Electrospray ionisation (revision 12)
Old revision·20:04, 6 Mar 2025·FormularyFrida
| Electrospray ionisationMass spectrometry | |
|---|---|
| Abbreviation | ESI |
| Produces | Multiply charged ions from solution |
| Nature | Soft; molecule survives intact |
| Compatible with | Liquid chromatography |
| Analytical method infobox · conventions | |
Electrospray ionisation transfers molecules from solution into the gas phase as ions. A solution is sprayed from a capillary held at high potential, producing charged droplets whose solvent evaporates until ions are released.[1]
It is a soft technique — the molecule survives largely intact — and it produces multiply charged ions from peptides and proteins, so a large molecule appears at a mass-to-charge ratio within the range of ordinary analysers. A single peptide therefore produces a series of peaks at successive charge states, deconvolved by software into one neutral mass.[2]
Because it works from flowing solution, it couples directly to chromatography, which is what makes LC-MS possible.[1]
Mechanism
[edit]The applied potential draws the liquid into a cone from whose tip a fine spray of charged droplets emerges. Solvent evaporates, the droplets shrink until charge repulsion exceeds surface tension, and they divide. Ions are ultimately released either by field-driven ejection from very small droplets or by complete evaporation of the solvent around them.[1]
The charge states observed depend on the number of ionisable sites and on solution conditions. Acidic mobile phases favour protonation and positive-mode operation, which is why peptide LC-MS is usually run in positive mode with an acidic modifier.[2]
Ion suppression is the characteristic difficulty: co-eluting species compete for charge, so a compound's signal depends on what else is eluting at the same time. This is why the technique is not quantitative without calibration.[3]
Practical consequences for peptide work
[edit]Mobile phases must be volatile. Trifluoroacetic acid, the standard modifier for peptide reverse-phase chromatography, suppresses electrospray signal appreciably, so LC-MS methods often substitute formic acid at some cost in peak shape.[2]
Salts and non-volatile buffers foul the source and suppress signal, so a sample containing substantial inorganic salt requires desalting before analysis.
The multiply charged envelope carries information beyond mass: its distribution shifts with conformation, so a partially unfolded protein produces a different envelope from a folded one. For small peptides this is of little consequence.[1]
See also
References
- ^ a b c d Fenn JB, Mann M, Meng CK, Wong SF, Whitehouse CM. "Electrospray ionization for mass spectrometry of large biomolecules." Science 246(4926):64–71 (1989). PMID 2675315.
- ^ a b c Aebersold R, Mann M. "Mass spectrometry-based proteomics." Nature 422(6928):198–207 (2003). PMID 12634793.
- ^ United States Pharmacopeia, General Chapter <1503>, Quality Attributes of Synthetic Peptide Drug Substances.