TOPIC 2: IB Chemistry
The Mass Spectrometer and Relative Atomic Mass
The mass of individual _______________ can be determined using a mass spectrometer. The principle behind
mass spectrometry is that a_______________ charged particle passing through a _______________ field is
deflected along a circular path on a radius that is proportional to the ________-to-___________ ratio, m/e. The
sample must first be _______________ either before it is injected into the spectrometer or immediately after it has
been injected. The vapour is then ________________. This is achieved by bombarding the vapour with a stream
of high-energy ______________ to generate _____________ ions:
M(g)
vaporized atoms
+
e
high-energy electron
______(g) +
unipositive ion
2e-
These positive ions then pass through holes in parallel plates, where they are ______________. The ions are then
deflected by an external magnetic field. The amount of the deflection depends both on the ________ of the ion
and on its ___________. Heavier and less highly charged ions will be deflected _______ than lighter and more
highly charged ions. Ions with a particular mass-to-charge ratio are then recorded on a _______________, which
measures both the mass-to-charge ratio and the relative amounts of all the ions present. In practice, the machine’s
electron beam energy can be adjusted so that only ___________ions with a ___________ charge are detected, so
that the mass-to-charge ratio is the __________ as the mass.
SUMMARY OF STAGES (VIADD):





STAGE 1 (VAPOURIZATION)  sample is heated and vaporized producing gaseous __________ or
______________
STAGE 2 (IONIZATION)  gaseous atoms bombarded by high energy ____________ resulting in
generation of _______________________ charged ions
STAGE 3 (ACCELERATION)  positive ions accelerated by _______________ charged plates in an
________________ field
STAGE 4 (DEFLECTION)  ions deflected by _______________ field ; angle of deflection depends on
charge to mass ratio
STAGE 5 (DETECTION)  detector detects ions and electrical signal is generated
Relative Atomic Mass Problems:
RECALL:
 The relative atomic mass, Ar, is the ratio of the average mass of the atom to 1/12 the mass of a C-12 atom.
(Alternately stated: ….the ratio of the average mass of the atom to the unified atomic mass unit.) *Relative atomic
mass is a ratio so it does not have units!)
 The unified atomic mass unit is a non SI unit of mass and is defined as one-twelfth the mass of a C-12 atom in its
ground state. This unit is used to express masses of atomic particles (1 amu = 1 u = 1.6605402 x 10-27 kg = the mass
of an H-1 atom)
1. Calculations:
a) Following mass spectrometry analysis, it is found that lead occurs naturally as four isotopes. These isotopes
206
207
208
are 204
82 Pb (1.37%, 204.0), 82 Pb (26.26%, 206.0 ), 82 Pb (20.82%, 207.0), and 82 Pb (51.55%, 208.0). Calculate the
relative atomic mass of lead.
b) An approximation is often made that naturally occurring chlorine contains 75% chlorine-35
and 25% chlorine-37. This would give a relative atomic mass of 35.5. However, according to the IB Data Booklet
the relative atomic mass of chlorine is 35.45. Calculate to two decimal places the actual percentage of chlorine-35
in naturally occurring chlorine.
c) Examine the mass spectrum on p. 49 of the text. Deduce the relative atomic mass of the element X from its
mass spectrum and identify X from the periodic table.