Mass Spectrometry Ionization Methods

Chemical ionization (CI)

CI uses a reagent ion to react with the analyte molecules to form ions by either a proton or hydride transfer:

MH + C₂H₅⁺ --> MH₂⁺ + C₂H₄

MH + C₂H₅⁺ --> M⁺ + C₂H₆

The reagent ions are produced by introducing a large excess of methane (relative to the analyte) into an electron impact (EI) ion source. Electron collisions produce CH₄⁺ and CH₃⁺ which further react with methane to form CH₅⁺ and C₂H₅⁺:

CH₄⁺ + CH₄ --> CH₅⁺ + CH₃

CH₃⁺ + CH₄ --> C₂H₅⁺ + H₂

Plasma and glow discharge

A plasma is a hot, partially-ionized gas that effectively excites and ionizes atoms.

A glow discharge is a low-pressure plasma maintained between two electrodes. It is particularly effective at sputtering and ionizing materials from solid surfaces.

Electron impact (EI)

An EI source uses an electron beam, usually generated fron a tungsten filament, to ionize gas-phase atoms or molecules. An electron from the beam knocks an electron off analyte atoms or molecules to create ions.

Electrospray ionization (ESI)

The ESI source consists of a very fine needle and a series of skimmers. A sample solution is sprayed into the source chamber to form droplets. The droplets carry charge when the exit the capillary and as the solvent vaporizes the droplets disappear leaving highly charged analyte molecules. ESI is particularly useful for large biological molecules that are difficult to vaporize or ionize.

Fast-atom bombardment (FAB)

In FAB a high-energy beam of netural atoms, typically Xe or Ar, strikes a solid sample causing desoprtion and ionization. It is used for large biological molecules that are difficult to get into the gas phase. FAB causes little fragmentation and usually gives a large molecular ion peak, making it useful for molecular weight determination.

The atomic beam is produced by accelerating ions from an ion source though a charge-exchange cell. The ions pick up an electron in collisions with netural atoms to form a beam of high energy atoms.

Laser ionization (LIMS)

A laser pulse ablates material from the surface of a sample and creates a microplasma that ionizes some of the sample constituents.

Matrix-assisted laser desorption ionization (MALDI)

MALDI is a LIMS method of vaporizing and ionizing large biological molecules such as proteins or DNA fragments. The biological molecules are dispersed in a solid matrix such as nicotinic acid.

A UV laser pulse ablates the matrix which carries some of the large molecules into the gas phase in an ionized form so they can be extracted into a mass spectrometer.

Plasma-desorption ionization (PD)

Decay of ₂₅₂Cf produces two fission fragments that travel in opposite directions. One fragment strikes the sample knocking out 1-10 analyte ions. The other fragment strikes a detector and triggers the start of data acquisition. This ionization method is especially useful for large biological molecules.

Resonance ionization (RIMS)

One or more laser beams are tuned in resonance to transistions of a gas-phase atom or molecule to promote it in a stepwise fashion above its ionization potential to create an ion. Solid samples must be vaporized by heating, sputtering, or laser ablation.

Further Information

/chem-ed/ms/ionizatn.htm, updated 11/3/96

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