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9 Cards in this Set
- Front
- Back
3 uses:
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detection of isotopes
determination of relative atomic mass determination of relative molecular mass |
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4 stages:
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ionisation, acceleration, deflection, detection.
A - gun, B, card slits, C curve, D detector. |
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First stage:
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Ionisation - an electron gun fires high energy electrons at the sample atoms (or mols) whcih knock electrons out of the gaseous particles so that they form positive ions. X(g) + e- -> X+(g) + 2e-. If energy of e-gun is too high, some 2+ ions may be formed. More energy needed so less likely. Migh energy electrons may also cause mols to fragment into smaller particles.
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Second stage:
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The possitive ions are accelerated twoards the -plates of the electric field and focussed into a beam as they pass through slits. Bigger voltage but small enough for only 1+ ions.
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Third stage.
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Deflection - +ions deflected by mag field. D depends on the m/z ratio - lighter have smaller and deflected most. heavier higher and deflected least. Vacuum pump removes air mols from chamber to avoid air-ion collisions that could also deflect the +ions
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4th stage:
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Detection - +ions attracted to -charged plate of the detector. When ion hits -plate electric current produced sends signal to computer to analyse. Lighter detected with small mag field. Strength can be changed.
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Ionization essetial because:
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only charged particles are accelerated, deflected and detected.
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Mass spectrum:
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y = relative abundance, x = m/z. Ion mass same as atom. Peak height = abundance.
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Molecules:
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molecular ions ionized in same way, although some will fragment. Assuming +1 charge, peak with highest M/z gives Mr of mol. This main use for mass spectrometer.
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