Advanced Chemistry Activity
Mass Spectrometry and Atomic Mass
Name:
Period:
INTRODUCTION
Atoms though very small, have a finite (nonzero) mass. If you know the mass of a single atom or molecule, you
can count the number of atoms or molecules in a sample by weighing it. Being able to count atoms in this way is
needed in understanding chemical reactions and in using chemical reactions in research, industrial technology,
and medicine.
An instrument called a mass spectrometer conveniently determines atomic and molecular masses and the presence
of isotopes. In a mass spectrometer, an electrical discharge knocks electrons off the atoms or molecules changing
them into positively charged ions. These ions then are accelerated in an electric field, and some property related to
their mass is measured. In some spectrometers, the property is the trajectory of the ions in a magnetic field. In
other spectrometers, the property is the time taken by the ions to move a known distance from the point of
ionization to a detector. An example of a mass spectrum is shown in the model.
THE MASS SPECTROMETER
Operation
The sample pressure of a typical mass spectrometer is kept extremely low so that
intramolecular fragmentations and re-arrangements can occur but the probability of any
reactions occurring between species is highly unlikely. The relative concentrations of ions is
dependent upon the relative rates of formation and subsequent fragmentation and
rearrangement of all the ions.
Instrumentation
A mass spectrometer consists of three units... an ion source, an analyzer and a detector.
Ion source
The ion source is maintained at an extremely low pressure and the sample is heated to a
temperature at which it will volatilize at the applied pressure. Very small samples (less than
10-3g) of material are needed. A filament, usually made of rhenium or tungsten, is heated until
it emits electrons; the energy of these electrons is very much higher than that required to break
the bonds within a molecule. The fragmentation ions are accelerated out of the ion source by
an electric field. Any parts of the sample that have not been ionized, plus any radicals or
neutral molecules formed as a result of fragmentation or rearrangement, will not be affected
since they are uncharged and are not accelerated.
Analyzer
The analyzer separates the ions according to their mass / charge (m/z). The radius of the path
depends on the value of m/z; ions of larger m/z values follow a curve of larger radius than
those of lower m/z. Since most of the ions originating from organic molecules are singly
charged (+1), the separation will be according to the mass of each fragment. Electric, magnetic
or a combination of electric and magnetic fields can be used for the separation.
Detector
Electric or photographic detection methods can be used to observe the presence of the ions. A
mass spectrum records the mass/charge (m/z) values and the relative abundance for all
fragments present. Instruments can have a range of sensitivities (resolving power). The greater
the sensitivity, the finer the difference in mass/charge they can resolve.
Olmsted and Williams (Chemistry, Wiley, 2002) pp. 51 - 57.
Advanced Chemistry Activity
Mass Spectrometry and Atomic Mass
Name:
Period:
INTERPRETATION OF SPECTRA
Isotopes
One of the first applications of the mass spectrum was the demonstration by F.W.
Aston, (Nobel Prize winner in 1922), that naturally occurring neon consisted of three isotopes
... 20Ne, 21Ne and 22Ne. By comparing the positions of the peaks, one can calculate the atomic
mass. In addition, the relative abundance of each isotope can be found by comparing the peak
intensities.
MODEL: DATA ON ATOMIC MASSES
Ion Counts per second
In a mass spectrum the number of ion counts over some period of time is plotted on the y-axis with the atomic
mass on the x-axis. The number of ion counts for different isotopes is proportional to the abundance of each
isotope in the sample. Since atomic masses are so small, a special mass unit is used. This unit is called the atomic
mass unit and is abbreviated amu. The atomic mass unit is defined so the mass of carbon-12 is exactly 12 amu.
The masses of all other atoms then are measured on relative to the mass of carbon-12. The factor for converting
grams to amu is 1.66054x10-24 g/amu.
SI (x)
% Mass in amu
Mass Spectrum of Boron
Olmsted and Williams (Chemistry, Wiley, 2002) pp. 51 - 57.
Advanced Chemistry Activity
Mass Spectrometry and Atomic Mass
Atomic Isotope
hydrogen
Symbol
1
1H
Name:
Period:
Table of Isotopic Data
Atomic Mass (amu)*
1.0078
Natural Abundance on Earth(%)
99.985
2
1H
2.0140
0.015
helium
4
2 He
4.00260
100.0
boron-10
10
5B
10.0129
19.78
boron-11
11
5B
11.0093
80.22
carbon-12
13
6C
Exactly 12 by definition
98.89
carbon-13
13
6C
13.0034
1.11
bromine-79
79
35 Br
78.9183
50.54
bromine-81
81
35 Br
80.9163
49.46
deuterium
KEY QUESTIONS
1. What mass unit is used to report the mass of atoms in the model?
2. How is this unit defined?
3. Are the mass number and the mass of an isotope (in amu units) exactly or only approximately the same?
4. Why is it more convenient to use atomic mass units (amu) rather than grams or kilograms (g or kg) in reporting
atomic masses?
5. Why is one peak in the mass spectrum for boron higher than the other?
6. What are the differences and similarities between isotopes of the same element?
7. If an element has several isotopes, what single value for the atomic mass could be used to characterize that
element? Justify your answer.
Olmsted and Williams (Chemistry, Wiley, 2002) pp. 51 - 57.
Advanced Chemistry Activity
Mass Spectrometry and Atomic Mass
Name:
Period:
EXERCISES
1. Show that the peak heights in the boron mass spectrum, shown in the model, are consistent with the natural
abundances given in the Table of Isotopic Data.
2. Draw a sketch to show what you expect the mass spectrum of bromine to look like.
3. If you could pick one carbon atom out of a natural sample, identify what its mass most likely would be.
Explain.
4. Determine the number of boron-11 atoms that you would expect to find in a natural sample of 10,000 boron
atoms.
Ion Counts per second
PROBLEMS
1. Obtain as much information as you can from the mass spectrum for nickel shown below.
SI (x)
% Mass in amu
Olmsted and Williams (Chemistry, Wiley, 2002) pp. 51 - 57.
Advanced Chemistry Activity
Mass Spectrometry and Atomic Mass
Name:
Period:
2. (a) The atomic mass of an isotope is 64.9278 amu. How many atoms are there in exactly 1 kg of this isotope?
(b) Since the mass of carbon-12 is defined as 12 amu, how many atoms are there in exactly 12 g of carbon-12?
3. In a nuclear fusion reaction, two deuterium atoms combine to produce one helium-4 atom. Is mass conserved in
this reaction? Explain.
Olmsted and Williams (Chemistry, Wiley, 2002) pp. 51 - 57.