Addressing a Common Misconception: Ammonium Acetate as Neutral pH "Buffer" for Native Electrospray Mass Spectrometry
1Department of Chemistry, The University of Western Ontario, London, ON, N6A 5B7, Canada. konerman@uwo.ca.
Journal of the American Society for Mass Spectrometry
|July 16, 2017
Summary
Ammonium acetate solutions, commonly used in native ESI-MS, do not buffer at neutral pH. These solutions can acidify in the ESI plume, a crucial consideration for biomolecular analysis.
Area of Science:
- Analytical Chemistry
- Biochemistry
- Mass Spectrometry
Background:
- Native electrospray ionization mass spectrometry (ESI-MS) transfers intact biomolecules from solution to the gas phase.
- Maintaining native conformation during ESI-MS is critical, making solution conditions paramount.
- Commonly used neutral aqueous ammonium acetate solutions are often mistakenly referred to as buffers at pH 7.
Purpose of the Study:
- To clarify the buffering capacity of ammonium acetate solutions in the context of native ESI-MS.
- To investigate the pH changes experienced by analytes during the electrospray process.
- To correct the misconception regarding ammonium acetate's buffering behavior at neutral pH.
Main Methods:
- Analysis of the acid-base properties of ammonium acetate in aqueous solutions.
- Theoretical assessment of pH stability in neutral ammonium acetate solutions.
- Evaluation of pH changes in the electrospray ionization plume.
Main Results:
- Ammonium acetate does not form a conjugate acid/base pair, thus it does not buffer at pH 7.
- Neutral ammonium acetate solutions are labile and can acidify, particularly in the positive ion mode ESI plume, towards pH 4.75.
- Despite not being a buffer at pH 7, ammonium acetate remains a useful volatile electrolyte for native ESI-MS.
Conclusions:
- The term "ammonium acetate buffer" for neutral solutions is inaccurate and should be replaced with "ammonium acetate solution".
- Understanding the potential acidification of ammonium acetate solutions is crucial for interpreting native ESI-MS data of proteins and biomolecular complexes.
- Ammonium acetate solutions offer a less drastic pH drop compared to pure water, retaining utility in native ESI-MS applications.
Keywords:
Acid/base chemistryDissociation equilibriumDroplet evaporationHenderson-Hasselbalch equationNoncovalent complexes in the gas phaseMore Related Videos
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