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The Mass
Spectrometer
Isotopes
• Most elements occur as mixtures of isotopes
• Isotopes of the same element have different masses.
• The listed atomic mass of an element is the average relative
mass of the isotopes of that element compared to the mass
of carbon-12 (exactly 12.0000 amu)
• Individual atomic
spectrometry
masses
are
determined
by
mass
F W Aston ~ Invented the Mass
Spectrometer
•Invented by F W Aston in 1919
•Working on separating the
isotopes of Neon
•Discovered two types of atom
•One type of mass 20
•Another type of mass 22
F W Aston ~ Invented the
Mass Spectrometer
• Both had 10 protons
• The first type had 10 neutrons
• Second type had 12 neutrons
• Isotopes [Greek for same place]
• Atoms of the same element that have the same
atomic number but different mass numbers due to
different numbers of neutrons
Aston’s Mass Spectrometer
ITS Chemistry
ITS Chemistry
Principle on which the Mass
Spectrometer is based
The Mass Spectrometer works on
the principal that positive ions are
separated by mass when moving in a
magnetic field
Structure of the Mass
Spectrometer
Sample Inlet
Negatively charged plates
Ionisation Chamber
Accelerator
Electron Gun
Variable magnetic field
Lighter particles
Recorder
Ion detector
Amplifier
Separation
Unit
Heavier
particles
Vacuum Pump
Five Stages in mass spectrometry
1) Vaporisation
2) Ionisation
3) Acceleration
4) Separation
5) Detection, Amplification and Display
Vaporisation
• Sample must be vaporised
• There is a Vacuum in the instrument so
liquids vaporise easily
• Solids may have to be heated in a
special unit
Vaporisation Chamber
Sample inlet A
Sample in syringe
injected into chamber
Sample may need to be
heated to vaporise it
Ionisation
• Turns atoms and molecules into ions
• Using an “electron gun”
• Fires high energy electrons at atoms
• These high energy electrons knock off normal electrons forming
positive ions
X + e- = X+ + 2 e• These ions can then be accelerated and deflected using electric
charge and a magnetic field
+
The positive ions are attracted by the
negative charge of the accelerator plates
and those
which knock
pass through
These
electrons
otherthe hole are
accelerated to high speeds and fired into
electrons
from the atom turning
the variable magnetic field
it into
a positive ion
This
is the
IONISATION CHAMBER
Now vaporised atoms are
where atomssprayed
are turned
into ions
into ionisation
chamber and are hit by the
high speed electrons
+
▬
Hot wire is negatively charged and boils
off electrons which are repelled by
negative and attracted to the positive
port at high speed
A
c
c
e
l
e
r
a
t
o
r
Accelerator
• Made of a series of Negative plates with a hole in the centre
• These attract the positive ions
• Some of these ions pass through the holes in the centre of the
plates
• And as they do so are accelerated to high speeds
• This produces a fine beam of positive ions which pass into analyser
• Vacuum allows the ions to move [no air molecules to stop or deflect
them]
Separation
• Magnetic field does the separation.
• In the Analyser
• All ions have the same kinetic energy
• light particles are deflected most
• heavy particles are deflected least
• Beam of particles is separated according to their masses
Separator or Analyser Unit
Magnetic Field on
Magnetic Field off
Heavy
Light Ion
+
No deflection
No magnetic
field
Small deflection Heavy Ion
Large deflection
Light Ion
Detector
• Very sensitive instrument.
• Responds to the number of ions hitting it.
• The more ions that hit it the bigger the reading
• Changing the magnetic field brings particles of
different masses to focus on the detector.
[Stronger the field the greater the particle mass]
• Detector needs to be calibrated
Display
• On computer screen
• Trace recorded as a series of peaks
• Trace called a Mass Spectrum
• When printed called a Mass Specrtrogram
Isotope 2
Abundance
Isotope 1
Isotope 3
0
Mass
Structure of the Mass
Spectrometer
Sample Inlet
Negatively charged plates
Ionisation Chamber
Accelerator
Electron Gun
Variable magnetic field
Lighter particles
Recorder
Ion detector
Amplifier
Separation
Unit
Heavier
particles
Vacuum Pump
Relative Atomic Mass
•
Because most elements have multiple isomers, the masses on
the periodic table cannot describe only 1 isotope’s individual
atomic mass.
•
Therefore, the mass numbers on the periodic table are
relative atomic masses:
– Relative atomic mass is the average mass of the individual
isotopes of an element, taking into account the naturally
occurring relative abundance of each.
– To find the relative atomic mass for an element, sum the
mass contributions from each isotope of the element.
2-
What is the relative atomic mass of Cl?
The relative abundances of Cl35 and Cl-37 are 75.77 and
24.23 respectively
Relative atomic mass of Cl
(35  75.77)  (37  24.23)
=
(75.55  24.23)
= 35.48
• Aston discovered 35Cl and
37Cl
• Determined the percentage
of each isotope present
• Won the Nobel Prize for
Chemistry 1922
• Discovered 212 of the 287
naturally occurring nuclides
Uses of the mass spectrometer
• Determining the Relative molecular mass of atoms
[isotopes] till 1970
• Determining mass of unknown organic compounds
and thus help determine their structures
• Gases from waste dump
• Trace organic pollutants in water