Hydrophilic Interaction Liquid Chromatography: Some Aspects of Solvent and Column Selectivity

Importance of the Topic

The analysis of small polar and hydrophilic compounds poses significant challenges in liquid chromatography. Conventional reversed-phase approaches often require ion-pair reagents, high buffer concentrations or highly aqueous mobile phases, compromising mass spectrometric detection and chromatographic performance. Hydrophilic interaction liquid chromatography (HILIC) offers an effective alternative by combining a polar stationary phase with a predominantly organic mobile phase to improve retention, selectivity and compatibility with MS detection.

Objectives and Study Overview

This study evaluated how mobile phase composition, column temperature and buffer concentration affect retention and selectivity on five distinct stationary phases under HILIC conditions. The goals were:

• To investigate retention mechanisms across different HILIC materials
• To understand the chromatographic processes driving separation
• To guide method development and optimize column selection for polar analytes

Methodology

A mixture of five basic compounds (uracil, adenosine, uridine, cytosine, cytidine) and a mixture of four acidic compounds (salicylamide, salicylic acid, aspirin, 3,4-dihydroxyphenylacetic acid) were analyzed on five stationary phases:

• Accucore HILIC (core-shell silica)
• Hypersil GOLD HILIC (weak anion-exchange polymeric phase)
• Syncronis HILIC (zwitterionic phase)
• Hypersil GOLD Silica (bare silica)
• Hypercarb (porous graphitic carbon)

The effects of acetonitrile content (10–100%), column temperature (20–70 °C) and ammonium acetate concentration (2.5–20 mM) were systematically studied. Retention factors and van’t Hoff plots were generated to derive thermodynamic parameters and assess retention mechanisms.

Used Instrumentation

An HPLC system with a quaternary pump, diode array detector, degasser, column heater and autosampler was used. Columns were 100 × 4.6 mm with particle sizes of 2.6–5 µm. Flow rate was 1.0 mL/min, injection volume 5 µL, and detection wavelengths were 228 nm (acidic mixture) and 248 nm (basic mixture).

Main Results and Discussion

• Column selectivity varied markedly between phases despite identical mobile phases. Syncronis HILIC showed high retention due to combined partitioning and electrostatic effects, while Hypercarb displayed minimal retention of highly polar compounds at low organic content.
• Increasing acetonitrile content generally enhanced retention for HILIC phases; Hypercarb exhibited reversed-phase behavior below 60% acetonitrile, demonstrating dual retention mechanisms.
• Van’t Hoff analysis revealed exothermic retention on Syncronis HILIC, Hypersil GOLD Silica and Accucore HILIC; endothermic retention was observed on Hypersil GOLD HILIC and Hypercarb, reflecting contributions from ion exchange and dispersive interactions.
• Higher buffer concentrations increased retention on partitioning-dominated phases by expanding the water-rich layer, while on ion-exchange Hypersil GOLD HILIC, salicylic acid retention decreased with buffer due to competitive displacement.

Benefits and Practical Applications

The insights into retention mechanisms and selectivity provide analytical chemists with:

• A systematic approach to HILIC method development and column selection
• Strategies to tailor mobile phase composition and temperature for optimal separation
• Improved compatibility with MS detection and reproducible analysis of polar analytes

Future Trends and Applications

Advances in HILIC are expected to include the design of hybrid stationary phases combining multiple interaction modes, refined thermodynamic models for retention prediction, and integration with high-resolution mass spectrometry. Emerging applications may target metabolomics, glycomics and environmental analysis where polar compound profiling is critical.

Conclusion

This comparative study of five HILIC materials under varied conditions highlighted the complex interplay of partitioning, electrostatic and dispersive interactions. Understanding these mechanisms enables more informed method optimization and enhances the separation of challenging polar analytes.

References

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• P. Hemström and K. Irgum, J. Sep. Sci. 29 (2006) 1784