1. The Mass Spectrometer
In order to measure the characteristics of individual molecules, a mass
spectrometer converts them to ions so that they can be moved about and
manipulated by external electric and magnetic fields. The three essential
functions of a mass spectrometer, and the associated components, are:
1. A small sample of compound is ionized, usually to cations by
loss of an electron. The Ion Source
2. The ions are sorted and separated according to their mass and
charge. The Mass Analyzer
3. The separated ions are then detected and tallied, and the
results are displayed on a chart. The Detector
Because ions are very reactive and short-lived, their formation and
manipulation must be conducted in a vacuum. Atmospheric pressure is around 760
torr (mm of mercury). The pressure under which ions may be handled is roughly 10-5
to 10-8 torr (less than a billionth of an atmosphere). Each of the
three tasks listed above may be accomplished in different ways. In one common
procedure, ionization is effected by a high energy beam of electrons, and ion
separation is achieved by accelerating and focusing the ions in a beam, which is
then bent by an external magnetic field.
The ions are then detected
electronically and the resulting information is stored and analyzed in a
computer. A mass spectrometer operating in this fashion is outlined in the
following diagram. The heart of the spectrometer is the ion source. Here
molecules of the sample (black dots) are bombarded by electrons (light blue
lines) issuing from a heated filament. This is called an EI
(electron-impact) source. Gases and volatile liquid samples are allowed to leak
into the ion source from a reservoir (as shown), but non-volatile solids and
liquids may be introduced directly.
Cations formed by the electron bombardment
(red dots) are pushed away by a charged repellor plate (anions are attracted to
it), and accelerated toward other electrodes, having slits through which the
ions pass as a beam. Some of these ions fragment into smaller cations and
neutral fragments. When the ion beam experiences a strong magnetic field
perpendicular to its direction of motion, the ions are deflected in an arc whose
radius is inversely proportional to the mass of the ion. Lighter ions are
deflected more than heavier ions. By varying the strength of the magnetic field,
ions of different mass can be focused progressively on a detector fixed at the
end of a curved tube (also under a high vacuum).
When a high energy electron collides with a molecule it often ionizes it by
knocking away one of the molecular electrons (either bonding or non-bonding).
This leaves behind a molecular ion (colored red in the following
diagram). Residual energy from the collision may cause the molecular ion to
fragment into neutral pieces (colored green) and smaller fragment ions
(colored pink and orange). The molecular ion is a radical cation, but the
fragment ions may either be radical cations (pink) or carbocations (orange),
depending on the nature of the neutral fragment. An animated display of this
ionization process will appear if you click on the ion source of the mass
spectrometer diagram.
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