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Chemistry Unit 1: 1.4:
Mass Spectrometry
- Quick definition:
- The mass spectrometer determines very
accurately, the relative atomic mass of an
isotope!
- The mass spectrometer determines very
accurately, the relative atomic mass of an
isotope!
- What Happens in a Mass
Spectrometer?
- In a nutshell: atoms are converted into
ions, (remember ions?) accelerated,
deflected before arriving at a detector.
More details? Okay then.
- The steps are:
Ionisation,
acceleration, deflection
then detection.
- The steps are:
Ionisation,
acceleration, deflection
then detection.
- Ionisation:
- a beam of high energy
electrons is fired from an
electron gun that collides with
the sample inside the MS and
knocks and electron from it.
- this forms positive (+1) ions.
- nearly all atoms lose 1 electron but in
some cases (around 5%) 2 electrons
can be lost forming 2+ ions.
- nearly all atoms lose 1 electron but in
some cases (around 5%) 2 electrons
can be lost forming 2+ ions.
- this forms positive (+1) ions.
- a beam of high energy
electrons is fired from an
electron gun that collides with
the sample inside the MS and
knocks and electron from it.
- Acceleration:
- the newly formed +ions are
attracted to -charged plates and
accelerated to a high speed.
- the speed they reach is
proportional to their mass: the
higher the mass, the slower they
travel, the lower the mass, the
faster they go.
- some ions pass through slits in
the plates, this forms the ions in a
beam (sounds pretty Sci-fi,
doesn't it?)
- some ions pass through slits in
the plates, this forms the ions in a
beam (sounds pretty Sci-fi,
doesn't it?)
- the speed they reach is
proportional to their mass: the
higher the mass, the slower they
travel, the lower the mass, the
faster they go.
- the newly formed +ions are
attracted to -charged plates and
accelerated to a high speed.
- Deflection:
- the beam of ions then
travels into a magnetic
field at right angles (90
degrees) to the direction
it's travelling.
- the magnetic field deflects the ion beam (pew! pew!) into an arc of a
circle. The deflection of an ion depends on it's m/z ratio, or mass to
charge ratio.
- in the m/z ratio, 'm' is
the mass of the ion and
'z' is the charge (usually
+1)
- in the m/z ratio, 'm' is
the mass of the ion and
'z' is the charge (usually
+1)
- the magnetic field deflects the ion beam (pew! pew!) into an arc of a
circle. The deflection of an ion depends on it's m/z ratio, or mass to
charge ratio.
- heavier ions are deflected less (as we know) than
lighter ones and 2+ ions are deflected twice as much as
+1 ions of the same mass.
- the beam of ions then
travels into a magnetic
field at right angles (90
degrees) to the direction
it's travelling.
- In a nutshell: atoms are converted into
ions, (remember ions?) accelerated,
deflected before arriving at a detector.
More details? Okay then.
- Detection:
- the magnetic field's strength is gradually increased
so that the ions of increasing mass enter the
detector one after the other.
- ions collide with the detector, accept
electrons, lose their charge and create a
current that is proportional to their
abundance.
- Note: 'abundance' simply means 'amount of'.
- Note: 'abundance' simply means 'amount of'.
- ions collide with the detector, accept
electrons, lose their charge and create a
current that is proportional to their
abundance.
- a computer
measures the
strength of the
current created and
works out the m/z
ratio. A read-out
called a mass
spectrum is produced
- this is normally a
graph.
- a mass spectrum
- a mass spectrum
- the magnetic field's strength is gradually increased
so that the ions of increasing mass enter the
detector one after the other.
- How to Calculate Low Resolution Mass
Spectrometry:
- Take the readings/mass numbers from your mass spectrum (graph), e.g. neon
has two isotopes of mass numbers 20 and 22 with abundances of 90% and 10%
respectively.
- the mass number is the
y axis, the abundance
(%) is the x axis.
- to work out the relative atomic mass
of an element, you do this: (90 x 20)
+ (10 x 22), then you divide the
answer, which is 2020, by the total
mass, which is 100.
- the answer is 20.2 which means the relative atomic mass of neon is 20.2
- the mass number is the
y axis, the abundance
(%) is the x axis.
- Take the readings/mass numbers from your mass spectrum (graph), e.g. neon
has two isotopes of mass numbers 20 and 22 with abundances of 90% and 10%
respectively.
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