Overview

Mobile phase additives such as ammonium acetate and ammonium formate are commonly used in HILIC methods because they provide:

• Volatile buffering capacity
• LC-MS compatibility
• Improved chromatographic reproducibility
• Enhanced control of analyte ionization

However, buffer concentration can play an important role in how acidic analytes behave on TYPE-C™ silica hydride columns, including:

• Diamond Hydride™
• Phenyl Hydride™
• Amide™
• Silica-C™

As a result, buffer concentration should be considered an important method-development variable when optimizing separations of organic acids and other acidic compounds.

Why Buffer Concentration Matters

The concentration of ammonium acetate or ammonium formate can influence both:

• The ionization state of acidic compounds
• Interactions occurring at the stationary phase surface

Changes in buffer concentration may alter:

• Retention
• Selectivity
• Peak shape
• Method reproducibility

For acidic analytes, even small changes in buffer concentration can sometimes produce measurable differences in chromatographic behavior.

Effects on Acidic Compound Retention

Many acidic compounds are sensitive to mobile phase chemistry.

Examples include:

• Organic acids
• Carboxylic acids
• Metabolites
• Pharmaceutical acids
• Environmental analytes

As buffer concentration increases, retention of acidic compounds may decrease in some HILIC methods.

This effect may be observed as:

• Earlier elution
• Reduced retention factors
• Changes in selectivity
• Altered separation profiles

Because each analyte behaves differently, retention effects should always be verified experimentally during method development.

Influence on LC-MS Performance

Buffer concentration affects more than chromatography.

Increasing buffer concentration may also lead to:

• Reduced MS sensitivity
• Increased ion suppression
• Greater source contamination
• More frequent maintenance requirements

For these reasons, LC-MS methods often benefit from using the lowest buffer concentration that still provides acceptable chromatographic performance.

Finding the Optimal Buffer Concentration

Method development generally requires balancing several competing factors.

Enough buffer should be present to provide:

• Stable ionization conditions
• Consistent retention
• Good peak shape
• Method reproducibility

At the same time, buffer levels should remain low enough to:

• Preserve LC-MS sensitivity
• Minimize source contamination
• Prevent unnecessary retention changes
• Reduce the risk of precipitation

For many HILIC LC-MS applications, lower buffer concentrations often provide the best overall balance.

Practical Method Development Guidelines

When developing HILIC methods for acidic compounds: 

Begin with a Low Buffer Concentration

• Start with a conservative concentration and increase only if necessary.

Evaluate Retention Carefully

• If acidic compounds elute too early or retention changes unexpectedly, buffer concentration may be contributing to the behavior.

Monitor MS Response

• Always evaluate chromatographic improvements alongside MS sensitivity.  Improved retention is not always beneficial if signal intensity decreases substantially.

Maintain Consistency

Once an optimal buffer concentration has been identified:

• Keep buffer preparation consistent.
• Use the same buffer concentration between batches.
• Maintain the same mobile phase preparation procedure.

Consistency promotes reproducible method performance.

HILIC Method Optimization

Because HILIC methods often operate with:

• High acetonitrile content
• Low water content
• Volatile additives

small changes in mobile phase composition can sometimes produce larger chromatographic effects than users expect.

Buffer concentration is therefore one of the most important variables to evaluate when optimizing separations of acidic analytes.

Key Takeaways

• Buffer concentration can significantly affect the retention of acidic compounds in HILIC methods.
• Ammonium acetate and ammonium formate are commonly used volatile additives for HILIC LC-MS applications.
• Higher buffer concentrations may reduce retention of some acidic analytes.
• Excessive buffer concentration can reduce LC-MS sensitivity and increase source contamination.
• Lower buffer concentrations often provide the best balance between chromatographic performance and MS response.
• Buffer optimization should be part of all HILIC method-development studies involving acidic compounds.