% with recovery RSDs ≤ 0.80% (n = 6). Memantine free base was not retained/detected by CAD due to volatility, but its counterion (chloride) was quantified reliably. The drug-related peaks were well resolved from counterion peaks.

Benefits and practical applications of the method

• Simultaneous measurement of both anionic and cationic inorganic counterions in a single HPLC run simplifies workflows versus separate ion chromatography methods for cations and anions.
• CAD provides a universal response for non-volatile ionic analytes that lack UV chromophores; when optimized (PFV etc.) it delivers strong linearity and reproducible quantitative performance compared with ELSD alternatives.
• The method demonstrated high accuracy and precision suitable for drug development, release testing, stoichiometry confirmation and batch release QC.
• Integration with Empower software and Waters CDS enables compliance-ready data handling, streamlined reporting and traceability for regulated environments.

Limitations and practical considerations

• CAD detects non-volatile particles generated after solvent evaporation; volatile drug substances or volatile counterions may be lost and therefore not detected (as demonstrated for memantine free base).
• CAD response is inherently non-linear; digital correction (PFV) and appropriate calibration weighting (1/x) are required to obtain linear calibration across the intended range.
• Method robustness depends on careful control of CAD parameters (evaporator temperature, ion trap voltage, PFV) and consistent mobile phase composition and sample solvent strength in HILIC mode.

Future trends and potential applications

The combination of HILIC separations and CAD detection is positioned to expand in pharmaceutical analytics where rapid, single-method counterion analysis is desirable. Anticipated directions include:

• Broader application to peptide and biologic salts, and to organic counterions, with method adaptations for larger/less volatile species.
• Automation and higher-throughput implementations (shorter columns, UHPLC/HILIC miniaturization) coupled with optimized CAD control for routine QC environments.
• Further software-driven correction and chemometric approaches to extend CAD linear dynamic range and simplify calibration strategy.
• Method transfer and qualification across laboratories and instruments to support regulatory submissions and multi-site QC harmonization.

Conclusion

A HILIC-CAD method using an Atlantis Premier BEH Z-HILIC column and Waters CAD was shown to separate and quantify seven common inorganic counterions simultaneously with excellent accuracy (99–103% recoveries), precision (RSDs ≤ 1.15% inter-day, ≤ 0.80% recovery RSD), and linearity (R2 ≥ 0.998 using PFV = 1.30 and 1/x weighting). Optimization of CAD parameters (evaporator temperature, ion trap voltage, PFV) was critical to achieving these results. The approach offers a robust, single-run alternative for counterion analysis in drug development and QC, with clear pathways for broader application and automation.

References

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