Introduction

Ammonium acetate is one of the most widely used volatile buffers in HPLC and LC-MS because it provides buffering capacity while maintaining compatibility with mass spectrometric detection. High-organic mobile phases containing ammonium acetate are frequently used for the separation of polar compounds in HILIC and Aqueous Normal Phase (ANP) chromatography.  The following procedure produces a mobile phase containing:

• 16 mM ammonium acetate
• Approximately 90% acetonitrile
• Approximately 10% aqueous phase

Using a concentrated stock solution can improve consistency and simplify routine mobile phase preparation.

Step 1: Prepare a 500 mM Ammonium Acetate Stock Solution

Materials Required

• Ammonium acetate
• DI water
• 100 mL volumetric flask
• 0.45 µm nylon membrane filter

Procedure

1. Weigh 3.85 g ammonium acetate.
2. Transfer the material into a 100 mL volumetric flask.
3. Add DI water to the 100 mL mark.
4. Mix until completely dissolved.
5. Filter the solution through a 0.45 µm nylon membrane filter.
6. Store refrigerated at 2-8°C when not in use.

Result

• The resulting solution is:  500 mM Ammonium Acetate Stock Solution.  This stock solution can be used to prepare multiple mobile phase batches with consistent buffer concentration.

Step 2: Prepare the Final Mobile Phase

Materials Required

• 500 mM ammonium acetate stock
• DI water
• Acetonitrile
• 1 L volumetric flask
• 0.45 µm nylon membrane filter (recommended)

Procedure

1. Transfer 32 mL of the 500 mM ammonium acetate stock solution into a 1000 mL volumetric flask.
2. Add 68 mL DI water.
3. Fill to the 1000 mL mark with acetonitrile.
4. Mix thoroughly.
5. Filter through a 0.45 µm nylon membrane filter before use if desired.

Final Mobile Phase Composition

The final buffer concentration is calculated as:  (32 mL ÷ 1000 mL) × 500 mM = 16 mM Ammonium Acetate.  Final solvent composition:

• Approximately 90% Acetonitrile
• Approximately 10% Aqueous Phase (buffer stock plus water)

Why Water Is Required

Ammonium acetate is not readily soluble in pure acetonitrile.  The aqueous portion of the mobile phase:

• Maintains buffer solubility
• Prevents precipitation
• Provides consistent buffer concentration
• Improves long-term mobile phase stability

Failure to maintain sufficient water content may result in buffer precipitation and potential chromatographic problems.

Applications

A 16 mM ammonium acetate mobile phase in 90:10 acetonitrile:water is commonly used for:

• HILIC separations
• Aqueous Normal Phase (ANP) chromatography
• LC-MS methods
• Analysis of polar compounds
• Metabolomics
• Biopharmaceutical applications
• Pharmaceutical impurity analysis

Best Practices

For maximum reproducibility:

• Use HPLC or LC-MS grade solvents.
• Filter all stock solutions before use.
• Store concentrated buffer stocks under refrigeration.
• Inspect mobile phases for any signs of precipitation.
• Thoroughly mix before using in analytical methods.

These practices help ensure consistent chromatographic performance and support method transferability.

Conclusion

Preparing a 16 mM ammonium acetate mobile phase using a 500 mM stock solution provides a simple and reproducible approach for HPLC and LC-MS analyses. The final composition of approximately 90% acetonitrile and 10% aqueous phase is commonly used for polar compound separations and helps ensure consistency with many published HILIC and ANP methods.