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9 Cards in this Set

  • Front
  • Back
3 uses:
detection of isotopes
determination of relative atomic mass
determination of relative molecular mass
4 stages:
ionisation, acceleration, deflection, detection.
A - gun, B, card slits, C curve, D detector.
First stage:
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.
Second stage:
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.
Third stage.
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
4th stage:
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.
Ionization essetial because:
only charged particles are accelerated, deflected and detected.
Mass spectrum:
y = relative abundance, x = m/z. Ion mass same as atom. Peak height = abundance.
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.