Chemistry, Third edition

3 · INSIDE THE ATOM

(apositive ion). The positive ions are separated out in a mass spectrometer accord-

ing to the ratio of the mass to charge m/eof the ion, where eis the relative charge of

the ion (e.g. e 1 for Oande 2 for Cl^2 ) and mis the mass of the ion in

atomic mass units. In the following discussion we will use the mass number of ions

instead of the exact ion masses. For example,

m

(^35 Cl)

35

 35

e 1

m

(^37 Cl^2 )

37

18.5

e 2

Stages involved in a mass spectrometer

As an example, suppose that neon gas (consisting of atoms of the isotopes neon-22,

neon-21 and neon-20) has been injected into the mass spectrometer. The purpose

of the main parts of the mass spectrometer are as follows:

1.The vaporizer is an oven which ensures that any sample is turned into a gas.

(This is unnecessary with neon, which is a gas at room temperature.)

2.Electrons are fired at the gaseous sample. If one of these electrons collides with

an atom, it may push out an electron from the atom, making a positive ion:

atome(from gun) ione(from gun) e(from atom)

2.See Fig. 3.5. For neon:

(^22) Ne(g)e (^22) Ne(g)ee
(^21) Ne(g)e (^21) Ne(g)ee
(^20) Ne(g)e (^20) Ne(g)ee
2.(Some doubly charged ions, such as^20 Ne^2 , will also be produced but these will
be less common than singly charged ions because more energy is required to
make them.)
3.The ions are accelerated using charged plates. A narrow beam of ions is allowed to
enter into the rest of the spectrometer ‘chamber’. The chamber is evacuated using
36
Fig. 3.5Ionization of a neon
atom by an electron ().
A magnetic scanning mass spectrometer. The
spectrometer chamber is wedged between the poles
of a powerful electromagnet, which bends the beam
of ions, and whose magnetic strength may be
varied. Because the curvature of the chamber is
fixed, only ions of one m/evalue reach the detector
for a particular magnetic field strength.