Introduction

Retention time shifts that appear after a mobile phase or buffer has been stored for an extended period are often attributed to the column, instrument, or method. However, in many cases the mobile phase itself may have changed during storage.

Even minor changes in solvent composition, buffer concentration, or additive concentration can alter chromatographic behavior and cause methods that previously passed system suitability requirements to fall outside acceptable limits.  Understanding how storage affects mobile phases can help improve reproducibility and reduce troubleshooting time.

Why Retention Times May Change After Buffer Storage

Many HPLC methods are validated using freshly prepared mobile phases. When the same mobile phase is stored for extended periods, its composition may gradually change.

Potential consequences include:

• Retention time shifts
• Increased retention
• Changes in peak tailing
• Poor reproducibility
• Increased %RSD values
• Failure of system suitability requirements

The extent of these changes depends on the mobile phase composition, storage conditions, container closure integrity, and storage duration.

Organic Solvent Evaporation

One of the most common causes of retention time changes is evaporation of the organic component of the mobile phase.

For example, if a mobile phase contains:

• Acetonitrile
• Methanol
• Another volatile organic solvent

small losses due to evaporation can change the mobile phase composition over time.  As the organic content decreases, the mobile phase becomes relatively more aqueous.

In reversed-phase chromatography, this typically results in:

• Increased retention of non-polar compounds
• Longer retention times
• Potential increases in peak tailing
• Changes in selectivity

These effects may become more pronounced as storage time increases.

Impact on Method Performance

In one commonly encountered scenario, a freshly prepared buffer may perform normally for days or weeks, while the same mobile phase stored for months may no longer meet established system suitability requirements.

Possible observations include:

• Higher retention times
• Greater retention time variability
• Increased tailing factors
• Failure of retention time %RSD limits
• Reduced method robustness

Unless stability studies have demonstrated long-term mobile phase stability under the actual storage conditions, extended storage should be approached cautiously.

Buffer Additive Concentration Changes

Mobile phase additives can also become more concentrated as solvent evaporation occurs.

This is particularly important for ion-pairing and buffering reagents such as:

• Tetrabutylammonium hydrogen sulfate
• Phosphate buffers
• Sulfonic acids
• Other non-volatile salts

As solvent volume decreases, the concentration of these additives increases.

This can alter:

• Retention
• Selectivity
• Peak shape
• Column operating conditions

Effects on Column Performance

Certain buffer components may place additional stress on the stationary phase when their concentration increases beyond the intended method conditions.

Increases in buffer or additive concentration may contribute to:

• Changes in chromatographic behavior
• Altered retention characteristics
• Reduced method reproducibility
• Increased stationary phase stress over time

For this reason, mobile phase composition should remain as close as possible to the originally validated conditions.

Vacuum Degassing Considerations

Manual vacuum degassing can also influence mobile phase composition.

When volatile solvents such as acetonitrile are present, excessive vacuum exposure may result in:

• Partial solvent loss
• Changes in solvent ratios
• Altered retention behavior

Although degassing is important for chromatographic performance, excessive solvent loss during the process should be avoided.

Best Practices for Mobile Phase Storage

To improve method consistency:

• Prepare mobile phases on a routine schedule.
• Use well-sealed reservoir bottles.
• Minimize unnecessary storage times.
• Document preparation dates.
• Verify mobile phase stability during method validation when long-term storage is anticipated.
• Use caution when storing mobile phases containing volatile organics.
• Monitor retention times for trends that may indicate composition changes.

These practices can help maintain consistent chromatographic performance.

Applications Most Susceptible to Storage Effects

Changes in mobile phase composition can be particularly important for:

• Reversed-phase HPLC methods
• Ion-pair chromatography
• Gradient methods
• Pharmaceutical assays
• Stability studies
• Trace impurity analyses
• Methods with tight system suitability requirements

These methods often rely on highly reproducible mobile phase composition.

Conclusion

Retention time shifts observed after prolonged mobile phase storage are frequently caused by changes in solvent composition resulting from evaporation or concentration changes in buffer additives. Volatile organic solvents such as acetonitrile are particularly susceptible to loss during storage and vacuum degassing. Proper mobile phase preparation, storage, and validation of stability can help maintain consistent retention times, peak shape, and overall method performance.