Introduction

Mobile phase additives play a critical role in HILIC method development. Acids such as formic acid, acetic acid, and trifluoroacetic acid (TFA) are frequently used to influence analyte ionization, peak shape, retention, and LC-MS response.

A common assumption during method development is that increasing acid concentration will automatically improve chromatographic performance. However, this is often not the case when using Cogent™ TYPE-C™ silica hydride columns.

In many HILIC applications, relatively low concentrations of acid provide the best balance of:

• Retention
• Peak shape
• Reproducibility
• LC-MS compatibility
• Method robustness

Understanding how acid concentration influences chromatography can prevent unnecessary optimization work and improve overall method performance.

Typical Acid Concentrations Used in HILIC Methods

For most applications, there is little reason to exceed modest acid concentrations.

Common starting conditions include:

Formic Acid

• 0.05%
• 0.10%
• Up to 0.50% when necessary

Acetic Acid

• 0.05%
• 0.10%
• Up to 0.50% when necessary

These levels are generally sufficient for most polar compound analyses performed under HILIC conditions.

Trifluoroacetic Acid (TFA) Considerations

TFA may occasionally be used for certain applications, particularly when peak shape improvements are being investigated.

However, typical concentrations are generally kept low:

• Approximately 0.01%
• Up to approximately 0.10% in specialized applications

Important LC-MS Consideration

TFA is often avoided in LC-MS methods because it can reduce ionization efficiency and contribute to:

• Ion suppression
• Lower sensitivity
• Reduced detection limits

For LC-MS workflows, formic acid and acetic acid are typically preferred.

More Acid Is Not Always Better

One of the more interesting observations when developing HILIC methods on TYPE-C™ columns is that increasing acid concentration often produces diminishing returns.

In some cases:

• Retention may decrease.
• Selectivity may change.
• Peak shape may not improve.
• LC-MS response may decline.

As a result, the lowest concentration that provides acceptable chromatographic performance is frequently the preferred choice.

Why Acid Optimization Matters

Acid concentration affects multiple aspects of chromatography simultaneously.

Changes in acid levels can influence:

• Analyte ionization
• Retention behavior
• Selectivity
• Peak symmetry
• Detector response
• LC-MS sensitivity

For this reason, acid concentration should be treated as an important method-development variable rather than simply maximizing the additive level.

Retention Trends on TYPE-C™ Columns

During HILIC method development, retention behavior is often highly sensitive to additive concentration.

Small changes in formic acid or acetic acid concentration may produce measurable changes in:

• Retention time
• Resolution
• Selectivity

This is one reason why careful optimization is recommended rather than simply increasing additive concentration.

Acid Concentration Comparison Charts

LC-MS Method Development Advantages

Using lower acid concentrations can offer several practical benefits for LC-MS applications:

• Improved ionization efficiency
• Increased analyte response
• Better signal-to-noise ratio
• Reduced source contamination
• Lower operating costs
• Simplified mobile phase preparation

These advantages become increasingly important during high-throughput analytical workflows.

Recommended Method Development Strategy

A practical approach is to: Start Low

Begin with:

• 0.05% Formic Acid or
• 0.05% Acetic Acid

Evaluate Performance

Review:

• Retention
• Peak shape
• Selectivity
• Sensitivity

Increase Only If Necessary

Adjust acid concentration only when a measurable improvement is needed.  This approach often results in simpler and more robust methods.

Key Takeaways

• Low concentrations of formic acid or acetic acid are often sufficient for HILIC methods on TYPE-C™ columns.
• Acid concentrations above 0.5% are rarely necessary.
• TFA can be useful in selected applications but is generally not recommended for LC-MS because of ion suppression.
• Increasing acid concentration does not automatically improve retention or peak shape.
• Acid concentration is an important method-development variable that should be optimized experimentally.
• The lowest concentration that achieves acceptable performance is often the preferred choice.