HILIC: Principles, Method Development, and Applications
Hydrophilic interaction liquid chromatography (HILIC) pairs a polar stationary phase with a high-organic mobile phase to retain polar compounds that a typical reversed-phase column cannot hold. Some chemists still ask, half-jokingly, “what the HILIC is HILIC,” and the short answer is that it is the chromatography technique that fills the retention gap left by reversed phase for very hydrophilic analytes.
Analysts working on metabolites, glycans, and small polar drugs turn to this chromatographic mode because it delivers strong retention where reversed phase falls short, and because its mobile-phase composition suits mass spectrometry detection directly. This guide walks through the retention mechanism, phase selection, common pitfalls, and applications, then compares HILIC against reversed-phase high-performance liquid chromatography (HPLC) so you can decide which mode fits your separation.
What Is HILIC?
HILIC is a variant of normal-phase liquid chromatography that separates polar compounds on a polar stationary phase using a water-miscible organic mobile phase, where water acts as the strong solvent. Andrew Alpert named and introduced hydrophilic interaction chromatography in 1990 to address the poor retention of polar compounds on reversed-phase columns, proposing the term specifically to distinguish it as a variation of normal-phase chromatography.
HILIC complements reversed-phase HPLC rather than replacing it, and its elution order runs opposite to a reversed-phase column: analytes travel from least polar to most polar, a distinctive pattern that makes HILIC easy to recognize on a chromatogram.
How HILIC Works: The Retention Mechanism
A water-enriched layer forms on the surface of the polar stationary phase when a column equilibrates with an acetonitrile-rich mobile phase, and polar analytes partition between this layer and the organic-rich bulk solvent. Retention is not governed by partitioning alone, though, so treat it as a multi-modal mechanism that blends partitioning in the mobile phase layers and interaction with the stationary phase, hydrogen bonding, dipole–dipole interactions, and electrostatic (ion-exchange-like) effects.
Water functions as the strong eluting solvent in HILIC, the opposite of reversed phase, so raising the water content of the mobile phase speeds elution rather than slowing it down.
HILIC vs. Reversed-Phase HPLC
HILIC and reversed-phase HPLC sit at opposite ends of the polarity spectrum, and choosing between them comes down to how hydrophilic the target analyte is. The table below lays out the practical differences an analyst weighs during method development.
HILIC and reversed-phase HPLC offer orthogonal selectivity. When a reversed-phase method shows poor retention or peak shape for a polar analyte, a HILIC column built for that analyte class, such as a Luna Omega HILIC, frequently restores retention and improves peak shape without a full method redesign.
HILIC Stationary Phases: How to Choose
Choosing a HILIC stationary phase starts with the dominant functional group on the analyte, since structure-guided selection makes method development more predictable and less dependent on trial and error. A comprehensive 2025 review in Molecules frames this clearly: “Understanding the relationship between the analyte structure and stationary phase is essential for rational HILIC method development. While empirical trial and error remains common practice, general structural rules can provide valuable guidance in selecting suitable stationary phases.”
The same 2025 review notes that basic analytes, including many antibiotics and nucleobases, “are prone to strong electrostatic interactions with residual silanols, leading to poor peak shape if not properly controlled,” and that zwitterionic or amide-type phases balance ionic and hydrogen-bonding interactions to improve reproducibility. The authors add that aligning stationary phase chemistry with an analyte’s dominant functional groups “can become more predictable and less reliant on empirical screening,” helping secure optimal selectivity, retention, and peak shape for highly polar compounds.
Follows a simple table that helps chromatographers to match analyte functional groups to the most suited HILIC stationary phase.
(-COOH, -SO3H)
With HILIC, there is no single stationary phase that fits every application. Analysts can choose from a variety of chemistries, including bare silica, amino, amide, zwitterionic, and diol phases, each offering distinct selectivity for polar compounds. For many method development projects, however, diol-based stationary phases provide an excellent starting point thanks to their broad applicability across a wide range of polar analytes. Columns such as Luna HILIC and Luna Omega HILIC are often considered versatile first choices, delivering balanced retention and selectivity for compounds ranging from small metabolites to pharmaceutical impurities. As one of the latest additions to the Phenomenex HILIC portfolio, Luna Omega HILIC is designed to enhance polar compound retention and separation through multiple interaction mechanisms, while its robust, thermally modified fully porous particles help support long column lifetime. The phase also offers additional selectivity for small polar analytes, providing analysts with greater flexibility when developing or optimizing challenging HILIC methods.
To check all the Phenomenex HILIC stationary phases and all the applications they can be used for you can reference to the Phenomenex HILIC page.
