Mass Spectrometry (MS) part:5

A mass spectrometer especially a multi-sector instrument is one of the most complex electronic and mechanical devices Therefore this means high costs at purchase and maintenance besides a specialized training for the operator(s). 

Measurement principles. In Figure  the essential parts of an analytical mass spectrometer are depicted. 

Its procedure is as follows: 

1. A small amount of a compound, typically one micromole or less, is evaporated. The vapour is leaking into the ionization chamber where a pressure is maintained of about 10-7 mbar.

2.The vapour molecules are now ionized by an electron-beam. A heated cathode, the filament, produces this beam. Ionization is achieved by inductive effects rather then strict collision. By loss of valence electrons, mainly positive ions are produced. 

3. The positive ions are forced out of the ionization chamber by a small positive charge (several Volts) applied to the repeller opposing the exit-slit (A). After the ions have left the ionization chamber, they are accelerated by an electrostatic field (A>B) of several hundreds to thousands of volts before they enter the analyzer. 

4. The separation of ions takes place in the analyzer, in this example a magnetic sector, at a pressure of about 10-8 mbar. A strong magnetic field is applied perpendicular to the motional direction of the ions. The fast moving ions then will follow a circular trajectory, due to the Lorentz acceleration, whose radius is determined by the mass/charge ratio of the ion and the strength of the magnetic field. Ions with different mass/charge ratios are forced through the exit-slit by variation of the accelerating voltage (A>B) or by changing the magnetic-field force. 

5. After the ions have passed the exit-slit, they collide on a collector-electrode. The resulting current is amplified and registered as a function of the magnetic-field force or the accelerating voltage. The applicability of mass-spectrometry to the identification of compounds comes from the fact that after the interaction of electrons with a given molecule an excess of energy results in the formation of a wide range of positive ions. The resulting mass distribution is characteristic (a fingerprint) for that given molecule. Here there are certain parallels with IR and NMR. Mass-spectrograms in some ways are easier to interpret because information is presented in terms of masses of structure-components. 

Cont,