Improved Results for LC/MS of Basic Compounds Using High pH Mobile Phase on a Gemini® C18 Column

Importance of the Topic

Study Objectives and Overview

Methodology and Instrumentation

• HPLC system: Agilent HP 1100 series with binary pump and autosampler
• Mass spectrometer: API 3000 triple quadrupole with TurboIonSpray source
• Column: Gemini 5 µm C18, 150 × 3.0 mm
• Low pH mobile phase: 0.1% formic acid in water (A) and 0.1% formic acid in acetonitrile (B)
• High pH mobile phase: 10 mM ammonium bicarbonate buffer adjusted to pH 7.8–11 (A) and acetonitrile (B)
• Gradient: 10% to 90% B over 10 min, hold 2 min, re-equilibrate 4 min at 0.6 mL/min
• Analyte range: 0.05–200 ng/mL; injection volume: 5 µL

Main Results and Discussion

• Retention and Selectivity: High pH buffers extended retention of polar bases and often altered elution order, revealing alternative stationary-phase selectivity.
• Peak Shape: Peak asymmetry decreased, producing sharper and more symmetrical chromatographic peaks.
• Ionization Response: Positive ion formation remained efficient at high pH, with many compounds showing higher signal intensities and signal-to-noise ratios versus pH 2.7.
• Sensitivity: Limits of quantitation as low as 50 fg on-column were achieved, with most analytes quantifiable at 1 pg or below.
• Linearity: Calibration curves over 0.05–100 ng/mL exhibited excellent linearity (R2 > 0.997).
• Precision and Accuracy: Inter-day precision was below 15% RSD at the lowest level and below 12% at 100 ng/mL; accuracy ranged between 80% and 120% across concentrations.

Benefits and Practical Applications

• Elimination of ion-pair reagents while maintaining retention of polar basic analytes.
• Enhanced chromatographic resolution and operational robustness.
• Improved MS compatibility due to higher organic content and stable pH buffering.
• Wide dynamic range suitable for trace-level analysis in pharmaceutical, clinical, and environmental studies.

Future Trends and Potential Applications

• Application to more challenging basic analytes and metabolite profiling.
• Integration with high-throughput workflows and automated sample preparation.
• Development of pH-stable stationary phases enabling operation at extreme pH values.
• Use in multidimensional LC separations and proteomic analyses of peptides.
• Adoption in regulated bioanalytical methods for drug development and therapeutic monitoring.

Conclusion

References

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