HILIC Mobile Phases, Buffers, and Injection Solvent
A HILIC mobile phase typically starts with a high percentage of aprotic organic solvents, and gradients raise the water content to increase elution strength since water is the strong solvent here. The exact starting ratio depends on the analyte and stationary phase.
Acetonitrile is strongly preferred as the organic component among common HPLC mobile phase solvents because it is aprotic and readily supports formation of the water-rich surface layer that drives HILIC retention as well as it is MS compatible. Protic solvents such as methanol are also used in HILIC, but they compete with the analyte for that water layer, which generally reduces retention and shifts selectivity, so methanol works best as a tool for fine-tuning selectivity rather than as the primary organic solvent.
Buffer choice is especially critical because concentration and pH strongly affect selectivity, and inconsistent buffer preparation is one of the fastest ways to destabilize a HILIC separation. Ammonium acetate and ammonium formate are the standard buffers because both are volatile, fully compatible with mass spectrometry detection, and soluble even in a mobile phase with a high percentage of acetonitrile.
HILIC is also unusually sensitive to injection solvent, so the sample should ideally dissolve in the same solvent as the starting mobile phase, or in a slightly weaker (more organic) solvent, because a too-aqueous injection solvent causes peak splitting or breakthrough. Columns also need longer equilibration than a typical reversed-phase column because the water layer that drives retention takes time to establish fully on the polar stationary phase.
For further method development support the HILIC method development guide is available on the Phenomenex website.
Applications of HILIC
HILIC has moved well beyond a niche technique and now covers several core areas of analytical work in pharmaceutical and life sciences:
- Pharmaceutical impurity profiling of polar drugs and process-related impurities, a domain where HILIC has seen extensive published use for over two decades.
- Metabolomics, where amino acids, sugars, nucleotides, and organic acids separate well by HILIC-MS.
- Glycan, peptide, and intact protein or glycoprotein characterization, taking advantage of HILIC’s compatibility with mass spectrometry.
- Oligonucleotide analysis and emerging mRNA and lipid nanoparticle workflows, where highly charged and polar species need a dedicated separation mode.
- Food science, clinical chemistry, and natural-product analysis of polar analytes such as vitamins, organic acids, and small carbohydrates.
Common HILIC Pitfalls and How to Avoid Them
Most troubleshooting traces back to a handful of recurring issues, and each has a fairly direct fix.
- Insufficient equilibration causes drifting retention, so allow extra column volumes before the first injection and between batches.
- An excessively aqueous injection solvent causes peak splitting or fronting, so match the injection solvent to the starting mobile phase composition or use a weaker diluent.
- Small buffer or pH changes shift selectivity unexpectedly, so fix and document buffer concentration and pH as tightly as you would for an ion exchange method.
- Low reproducibility often comes from inconsistent water content between mobile phase preparations, so measure and verify accurately the aqueous percentage it.
- Salt precipitation can occur when a high-aqueous sample meets a high-organic mobile phase at the needle, so keep injection solvent composition compatible with the starting conditions.
FAQs on HILIC
For what is HILIC used?
HILIC separates polar, hydrophilic, and ionic compounds that reversed-phase chromatography cannot retain well, including metabolites, glycans, peptides, and polar pharmaceutical impurities. It is a common choice whenever an analyte elutes too close to the void volume on a reversed-phase column.
What is the difference between HILIC and reversed-phase chromatography?
HILIC uses a polar stationary phase with a high-organic mobile phase, and water acts as the strong solvent. Reversed-phase chromatography uses an apolar stationary phase with a mostly aqueous mobile phase, and the elution order in each mode runs opposite to the other.
Why is acetonitrile used in HILIC?
Acetonitrile is aprotic, so it does not compete with the analyte for the water-enriched layer that forms on the polar stationary phase. That property makes it the preferred organic solvent, while protic solvents such as methanol are reserved for adjusting selectivity.
Is water the strong or weak solvent in HILIC?
Water is the strong solvent in HILIC, the opposite of reversed phase. Increasing the water percentage in the mobile phase speeds elution rather than slowing it down.
What compounds are best analyzed by HILIC?
HILIC suits polar, hydrophilic, and ionic compounds such as amino acids, sugars, nucleotides, and small polar drug metabolites. Zwitterionic and amide phases extend this range to peptides and basic analytes that show poor peak shape elsewhere.
Why does my HILIC method have poor peak shape or drifting retention?
Poor peak shape often traces back to a mismatched injection solvent, insufficient equilibration, or shifting buffer pH. Match the injection solvent to the mobile phase, extend equilibration time, and lock down buffer concentration and pH before troubleshooting further.
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