MASS
SPECTROMETRY
A guide for A level students
KNOCKHARDY PUBLISHING
2008
SPECIFICATIONS
KNOCKHARDY PUBLISHING
MASS SPECTROMETRY
INTRODUCTION
This Powerpoint show is one of several produced to help students understand
selected topics at AS and A2 level Chemistry. It is based on the requirements of
the AQA and OCR specifications but is suitable for other examination boards.
Individual students may use the material at home for revision purposes or it may
be used for classroom teaching if an interactive white board is available.
Accompanying notes on this, and the full range of AS and A2 topics, are available
from the KNOCKHARDY SCIENCE WEBSITE at...
www.knockhardy.org.uk/sci.htm
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MASS SPECTROMETRY
CONTENTS
• Prior knowledge
• Background information
• The basic parts of a mass spectrometer
• The four stages of obtaining a spectrum
• How different ions are deflected
• Calculating molecular masses using mass spectra
• Example questions
• Test questions
• Other uses of mass spectrometry
• Check list
MASS SPECTROMETRY
Before you start it would be helpful to…
• know that atoms are made up of protons, neutrons and electrons
• know that like charges repel
MASS SPECTROMETRY
The first mass spectrometer was built in 1918 by Francis W Aston,
a student of J J Thomson, the man who discovered the electron.
Aston used the instrument to show that there were different forms
of the same element. We now call these isotopes.
In a mass spectrometer, particles are turned into positive ions,
accelerated and then deflected by an electric or magnetic field. The
resulting path of ions depends on their ‘mass to charge’ ratio (m/z).
Francis Aston
Particles with a large m/z value are deflected least
those with a low m/z value are deflected most.
The results produce a mass spectrum which portrays the different
ions in order of their m/z value.
USES
Mass spectrometry was initially used to show the identity of isotopes.
It is now used to calculate molecular masses and characterise new compounds
A MASS SPECTROMETER
DETECTOR
ION SOURCE
ANALYSER
A mass spectrometer consists of ... an ion source, an analyser and a detector.
PARTICLES MUST BE IONISED SO THEY
CAN BE ACCELERATED AND DEFLECTED
HOW DOES IT WORK?
DETECTOR
ION SOURCE
ANALYSER
IONISATION
• gaseous atoms are bombarded by electrons from an electron gun and are IONISED
• sufficient energy is given to form ions of 1+ charge
HOW DOES IT WORK?
DETECTOR
ION SOURCE
ANALYSER
IONISATION
• gaseous atoms are bombarded by electrons from an electron gun and are IONISED
• sufficient energy is given to form ions of 1+ charge
ACCELERATION
• ions are charged so can be ACCELERATED by an electric field
HOW DOES IT WORK?
DETECTOR
ION SOURCE
ANALYSER
IONISATION
• gaseous atoms are bombarded by electrons from an electron gun and are IONISED
• sufficient energy is given to form ions of 1+ charge
ACCELERATION
• ions are charged so can be ACCELERATED by an electric field
DEFLECTION
• charged particles will be DEFLECTED by a magnetic or electric field
HOW DOES IT WORK?
DETECTOR
ION SOURCE
ANALYSER
IONISATION
• gaseous atoms are bombarded by electrons from an electron gun and are IONISED
• sufficient energy is given to form ions of 1+ charge
ACCELERATION
• ions are charged so can be ACCELERATED by an electric field
DEFLECTION
• charged particles will be DEFLECTED by a magnetic or electric field
DETECTION
• by electric or photographic methods
HOW DOES IT WORK?
DETECTOR
ION SOURCE
ANALYSER
IONISATION
• gaseous atoms are bombarded by electrons from an electron gun and are IONISED
• sufficient energy is given to form ions of 1+ charge
ACCELERATION
• ions are charged so can be ACCELERATED by an electric field
DEFLECTION
• charged particles will be DEFLECTED by a magnetic or electric field
DETECTION
• by electric or photographic methods
HOW DOES IT WORK? - Deflection
20Ne
21Ne
22Ne
HEAVIER ISOTOPES ARE
DEFLECTED LESS
• the radius of the path depends on the value of the mass/charge ratio (m/z)
• ions of heavier isotopes have larger m/z values so follow a larger radius curve
• as most ions are 1+charged, the amount of separation depends on their mass
HOW DOES IT WORK? - Deflection
20Ne
2+ ions
22Ne
1+ ions
20Ne
ABUNDANCE
21Ne
22Ne
HEAVIER ISOTOPES ARE
DEFLECTED LESS
0
4
8
12
16
20
m/z values
Doubling the charge, halves the m/z value
Abundance stays the same
• the radius of the path depends on the value of the mass/charge ratio (m/z)
• ions of heavier isotopes have larger m/z values so follow a larger radius curve
• as most ions are 1+charged, the amount of separation depends on their mass
• if an ion acquires a 2+ charge it will be deflected more; its m/z value is halved
WHAT IS A MASS SPECTRUM?
