Which ionization method produces multiply charged ions in MS?
Electrospray ionization is a soft ionization method that transfers large, nonvolatile biomolecules from solution into the gas phase without extensive fragmentation. Proteins in mildly acidic spray acquire protons at multiple basic residues such as lysine, arginine and histidine, generating a characteristic envelope of multiply charged ions [M+nH]n+ . This charge multiplication reduces mass-to-charge ratio into the accessible range of quadrupole or time-of-flight analyzers, allowing accurate deconvolution. Electron ionization and fast atom bombardment mainly produce singly charged species, while MALDI predominantly yields singly charged ions, making ESI uniquely suited for intact protein analysis.
Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.