Introduction

Aqueous mobile phases provide an environment that can support bacterial and microbial growth if stored improperly or used for extended periods. While organic solvents generally inhibit microbial activity, water-rich mobile phases and neutral pH buffers are more susceptible to contamination.

Microbial growth can affect both HPLC system performance and analytical results, making preventive practices an important part of routine laboratory operation.

Why Microbial Growth Is a Concern

Microorganisms can proliferate in aqueous solvents and buffers over time, especially when:

• Mobile phases are stored for long periods.
• Buffers are prepared in large batches.
• Reservoirs are not cleaned regularly.
• Solutions are maintained near neutral pH.
• Laboratory temperatures are elevated.

Microbial contamination can contribute to:

• Increased system backpressure
• Blocked inlet frits and filters
• Clogged tubing
• Baseline instability
• Ghost peaks
• Reduced column performance
• Shortened column lifetime

Preventing contamination is generally easier than troubleshooting its effects after it occurs.

Acidified Mobile Phases Can Reduce Growth

One common approach is to acidify aqueous mobile phases when method conditions permit.

Examples include:

• Formic acid
• Acetic acid
• Trifluoroacetic acid (when appropriate)

For example:  0.1% formic acid is frequently used in HPLC and LC-MS methods and may provide the added benefit of discouraging microbial growth within the aqueous phase.

Many chromatographic methods already utilize acidified mobile phases to improve:

• Peak shape
• Reproducibility
• Ionization efficiency
• Method robustness

Reduced microbial activity can be an additional advantage.

Organic Solvents Help Inhibit Contamination

The presence of organic solvents often suppresses microbial growth.

Common solvents include:

• Acetonitrile
• Methanol
• Isopropanol

Even relatively small percentages of organic solvent may help reduce bacterial proliferation compared to purely aqueous solutions.  For many chromatographic methods, mixed aqueous-organic mobile phases naturally present a lower contamination risk than water-only systems.

Special Considerations for Neutral pH Buffers

Buffers prepared near neutral pH are generally more susceptible to microbial contamination.

Examples include:

• Phosphate buffers
• Acetate buffers
• Various biological buffers

When neutral pH conditions are required:

Recommended Practice

• Prepare concentrated stock solutions.
• Store stock solutions under refrigerated conditions when appropriate.
• Prepare fresh working dilutions as needed.
• Discard aged mobile phases according to laboratory procedures.

Fresh preparation often provides the best protection against contamination-related issues.

Avoid Cross-Contamination

When replacing aqueous mobile phases:

Do

• Completely empty the solvent reservoir.
• Clean the reservoir when appropriate.
• Refill with fresh solution.

Avoid

• Topping off old mobile phase with new solution.
• Mixing fresh and aged buffer preparations.
• Leaving residual solution in the reservoir for extended periods.

Mixing old and new solutions may introduce microorganisms into the fresh mobile phase and accelerate contamination.

Additional Best Practices

To help minimize microbial growth:

• Use high-purity water.
• Filter mobile phases when appropriate.
• Replace mobile phases regularly.
• Keep solvent reservoirs covered.
• Clean solvent reservoirs routinely.
• Inspect inlet filters and tubing periodically.
• Follow laboratory SOPs for mobile phase storage and disposal.

These practices help maintain reliable chromatographic performance and reduce unnecessary maintenance.

Key Takeaways

• Microbial growth is most common in water-rich mobile phases and neutral pH buffers.
• Contamination can contribute to backpressure increases, baseline issues, and system fouling.
• Acidification with agents such as formic acid may help suppress microbial activity.
• Organic solvents such as acetonitrile and methanol can inhibit bacterial growth.
• Neutral pH buffers should be prepared and stored carefully.
• Always replace old mobile phases rather than topping them off with fresh solution.
• Good solvent handling practices improve both system reliability and analytical performance.