How does ionisation occur in a mass spectrometry (process of ionisation)?

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Jun 3, 2017

It depends on your ionization method. After the ionization, the analyte ions are in general sent to an electromagnetic sector of the mass spectrometer for separation by #m"/"z# ratio.

It may help if you look at this overview of mass spectroscopy.


There are many methods, but some common ionization methods are:

  • Electrospray ionization
  • Matrix-Assisted Laser Desorption Ionization
  • Electron Ionization

Often the method is chosen in favor of so-called soft ionization (rather than hard ionization).

Soft ionization better-retains the parent peak during fragmentation, so that you can find the peak that corresponds to the molecular mass of the original ion.

ELECTROSPRAY IONIZATION (ESI)

http://www.chm.bris.ac.uk/

Basically...

  1. Analyte solution is passed through the spray needle, which is connected to a counterelectrode with a large potential difference in between them.
  2. It comes out as a charged droplet that evaporates on its way to the source-sampling counterelectrode.
  3. Solvent evaporation stresses the surface tension of the droplet until it breaks into smaller charged droplets (reaching the "Rayleigh" limit). This rapidly repeats until analyte ions are left.

These analyte ions have little residual energy, so this ionization method is soft.

MATRIX-ASSISTED LASER DESORPTION IONIZATION (MALDI)

http://www.chm.bris.ac.uk/

The general process was:

  1. An easily-sublimable solid sample matrix surrounds the analyte molecules, isolating them from each other.
  2. A laser of the appropriate wavelength excites the matrix surface, ejecting clusters in which the analyte molecules are surrounded by the matrix ions.
  3. The (presumably) easily-sublimed matrix ions then vaporize off, leaving gaseous analyte ions for sample analysis.

This is also a soft ionization method, since the laser does not directly ionize the sample itself (which would have overly fragmented some of the sample).

ELECTRON IONIZATION (EI)

http://www.chm.bris.ac.uk/

Sometimes labeled the "classical" method. The general idea is:

  1. Previously-vaporized analyte solution is sent into the electron ionization chamber.
  2. A hot (rhenium or tungsten) filament sends a beam of spiraling electrons towards an electron trap.
  3. What happens most often is that energy is transferred from the electron beam into the analyte gas, fragmenting it into ions.

This is a hard ionization method, since energy is directly transferred from the electron beam and generally little sample consists of intact molecular ions.


After the ionization, the analyte ions are in general sent to an electromagnetic sector of the mass spectrometer for separation by #m"/"z# ratio.

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