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Electrospray ionization (ESI) turns analytes in solution into gas-phase ions by spraying the liquid through a charged capillary tip at atmospheric pressure. It is a soft ionization method: it is widely used to deliver intact molecules, including proteins and noncovalent complexes, to the mass analyzer. This page is for researchers, core-facility users and lab staff who need to understand what the source does, what the spectrum will look like, and what a sample must contain (and avoid) before it goes on an electrospray instrument.
Short answer
In ESI, a high voltage applied to a liquid emerging from a capillary forms a Taylor cone and a spray of charged droplets. As the solvent evaporates, the droplets shrink and shed smaller droplets until analyte ions are released into the gas phase and enter the mass spectrometer. Large molecules usually appear as a series of multiply charged ions, so one compound gives several peaks at different m/z values. Salts, detergents and non-volatile additives interfere with the process, which is why core facilities publish sample-preparation rules.
Definition and scope
Electrospray ionization is an ion source, not a mass analyzer. It is paired with an analyzer (quadrupole, time-of-flight, ion trap, Orbitrap and others) and is commonly fed either by direct infusion or by a liquid chromatography column. For the instrument as a whole, see What Is a Mass Spectrometer?; for how the two are connected, see LC-MS explained.
This page covers the principle and the user-facing consequences. For choosing between ESI and other atmospheric-pressure sources, see ESI vs APCI vs APPI; for the instrument-side settings, see ESI source parameters and tuning.
How ESI forms ions
The mechanism below follows Wilm’s review of ESI principles (Mol Cell Proteomics 2011;10(7):M111.009407).
- Taylor cone and spray. At a threshold voltage the liquid at the capillary tip forms a cone, the Taylor cone (opening angle about 49.3 degrees), and the spray is emitted from its tip.
- Charged droplets. The droplets leave carrying charge near the Rayleigh limit, the point at which charge repulsion balances surface tension. As solvent evaporates they shed smaller secondary droplets, roughly one-tenth the diameter.
- Release of gas-phase ions. Two models describe the last step. In the charged residue model (CRM), a final droplet holds about one analyte molecule, which is left as an ion when the solvent evaporates. This is considered the main route for large ions and explains why noncovalent complexes can survive. In the ion evaporation model (IEM), solvated ions are expelled from very small droplets (about 20 nm) by a strong surface field, mainly for smaller ions.
ESI compared with other ionization: soft versus hard
Electron ionization (EI) is a hard technique: in the vapor phase, electrons strike the molecule, which commonly breaks into fragments. ESI is a soft technique that starts from a liquid and usually leaves the molecule intact as an ion such as [M+H]+, so the spectrum reports molecular mass first and fragments only if you deliberately fragment the ion in the instrument. To read the resulting spectrum, see How to Read a Mass Spectrum. Another soft method, which uses a solid matrix and a laser rather than a liquid spray, is covered in MALDI-TOF.
Charge states and adducts: what you see in the spectrum
The table maps each mechanistic point to something a user observes. Every row is taken from Wilm (2011).
| Mechanism or condition | What a user sees or should expect |
|---|---|
| Charge is carried by protons on basic sites (positive mode) | Analytes with basic sites give protonated ions. Where there is no basic site, sodium attaches instead, so [M+Na]+ can appear. |
| Denatured, elongated proteins | They carry more charge than folded ones, so the charge-state envelope shifts toward higher charge (lower m/z). |
| Charged residue model | Large analytes and noncovalent complexes can reach the gas phase intact. |
| Native complexes | They carry water and buffer adducts, which broaden peaks. |
| Surface activity of the analyte | Surface-active, hydrophobic analytes are detected more readily than hydrophilic ones. |
| Nano-electrospray (much smaller droplets) | Better tolerance to salt than conventional flow-rate sources. |
Sample and solvent compatibility checklist
These points come from the University of Illinois Proteomics core sample-submission page (read 2026-11-07), which states the samples should be as free from salts, detergents and non-volatile components as possible. Facilities differ in what they accept, so check your own core’s page and give the staff the exact buffer composition.
- Not acceptable for intact-mass work at that core: the detergents Brij-35, Triton X-100, CHAPS, Tween-20 and SDS; glycerol; PEG; DMF and DMSO; and salts or Tris and phosphate buffers above 100 mM.
- Acceptable at that core: formic and acetic acid; methanol, ethanol and acetonitrile.
- Always: report the full buffer recipe with the sample, since the core, not the submitter, judges whether it can run.
Common misunderstandings
- “One compound gives one peak.” A large molecule typically gives a series of charge states, and each is a separate peak at a different m/z value.
- “Soft means no fragmentation, ever.” Soft means the ionization step itself tends to leave the molecule intact; fragmentation can still be introduced later in the instrument.
- “Any buffer will do.” Non-volatile salts and detergents are the usual reason for lost signal; one core’s list of unacceptable additives is above.
- “Extra peaks mean impurity.” Sodium or buffer adducts are a property of the ionization, not necessarily of the sample.
Frequently asked questions
What is the principle of electrospray ionization mass spectrometry?
A charged liquid spray forms droplets that evaporate and shrink until analyte ions are released into the gas phase and analyzed by their m/z (Wilm, 2011).
What is the difference between soft and hard ionization?
Soft methods such as ESI generally leave the molecule intact as an ion; hard methods such as EI fragment it on ionization. The ESI-versus-EI choice is therefore also a choice of sample type: solution versus vapor.
What is ESI versus EI in mass spectrometry?
ESI ionizes dissolved analytes at atmospheric pressure from a charged spray; EI ionizes vapor-phase molecules with electrons. For other atmospheric-pressure options, see ESI vs APCI vs APPI.
Is electrospray ionization the same as electrospray ionization mass spectrometry?
Electrospray ionization is the ion source; ESI-MS is the source coupled to a mass analyzer.
Sources
- Wilm M. Principles of Electrospray Ionization. Mol Cell Proteomics. 2011;10(7):M111.009407. pmc.ncbi.nlm.nih.gov/articles/PMC3134074 (read 2026-11-07).
- University of Illinois Urbana-Champaign, Proteomics core, sample submission. biotech.illinois.edu/proteomics-core/proteomics-sample-submission (read 2026-11-07).