20Ne
90.92%
MASS
SPECTRUM
OF NEON
21Ne
0.26%
22Ne
19
20
21
22
8.82%
23
In early research with a mass spectrograph, Aston (Nobel Prize, 1922) demonstrated
that naturally occurring neon consisted of three isotopes ... 20Ne, 21Ne and 22Ne.
• positions of the peaks gives atomic mass
• peak intensity gives the relative abundance
• highest abundance is scaled to 100% and other values are adjusted accordingly
EXAMPLE CALCULATION (1)
Calculate the average relative atomic mass of neon using data on the previous page.
Out of every 100 atoms...
Average =
90.92 are
20Ne
, 0.26 are
21Ne
and 8.82 are
22Ne
(90.92 x 20) + (0.26 x 21) + (8.82 x 22) = 20.179 Ans. = 20.18
100
TIP
In calculations of this type...
multiply each relative mass by its abundance
add up the total of these values
divide the result by the sum of the abundances
* if the question is based on percentage abundance, divide by 100 but if
it is based on heights of lines in a mass spectrum, add up the heights
of the lines and then divide by that number (see later).
EXAMPLE CALCULATION (2)
Naturally occurring potassium consists of potassium-39 and potassium-41. Calculate
the percentage of each isotope present if the average is 39.1.
Assume that there are x nuclei of
so
39x + 41 (100-x) = 39.1
39K
in every 100; there will then be (100-x) of
therefore
39 x + 4100 - 41x
= 3910
100
thus
- 2x = - 190
so x = 95
Ans.
95% 39K and 5%
41K
41K.
TEST QUESTION
Redraw the diagram with the most abundant
isotope scaled up to 100%.
Calculate the average relative atomic mass.
100%
ABUNDANCE
A mass spectrum shows the presence of two
isotopes of m/z values 38 and 40. Both have been
formed as unipositive ions.
60%
40%
0 10 20 30 40 m/z values
What would be a) the m/z values and b) the
abundance if 2+ ions had been formed?
ANSWERS ON NEXT PAGE
TEST QUESTION
Redraw the diagram with the most abundant
isotope scaled up to 100%.
Calculate the average relative atomic mass.
100%
ABUNDANCE
A mass spectrum shows the presence of two
isotopes of m/z values 38 and 40. Both have been
formed as unipositive ions.
60%
40%
0 10 20 30 40 m/z values
What would be a) the m/z values and b) the
abundance if 2+ ions had been formed?
New scale atoms of mass 38; abundance = 100
atoms of mass 40; abundance = 66.7
Average =
(100 x 38) + (66.7 x 40) = 38.80
166.7
By doubling the charge to 2+, m/z value is halved;
new peaks at 19 and 20. The abundance is the same.
100%
ABUNDANCE
The new values are 100 and 66.7 (see diagram)
100%
66.7%
0 10 20 30 40 m/z values
OTHER USES OF MASS SPECTROMETRY
- MOLECULAR MASS DETERMINATION Nowadays, mass spectrometry is used
to identify unknown or new compounds.
IONISATION
When a molecule is ionised it forms a
MOLECULAR ION which can also
undergo FRAGMENTATION or REARRANGEMENT to produce particles of
smaller mass.
MOLECULAR ION
FRAGMENTION
Only particles
with a positive charge
will be deflected and detected.
RE-ARRANGEMENT
FRAGMENTION
The resulting spectrum has many peaks.
The final peak (M+) shows the molecular
ion (highest m/z value) and indicates the
molecular mass. The rest of the
spectrum provides information about
the structure.
MASS SPECTRUM OF C2H5Br
The final peak in a mass spectrum is due to the molecular ion. In
this case there are two because Br has two main isotopes. As each
is of equal abundance, the peaks are the same size.
molecular ion contains...79Br
81Br
IDENTIFY THE PEAKS
IDENTIFY THE PEAKS
IDENTIFY THE PEAKS
OTHER USES OF MASS SPECTROMETRY
- SPACE EXPLORATION Mass spectrometry is used on space probes to identify elements
and compounds on the surface of planets.
In August and September 1975 the USA launched Viking 1 and 2.
Both probes entered Mars orbit to map the planet, dropping
landers that transmitted pictures, acted as weather and scientific
stations and analysed the Martian soil.
VIKING LANDER
THE MARTIAN SURFACE
REVISION CHECK
What should you be able to do?
Recall the four basic stages in obtaining a mass spectrum
Understand what happens during each of the above four stages
Understand why particles need to be in the form of ions
Recall the the meaning of mass to charge ratio (m/z)
Explain how the mass/charge value affects the path of a deflected ion
Interpret a simple mass spectrum and calculate the average atomic mass
Understand how mass spectrometry can be used to calculate molecular mass
Recall that mass spectrometry can be used to in space exploration
Recall other uses of mass spectrometry
CAN YOU DO ALL OF THESE?
YES
NO
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relevant topic(s) again
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WELL DONE!
Try some past paper questions
MASS
SPECTROMETRY
The End
© 2008 KNOCKHARDY PUBLISHING