Introduction
One of the most important concepts in HPLC method development is understanding how analytes interact with the stationary phase and mobile phase. The three most commonly discussed chromatographic modes are:
- Normal Phase (NP)
- Reversed Phase (RP)
- HILIC (Hydrophilic Interaction Liquid Chromatography)
Each mode provides different retention characteristics and is best suited for particular classes of compounds.
For polar compounds that are difficult to retain by conventional reversed phase methods, HILIC has become an important analytical tool. Cogent™ TYPE-C™ silica hydride columns, including Diamond Hydride™, Phenyl Hydride™, Amide™, and Silica-C™, can all be used under HILIC conditions while providing unique selectivity compared to many conventional HILIC stationary phases.
Historically, MICROSOLV referred to this retention behavior as Aqueous Normal Phase (ANP). Today, HILIC is the preferred industry terminology for these types of polar compound separations.
Normal Phase Chromatography (NP)
Normal Phase chromatography uses:
Stationary Phase
- Polar
Mobile Phase
- Non-polar
Typical solvents include:
- Hexane
- Heptane
- Isopropyl alcohol
- Other non-polar solvent mixtures
In Normal Phase chromatography:
- Polar compounds are retained most strongly.
- Retention increases as the mobile phase becomes less polar.
- Polar analytes interact strongly with the stationary phase.
Normal Phase methods are often used when analytes are highly polar and soluble in organic solvents.
Reversed Phase Chromatography (RP)
Reversed Phase chromatography is the most widely used HPLC mode.
Stationary Phase
- Hydrophobic or non-polar
Examples:
- C18
- C8
- Phenyl phases
Mobile Phase
- Polar
Typical solvents include:
- Water
- Acetonitrile
- Methanol
In Reversed Phase chromatography:
- Hydrophobic compounds are retained most strongly.
- Retention generally increases as water content increases.
- Polar compounds often elute near the column void volume.
RP methods are excellent for many pharmaceutical, environmental, food, and industrial applications.
HILIC Chromatography
HILIC is designed specifically to improve retention of polar compounds.
Stationary Phase
Often uses materials capable of retaining hydrophilic analytes under high-organic mobile phase conditions.
Mobile Phase
Typically contains:
- High acetonitrile content
- Low water content
- Volatile buffers or modifiers
Examples:
- Acetonitrile / Water
- Acetonitrile / Formic Acid
- Acetonitrile / Ammonium Formate
In HILIC:
- Polar compounds are strongly retained.
- Retention is generally greatest at high organic content.
- Retention decreases as aqueous content increases.
This retention pattern often appears opposite to what is observed in Reversed Phase chromatography.
HILIC with TYPE-C™ Columns
All Cogent™ TYPE-C™ columns can be used under HILIC conditions, including:
- Diamond Hydride™
- Phenyl Hydride™
- Amide™
- Silica-C™
These columns provide strong retention of polar analytes while offering a selectivity profile that differs from many traditional HILIC materials.
Typical analytes include:
- Organic acids
- Amines
- Metabolites
- Peptides
- Carbohydrates
- Polar pharmaceuticals
- Zwitterions
The ability to retain these compounds effectively makes TYPE-C™ columns especially useful for LC-MS applications.
Comparing the Three Modes
Normal Phase
Stationary Phase: Polar
Mobile Phase: Non-polar
Retention Increases With: Less polar mobile phase
Best For: Polar compounds soluble in organic solvents
Reversed Phase
Stationary Phase: Non-polar
Mobile Phase: Polar
Retention Increases With: More aqueous mobile phase
Best For: Non-polar and moderately polar compounds
HILIC
Stationary Phase: Optimized for polar compound retention
Mobile Phase: High organic, low aqueous
Retention Increases With: Higher organic content
Best For: Polar, hydrophilic, and ionic compounds
Why HILIC Is Important
Many compounds show little useful retention in reversed phase chromatography.
Examples include:
- Small polar metabolites
- Charged molecules
- Amino acids
- Hydrophilic pharmaceuticals
- Sugars
HILIC provides:
- Increased retention
- Improved selectivity
- Enhanced LC-MS sensitivity
- Better separation of difficult polar analytes
These benefits have made HILIC one of the most important chromatographic modes for modern analytical laboratories.
Key Takeaways
- Normal Phase uses polar stationary phases and non-polar mobile phases.
- Reversed Phase uses non-polar stationary phases and aqueous mobile phases.
- HILIC is optimized for the retention of polar compounds using organic-rich mobile phases.
- TYPE-C™ columns can operate under HILIC conditions while providing unique selectivity for polar analytes.
- HILIC is especially valuable for LC-MS applications and compounds that exhibit poor retention in reversed phase methods.
- Diamond Hydride™, Phenyl Hydride™, Amide™, and Silica-C™ columns all support HILIC separations.
Additional Resources
For HILIC method development guidance, TYPE-C™ column specifications, LC-MS applications, product images, and ordering information, view: Cogent™ Diamond Hydride™, Phenyl Hydride™, Amide™, and Silica-C™ Column Specifications, HILIC Applications, and Ordering Information
