Basic Type
Name and Acronym
Ionizing Agent
Gas Phase Electron Impact (EI) Energetic electrons

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Chemical Ionization (CI) Reagent gaseous ions

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Field Ionization (FI) High-potential electrode

Desorption

Field Desorption (FD) High-potential electrode

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Electrospray ionization (ESI) High electrical field

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Matrix-assisted Laser beam desorption/ionization (MALDI)  
_ Plasma Desorption (PD) Fission fragments from radioisotope
_ Fast Atom Bombardment Energetic atomic beam
_ Secondary Ion Mass Energetic beams of ions Spectrometry (SIMS)
_ Thermospray Ionization High temperature

Field GC/MS instruments used for environmental analysis typically use gas phase ionization sources such as electron impact.