Introduction

Mobile phase salts are commonly used in chromatography to influence:

• Retention
• Selectivity
• Peak shape
• Reproducibility
• Analyte ionization

In some chromatographic modes, particularly conventional HILIC and ion-exchange methods, relatively high salt concentrations are often used to achieve adequate retention and separation.

While these approaches may be effective chromatographically, elevated salt concentrations can create challenges for both LC-MS systems and stationary phase performance.  For Cogent™ TYPE-C™ columns, effective separations are often achieved with considerably lower additive concentrations, making method development simpler and more LC-MS friendly.

Why Salt Concentration Matters

Mobile phase additives such as:

• Ammonium acetate
• Ammonium formate
• Other volatile buffers

can significantly influence chromatographic behavior.

However, as salt concentration increases, several disadvantages may occur:

• Increased LC-MS source contamination
• Ion suppression
• Reduced sensitivity
• More frequent instrument maintenance
• Longer equilibration times
• Potential changes in stationary phase behavior

Because of these effects, unnecessarily high salt levels should generally be avoided.

Recommended Maximum Concentration

For most applications using Cogent™ TYPE-C™ columns:  10 mM should be considered the practical upper limit for routine mobile phase buffer concentrations.

This recommendation applies to both:

• HPLC methods
• LC-MS methods

In many cases, substantially lower concentrations may provide satisfactory retention and selectivity.

Typical starting points include:

• 1 mM to 5 mM ammonium acetate
• 1 mM to 5 mM ammonium formate
• Low concentrations of formic acid
• Low concentrations of acetic acid

Method developers should use the lowest concentration that achieves acceptable chromatographic performance.

Considerations for LC-MS Methods

Salt concentration is particularly important in LC-MS applications.

As buffer concentration increases:

• Ionization efficiency may decrease.
• Signal-to-noise ratio may decline.
• Source contamination can increase.
• Detector sensitivity may decrease.

These effects are often most noticeable during long analytical sequences where additive residues gradually accumulate in the ion source.

Lower additive concentrations help support:

• Better sensitivity
• Cleaner mass spectra
• Reduced maintenance requirements
• Improved instrument uptime

Retention Without High Salt Loads

One advantage of TYPE-C™ silica hydride technology is that many HILIC methods can be developed without relying on high concentrations of salts or buffers.

This often provides:

• Simpler mobile phases
• Improved LC-MS compatibility
• Faster method development
• Reduced operating costs
• Easier column maintenance

As a result, high-salt methods commonly used on some conventional HILIC stationary phases are often unnecessary.

Long-Term Effects of High Salt Concentrations

Very high salt concentrations may also create long-term challenges.

Potential concerns include:

• Salt accumulation within the chromatographic system
• Buffer precipitation
• Increased maintenance requirements
• Altered chromatographic behavior over time

In severe cases, residual additives may persist despite extensive washing procedures.  For this reason, routine operation at unnecessarily high concentrations is generally discouraged.

Method Development Strategy

When developing methods:

Start Low

Begin with:

• 0.1% Formic Acid
• 0.1% Acetic Acid
• 1-5 mM Ammonium Acetate
• 1-5 mM Ammonium Formate

Evaluate Performance

Review:

• Retention
• Resolution
• Peak shape
• Signal intensity
• Method reproducibility

Increase Only If Necessary

Raise additive concentration only when measurable improvements justify the change.  This approach frequently results in more robust and LC-MS-compatible methods.

Key Takeaways

• Most TYPE-C™ methods do not require high salt concentrations.
• A practical maximum limit for routine applications is approximately 10 mM.
• Lower concentrations are often sufficient for retention and selectivity.
• Excessive salt concentrations can reduce LC-MS sensitivity and increase maintenance requirements.
• High salt levels may contribute to source contamination and buffer accumulation.
• Using the lowest effective additive concentration generally produces the most robust methods.
• TYPE-C™ columns frequently provide excellent chromatographic performance without the high salt requirements associated with some conventional HILIC methods.