Electron Ionization

For EI, a sample is introduced to a mass spectrometer either via capillary GC or via a direct insertion probe. The stream of vaporized sample molecules at a pressure of approximately 10-5 torr entering the source interacts with the beam of electrons, one or more secondary electrons are ejected, and relatively unstable odd-electron or multiply charged positive ions are produced. Negative ions may also be produced when samples have high electron affinities.

M + e → M+• + 2e

Electrons are produced by heating a metal filament (cathode), usually made from a fine wire of tungsten or rhenium.

Picture. EI filament upon electron microscope, at ×100 and ×500 magnification.

These electrons are accelerated by a potential difference between the filament and the ions source box, pass through entrance and exit apertures in the box, and are collected by a trap electrode (anode). The ions are pushed out of the source by a repeller potential and then are accelerated into the mass analyzer. The efficiency of EI is only ~1/105; the remaining sample molecules are removed by vacuum pumps on the ion-source housing.

For analytical applications, an electron energy of 70eV has been adopted as the standard. At these ionization energies, a significant part of the ionized molecules is formed with such high internal energy that they fragment before leaving the ion source; therefore EI is called the "hard" ionization technique. In EI the energy lost in the ionizing collision between a neutral molecule and an electron is shared between the ion and the secondary electron. This leads to a broad internal energy distribution (typically 10-20 eV) and virtually all energetically allowed fragments are observed. EI mass spectra observed under standard conditions are reproducible and largely independent of the instrument used. The major applications of EI are the determination o

f molecular weight and structure of molecules that are stable enough to survive vaporization without decomposition. Consequently, the major limitation of EI mass spectrometry is the requirement for stable neutral molecules in the gas phase prior to ionization. Most biomolecules, polymers, and other large organic molecules are simply not stable at elevated temperatures and can not be vaporized and analyzed by EI mass spectrometry.

Direct introduction of a sample into the ion source is preferred for compounds of low volatility. In this approach, samples are vaporized from a heated wire or small vial, which is inserted directly into the path of the electron beam. The probe is inserted into an ion source through the vacuum interlock.

A mass spectrometer coupled to a gas chromatograph (GC/MS) is an important tool for analyses of volatile and semivolatile mixtures of organic compounds.