1.4- The Mass Spectrometer

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  • Created by: Heather
  • Created on: 04-10-14 15:02
What does a mass spectrometer measure?
The Relative Atomic masses of an atom
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What scale are the atomic masses measured?
The scale on which the mass of the atom of Carbon-12 is defined as exactly 12,
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Why is carbon-12 used as the scale?
It is the only isotope that has an exact atomic mass that is a whole number
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Give an example of a use of a mass spectrometer?
They are used by forensic scientists to help identify substances like illegal dugs
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What conditions is the spectroemeter kept under and why?
In a high vacuum so the ions dont collide with air molecules which might stop them reaching the detector,
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What state is the sample put into the mass spectrometer?
In a gaseous state and if a solid, it is vapourised first by heating,
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What is an outline of what happens in a mass spectrometer?
Atoms are converted to ions, accelerated and deflected accordin to their masses adn their charges and arrive at a detector.
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What is the first stage of the mass sepctrometer?
Ionisation- A beam of electrons from an 'electron gun' knocks out electrons from atoms or molecules of the sample so they form positive ions. Majority of atoms lose one electron but 5% lose two electrons
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What is the second stage of the mass spectrometer?
Acceleration- These positive ions are attracted towards the negatively charged plates and are accelerated to a high speed. The speed they reach depends on their mass- the lighter the ions the faster they go
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What is the third stage of the mass spectrometer?
Deflection- Beam of ions moves into a magnetic field at right angles to its direction of travel. The magnetic field deflects the beam of ion into an arc of a circle.
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What does the deflection of the ion depend on?
It depends on the ratio of its mass to charge (m/z). Heavier atoms are delfected less than lighter ones and 2+ ions are deflected twice as much as 1+ ions. it also depend on the magnetic field strength- the strogner the field, the greater deflection
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What is the fourth stage of the mass spectrometer?
Detection- The magnetic filed is graudually increased so that ions of increasing mass enter the detector one after the other. Ions stirke the detector, accept electrons, lose their charge and create a current,
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What is the current produced proportional to?
Current is proportional to the abundance of each ion,
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What do the cimputer do when the ion hits the detector? What is produced?
From the strength of the magentic fild at which a partiuclar ion hits the detector, a computer works out the value of the mass to charge ratio, m/z, of the original ion and a mass spectrum is produced,
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What is a mass spectrum?
A graph of relative abundance of ions against the mass to charge ration (m/z)
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So what are the four key stages in a mass spectrometer?
Ionisation, acceleration, deflection and detection
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How resolute is a mass spectrometer?
It can measure to 5 decimal places which is called High Resolution mass spectrometry and low resolution mass spectrometry is to the nearest whole number,
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How are mass spectrometers used in space?
In space probes like the Viking Martian Lander which are used to identify the elements in rock samples
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Other cards in this set

Card 2

Front

What scale are the atomic masses measured?

Back

The scale on which the mass of the atom of Carbon-12 is defined as exactly 12,

Card 3

Front

Why is carbon-12 used as the scale?

Back

Preview of the front of card 3

Card 4

Front

Give an example of a use of a mass spectrometer?

Back

Preview of the front of card 4

Card 5

Front

What conditions is the spectroemeter kept under and why?

Back

Preview of the front of card 5
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