Why Is the Protonated Molecular Ion Observed as [M+H]⁺ in ESI Mass Spectra?
Electrospray ionization (ESI) is a widely employed soft ionization technique in mass spectrometry, particularly for the analysis of biomolecules such as proteins, peptides, and nucleic acids. In ESI, the sample solution is dispersed through a charged electrospray needle, generating fine droplets that undergo ionization under a high-voltage electric field.
Why [M+H]⁺
1. Protonation Mechanism
In the electrospray process, analyte molecules in the droplets readily capture protons (H⁺), primarily due to the acidic microenvironment near the spray needle, which facilitates protonation.
2. Soft Ionization Nature
ESI is classified as a soft ionization technique, meaning it generally does not induce significant fragmentation of the analyte. Consequently, the predominant species observed is often the protonated molecular ion, [M+H]⁺.
3. Solution Chemistry
Under ESI conditions, the sample is typically in an acidic or near-neutral solution, favoring protonation over cationization by species such as Na⁺ or K⁺.
4. Formation of Multiply Charged Ions
For large biomolecules, such as proteins, ESI commonly produces multiply charged ions, represented as [M+nH]ⁿ⁺, where n is an integer greater than 1.
5. Negative Ion Mode Consideration
In negative ion mode, deprotonated species such as [M-H]⁻ may be observed. However, positive ion mode is generally preferred due to the higher efficiency of protonation and greater sensitivity of most mass spectrometers in this mode.
Thus, the appearance of an [M+H]⁺ peak in an ESI mass spectrum typically indicates that the analyte has undergone protonation, forming a positively charged species. This represents one of the fundamental ionization mechanisms in ESI mass spectrometry.
MtoZ Biolabs, an integrated chromatography and mass spectrometry (MS) services provider.
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