MASS SPECTROMETRY

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MASS SPECTROMETRY
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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

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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). 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.
Francis Aston
USES Mass spectrometry was initially used to show
the identity of isotopes. It is now used to
calculate molecular masses and characterise new
compounds
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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
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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

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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

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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

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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

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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

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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 1charged, the amount of
  separation depends on their mass

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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 1charged, 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

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WHAT IS A MASS SPECTRUM?
MASS SPECTRUM OF NEON

• 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

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EXAMPLE CALCULATION (1)
Calculate the average relative atomic mass of
neon using data on the previous page.
Out of every 100 atoms... 90.92 are 20Ne
, 0.26 are 21Ne and 8.82 are 22Ne
Average (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 (100 in this
case) 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).
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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 39K in every
100 there will then be (100-x) of 41K.
so 39x 41 (100-x) 39.1
therefore 39 x 4100 - 41x 3910
100 thus - 2x - 190 so x
95
Ans. 95 39K and 5 41K
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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
CAN YOU DO ALL OF THESE? YES NO