Introduction
Mobile phase selection is one of the most important variables in HILIC method development. Unlike reversed phase chromatography, where increasing organic solvent generally increases elution strength, HILIC methods follow a very different pattern.
Because Cogent™ TYPE-C™ columns are frequently used for the retention and separation of polar compounds, understanding the relative strength of common solvents can simplify method development and improve chromatographic performance.
Proper solvent selection influences:
- Retention
- Selectivity
- Resolution
- Run time
- Column cleanliness
- LC-MS compatibility
A clear understanding of solvent strength allows chromatographers to make informed adjustments during optimization.
Solvent Strength Order in HILIC Methods
When used under HILIC conditions on TYPE-C™ columns, the relative solvent strength generally follows the order:
DI Water > Methanol > Isopropanol (IPA) > Acetone > Acetonitrile
Strongest Solvent - Water
Produces the greatest reduction in retention for many polar compounds.
Weakest Solvent - Acetonitrile
Typically produces the strongest retention of polar analytes. This ordering is fundamentally different from what chromatographers commonly observe in reversed phase HPLC.
How Solvent Strength Affects Retention
The solvent strength relationship has direct practical implications.
Increasing Water Content
Increasing the proportion of water generally:
- Decreases retention
- Accelerates elution
- Shortens run times
This effect can be useful when compounds are excessively retained.
Increasing Acetonitrile Content
Increasing acetonitrile generally:
- Increases retention
- Improves separation of early-eluting polar compounds
- Enhances selectivity for many hydrophilic analytes
This is one reason acetonitrile-rich mobile phases are commonly used in HILIC methods.
Implications for Polar Compound Analysis
Many analytes that are poorly retained in reversed phase chromatography exhibit excellent retention under HILIC conditions.
Examples include:
- Amino acids
- Organic acids
- Polar pharmaceuticals
- Metabolites
- Sugars
- Charged compounds
For these analytes, adjusting solvent strength can have a substantial impact on chromatographic performance.
A small change in water content may produce significant changes in:
- Retention time
- Resolution
- Peak spacing
- Selectivity
Why Methanol and IPA Are Frequently Used
Although water is the strongest solvent in HILIC methods, many mobile phases include methanol or isopropanol in the aqueous component.
These alcohols provide benefits beyond simple retention control.
Advantages of Methanol and IPA
They can help:
- Reduce buildup of matrix components
- Improve long-term reproducibility
- Minimize carryover
- Maintain column cleanliness
- Improve performance with biological samples
These benefits are especially important for:
- Metabolomics
- Bioanalysis
- Food and beverage testing
- Complex sample matrices
Important Practical Note
Methanol or IPA is typically incorporated into the aqueous mobile phase component.
HILIC methods generally do not rely solely on water and methanol as the complete mobile phase system.
Acetone Versus Acetonitrile
Acetone is sometimes used as an alternative to acetonitrile, particularly in LC-MS applications. Although both solvents can function well in HILIC methods, they should not be considered interchangeable.
Potential Differences
Some analytes may exhibit:
- Different retention times
- Different selectivity
- Different elution order
When replacing acetonitrile with acetone, method developers should re-evaluate the separation rather than assuming equivalent performance.
Acetone in LC-MS Applications
Acetone can be particularly attractive in some LC-MS methods because:
- It is highly volatile.
- It can provide alternative selectivity.
- It may be useful during acetonitrile shortages or method optimization.
However, suitability should always be verified experimentally for the analytes of interest.
Using Solvent Strength During Method Development
Understanding solvent strength provides a powerful optimization tool.
When Retention Is Too Low
Consider:
- Increasing acetonitrile content
- Reducing water content
When Retention Is Too High
Consider:
- Increasing water content
- Increasing solvent strength
When Selectivity Needs Improvement
Evaluate:
- Methanol
- IPA
- Acetone
as alternative modifiers.
These changes often produce selectivity differences that may improve difficult separations.
Practical Method Development Guidelines
Stronger Solvents Such as Water Can Be Used For:
- Reducing excessive retention
- Shortening run times
- Column washing
- Method cleanup steps
Weaker Solvents Such as Acetonitrile Can Be Used For:
- Maximizing retention of polar compounds
- Improving resolution
- Separating complex metabolite mixtures
- Enhancing retention of early-eluting analytes
A systematic evaluation of solvent composition is often one of the most effective method-development strategies.
Key Takeaways
- Solvent strength in HILIC methods differs significantly from reversed phase chromatography.
- The general solvent strength order is: Water > Methanol > IPA > Acetone > Acetonitrile.
- Increasing water content generally decreases retention.
- Increasing acetonitrile content generally increases retention.
- Methanol and IPA are often used to improve column cleanliness and robustness.
- Acetone can serve as an alternative organic modifier but may alter selectivity.
- Understanding solvent strength simplifies HILIC method development and optimization.
Related Resources
