Mass spectrometer can be considered as a huge landmark in modern chemistry. It was designed by scientist F.W. Aston in 1919 to analyse positive rays. This was later modified by Aston and A.J Dempster to increase its sensitivity.
What is a mass spectrometer?
The mass spectrometer is an instrument which can be used to measure,
-masses of atoms and molecules
-relative concentrations of atoms and molecules
It makes use of the basic magnetic field acting on a moving charged particle. Thereby we get a magnetic force acting on the particle..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.
How is it done?
Suppose you had a cannonball travelling 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 travelling 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.
It is the same principle applied in this case too.
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Atoms and molecules cannot be deflected by magnetic fields. Electrically charged particles are affected by a magnetic field which means that its the ions that are deflected.
Mass spectrometer protocols
1) Ionization
The atom or molecule is ionized by removing 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). Most mass spectrometers work with positive ions.Therefore, in order to ionize a particular atom or molecule we must supply the necessary ionization enthalphy. This is done in the ionization chamber in the mass spectrometer.
.2)Acceleration
The ions are accelerated so that they all have the same kinetic energy.
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.
In other words deflection depends on the charge/mass (e/m) ratio. If e/m is high then the deflection is high.
4)Detection
The beam of ions passing through the machine is detected electrically
Special points
Neccesity of a vacuum chamber.
It's important that the ions produced in the ionization chamber have a free run through the machine without hitting air molecules.
Ionization chamber
The vaporised sample passes into the ionisation 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. The second ionization energy is comparatively higher.
These positive ions are forced out into the rest of the machine by the ion repeller which is another metal plate carrying a slight positive charge.
As you will see in a moment, the whole ionisation chamber is held at a positive voltage of about 10,000 volts. Where we are talking about the two plates having positive charges, these charges are in addition to that 10,000 volts.
Acceleration mechanism
The positive ions are repelled away from the very positive ionisation 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 mechanism
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 charge/mass ratio. It is given the symbol e/m.
In the above diagram, ion stream A is most deflected - it will contain ions with the largest e/m ratio. Ion stream C is the least deflected - it contains ions with the lowest e/m ratio.
Assuming 1+ ions, stream A has the lightest ions, stream B the next lightest and stream C the heaviest. Lighter ions are going to get 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 among 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. When more ions are detected more would be the current too.
Detection of 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 greatest value of e/m (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 smaller e/m 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/e directly. (Here the reciprocal of e/m is considered)The mass is measured on the 12C scale.
Mass spectrometer output
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.
Above recorded is a chart of Molibdium.
You may find diagrams in which the vertical axis is labelled 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.
Therefore, from all the above facts we can conclude that mass spectrometer is a great invention which helped chemists to find out about isotopes.