Chemistry As Level - 10.1.2 - Mass Spectrometer

Front 1

3 uses:

Back 1

detection of isotopes
determination of relative atomic mass
determination of relative molecular mass

Front 2

4 stages:

Back 2

ionisation, acceleration, deflection, detection.
A - gun, B, card slits, C curve, D detector.

Front 3

First stage:

Back 3

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.

Front 4

Second stage:

Back 4

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.

Front 5

Third stage.

Back 5

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

Front 6

4th stage:

Back 6

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.

Front 7

Ionization essetial because:

Back 7

only charged particles are accelerated, deflected and detected.

Front 8

Mass spectrum:

Back 8

y = relative abundance, x = m/z. Ion mass same as atom. Peak height = abundance.

Front 9

Molecules:

Back 9

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.