How the Mass Spectrometer Works
How the Mass Spectrometer Works
This page describes how a mass spectrum is produced using a mass spectrometer.
How a mass spectrometer works
The basic principle
If
something is moving and you subject it to a sideways force, instead of
moving in a straight line, it will move in a curve - deflected out of
its original path by the sideways force.
Suppose
you had a cannonball traveling past you and you wanted to deflect it as
it went by you. All you've got is a jet of water from a hose-pipe that
you can squirt at it. Frankly, its not going to make a lot of
difference! Because the cannonball is so heavy, it will hardly be
deflected at all from its original course.
But
suppose instead, you tried to deflect a table tennis ball traveling at
the same speed as the cannonball using the same jet of water. Because
this ball is so light, you will get a huge deflection.
The
amount of deflection you will get for a given sideways force depends on
the mass of the ball. If you knew the speed of the ball and the size of
the force, you could calculate the mass of the ball if you knew what
sort of curved path it was deflected through. The less the deflection,
the heavier the ball.
You can apply exactly the same principle to atomic sized particles.
An outline of what happens in a mass spectrometer
Atoms
can be deflected by magnetic fields - provided the atom is first turned
into an ion. Electrically charged particles are affected by a magnetic
field although electrically neutral ones aren't.
The sequence is :
- Stage 1: Ionization: The atom is ionised by knocking one or more electrons off to give a positive ion. This is true even for things which you would normally expect to form negative ions (chlorine, for example) or never form ions at all (argon, for example). Mass spectrometers always work with positive ions.
- Stage 2: Acceleration: The ions are accelerated so that they all have the same kinetic energy.
- Stage 3: Deflection: The ions are then deflected by a magnetic field according to their masses. The lighter they are, the more they are deflected. The amount of deflection also depends on the number of positive charges on the ion - in other words, on how many electrons were knocked off in the first stage. The more the ion is charged, the more it gets deflected.
- Stage 4: Detection: The beam of ions passing through the machine is detected electrically.
A full diagram of a mass spectrometer
Understanding what's going on
The need for a vacuum
It's
important that the ions produced in the ionization chamber have a free
run through the machine without hitting air molecules.
Ionization
The
vaporized sample passes into the ionization chamber. The electrically
heated metal coil gives off electrons which are attracted to the
electron trap which is a positively charged plate.
The
particles in the sample (atoms or molecules) are therefore bombarded
with a stream of electrons, and some of the collisions are energetic
enough to knock one or more electrons out of the sample particles to
make positive ions.
Most
of the positive ions formed will carry a charge of +1 because it is
much more difficult to remove further electrons from an already positive
ion. These positive ions are persuaded out into the rest of the machine
by the ion repeller which is another metal plate carrying a slight
positive charge.
Acceleration
The
positive ions are repelled away from the very positive ionization
chamber and pass through three slits, the final one of which is at 0
volts. The middle slit carries some intermediate voltage. All the ions
are accelerated into a finely focused beam.
Deflection
Different ions are deflected by the magnetic field by different amounts. The amount of deflection depends on:
- the mass of the ion. Lighter ions are deflected more than heavier ones.
- the charge on the ion. Ions with 2 (or more) positive charges are deflected more than ones with only 1 positive charge.
These two factors are combined into the mass/charge ratio. Mass/charge ratio is given the symbol m/z (or sometimes m/e).
For
example, if an ion had a mass of 28 and a charge of 1+, its mass/charge
ratio would be 28. An ion with a mass of 56 and a charge of 2+ would
also have a mass/charge ratio of 28.
In
the last diagram, ion stream A is most deflected - it will contain ions
with the smallest mass/charge ratio. Ion stream C is the least
deflected - it contains ions with the greatest mass/charge ratio.
It
makes it simpler to talk about this if we assume that the charge on all
the ions is 1+. Most of the ions passing through the mass spectrometer
will have a charge of 1+, so that the mass/charge ratio will be the same
as the mass of the ion.
Assuming
1+ ions, stream A has the lightest ions, stream B the next lightest and
stream C the heaviest. Lighter ions are going to be more deflected than
heavy ones.
Detection
Only
ion stream B makes it right through the machine to the ion detector.
The other ions collide with the walls where they will pick up electrons
and be neutralised. Eventually, they get removed from the mass
spectrometer by the vacuum pump.
When
an ion hits the metal box, its charge is neutralised by an electron
jumping from the metal on to the ion (right hand diagram). That leaves a
space amongst the electrons in the metal, and the electrons in the wire
shuffle along to fill it.
A
flow of electrons in the wire is detected as an electric current which
can be amplified and recorded. The more ions arriving, the greater the
current.
Detecting the other ions
How might the other ions be detected - those in streams A and C which have been lost in the machine?
Remember
that stream A was most deflected - it has the smallest value of m/z
(the lightest ions if the charge is 1+). To bring them on to the
detector, you would need to deflect them less - by using a smaller
magnetic field (a smaller sideways force).
To
bring those with a larger m/z value (the heavier ions if the charge is
+1) on to the detector you would have to deflect them more by using a
larger magnetic field.
If
you vary the magnetic field, you can bring each ion stream in turn on
to the detector to produce a current which is proportional to the number
of ions arriving. The mass of each ion being detected is related to the
size of the magnetic field used to bring it on to the detector. The
machine can be calibrated to record current (which is a measure of the
number of ions) against m/z directly. The mass is measured on the 12C scale.
What the mass spectrometer output looks like
The
output from the chart recorder is usually simplified into a "stick
diagram". This shows the relative current produced by ions of varying
mass/charge ratio. The stick diagram for molybdenum looks like this:
You
may find diagrams in which the vertical axis is labeled as either
"relative abundance" or "relative intensity". Whichever is used, it
means the same thing. The vertical scale is related to the current
received by the chart recorder - and so to the number of ions arriving
at the detector: the greater the current, the more abundant the ion.
As
you will see from the diagram, the commonest ion has a mass/charge
ratio of 98. Other ions have mass/charge ratios of 92, 94, 95, 96, 97
and 100. That means that molybdenum consists of 7 different isotopes.
Assuming that the ions all have a charge of 1+, that means that the
masses of the 7 isotopes on the carbon-12 scale are 92, 94, 95, 96, 97,
98 and 100.
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