Chemistry Reference
In-Depth Information
The mass spectrometer
How do we know isotopes exist? They were first
discovered by scientists using apparatus called a
mass spectrometer (Figure 3.6). The first mass
spectrometer was built by the British scientist
Francis Aston in 1919 and enabled scientists to
compare the relative masses of atoms accurately for
the first time.
35
17
37
17
Cl
Cl
vacuum chamber
17 protons
17 protons
ion
detector
sample
inlet
18 neutrons
20 neutrons
drift region
Figure 3.4
The two isotopes of chlorine.
ionisation
area
acceleration
area
flight path
time measurement
Table 3.4
Some atoms and their isotopes.
Figure 3.6
A diagram of a mass spectrometer.
Element
Symbol
Particles present
Hydrogen
1
H
1 e, 1 p, 0 n
A vacuum exists inside the mass spectrometer. A
sample of the vapour of the element is injected into
the ionisation chamber where it is bombarded by
electrons. The collisions which take place between
these electrons and the injected atoms cause an
electron to be lost from the atom, which becomes a
positive ion with a +1 charge. These positive ions are
then accelerated towards a negatively charged plate,
in the acceleration area. The spectrometer is set up
to ensure that when the ions leave the acceleration
area they all have the same kinetic energy, regardless
of the mass of the ions. This means that the lighter
ions travel faster than the heavier ones, and effectively
separates the ions according to their mass. Having left
the acceleration area, the time for the ions to reach
the detector is recorded. The detector counts the
number of each of the ions which fall upon it and so a
measure of the percentage abundance of each isotope
is obtained. A typical mass spectrum for chlorine is
shown in Figure 3.7.
(Deuterium)
1
H
1 e, 1 p, 1 n
(Tritium)
1
H
1 e, 1 p, 2 n
Carbon
1
6
C
6 e, 6 p, 6 n
1
6
C
6 e, 6 p, 7 n
1
6
C
6 e, 6 p, 8 n
Oxygen
1
8
O
8 e, 8 p, 8 n
1
8
O
8 e, 8 p, 9 n
1
8
O
8 e, 8 p, 10 n
Strontium
8
38
Sr
38 e, 38 p, 48 n
8
38
Sr
38 e, 38 p, 50 n
9
38
Sr
38 e, 38 p, 52 n
Uranium
23
92
U
92 e, 92 p, 143 n
23
92
U
92 e, 92 p, 146 n
100
75
35
Cl (75%)
50
25
37
Cl (25%)
0
10
20
30
40
50
Figure 3.5
Cobalt-60 is used in radiotherapy treatment.
mass
Figure 3.7
The mass spectrum for chlorine.
