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 material 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 of 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 evaporates, 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 desorption 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.

Field ionization

Molecules can lose an electron when placed in a very high electric field. High fields can be created in an ion source by applying a high voltage between a cathode and an anode called a field emitter. A field emitter consists of a wire covered with microscopic carbon dendrites, which greatly amplify the effective field at the carbon points.

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. The laser pulse accomplishes both vaporization and ionization of the sample.

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.

Secondary ionization (SIMS)

A primary ion beam; such as ³He⁺,¹⁶O⁺, or ⁴⁰Ar⁺; is accelerated and focused onto the surface of a sample and sputters material into the gas phase. Approximately 1% of the sputtered material comes off as ions, which can then be analyzed by a mass spectrometer. SIMS has the advantage that material can be continually sputtered from a surface to determine analyte concentrations as a function of distance from the original surface (depth profiling).

Spark source

A spark source ionizes analytes in solid samples by pulsing an electric current across two electrodes. If the sample is a metal it can serve as one of the electrodes, otherwise it can be mixed with graphite and placed in a cup-shaped electrode.

Thermal ionization (TIMS)

Thermal ionization is used for elemental or refractory materials. A sample is deposited on a metal ribbon, such as Pt or Re, and an electric current heats the metal to a high temperature. The ribbon is often coated with graphite to provide a reducing effect.

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