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Electrospray Ionization (ESI) Mass Spectrometry
Higher molecular weight biomolecules are nonvolatile compounds that may decompose before ionizing or vaporizing during mass analysis with conventional…
Biomolecules such as proteins and nucleic acids are low volatility compounds with high molecular weights. Conventional electron-impact ionization mass spectrometry faces challenges with such molecules, including severe molecular fragmentation.
Electrospray ionization is an alternative "soft" ionization method that generates nonradical molecular ions for the mass spectrometry of such biomolecules.
In this method, the sample is mixed with a polar solvent such as methanol, methanol-water, or acetonitrile with or without added salts.
The mixture is sprayed via a high voltage capillary into a vacuum chamber. The rapid evaporation of solvent from the fine spray of charged droplets ultimately leads to the charged gaseous sample molecule.
This charged sample molecule is detected in the mass spectrum as the sum of the sample molecular mass and the masses of the associated ions.
For example, if sodium ions are present in the ionic solution, the spectrum typically features an M+23 peak.
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Q1: Why is electrospray ionization better than electron-impact ionization for biomolecules?
Electron-impact ionization causes severe molecular fragmentation of high molecular weight biomolecules like proteins and nucleic acids. Electrospray ionization is a soft ionization method that generates nonradical molecular ions, preserving the intact biomolecule structure. This allows mass spectrometry analysis of biomolecules without excessive decomposition.
Q2: How does electrospray ionization transfer ions from liquid to gas phase?
The sample is mixed with a polar solvent and sprayed through a high-voltage capillary into a vacuum chamber. Electrical energy applied between the capillary and analysis chamber entrance moves charged droplets toward the chamber. Rapid solvent evaporation from the fine aerosol leaves behind charged gaseous sample molecules ready for mass detection.
Q3: What role does the ionic liquid play in electrospray ionization?
The ionic liquid serves as the ionization source in the sample solution. It provides cations that associate with the analyte biomolecule during ionization. The type and mass of these cations directly affect the m/z values detected in the mass spectrum, determining peak positions.
Q4: What does an M+23 peak indicate in an ESI mass spectrum?
An M+23 peak indicates that sodium ions from the ionic liquid have associated with the analyte molecule during ionization. The m/z value equals the sample's molecular mass plus 23, the atomic mass of sodium. Different cations produce different mass shifts, allowing identification of which ions are present.
Q5: How does solvent composition affect electrospray ionization performance?
The sample is mixed with polar solvents such as methanol, methanol-water, or acetonitrile, with or without added salts. These solvents facilitate the formation of charged droplets and enable efficient solvent evaporation. The choice of solvent and salt composition influences ionization efficiency and the types of cations available for association.
Q6: How does ESI handle multiple cation associations with a single biomolecule?
A biomolecule can associate with multiple cations during electrospray ionization, resulting in variably protonated or ionized species. Each association produces a distinct m/z value in the mass spectrum. The sum of the molecular mass and all associated ion masses determines the detected m/z, allowing analysis of chemical ionization mass spectrometry patterns.
Q7: What is the relationship between capillary voltage and charged droplet movement in ESI?
The voltage difference applied between the capillary and the analysis chamber entrance creates an electric field that drives charged droplets toward the chamber. Higher voltage increases droplet acceleration and ionization efficiency. This electrical energy is fundamental to transferring ions from the liquid phase into the gaseous phase for mass analysis.