Module 2 Mass Spectroscopy
Diagram of a mass spectrometer
Stage 1: Ionisation
The sample is injected as a vapour and the atoms are bombarded by fast moving electrons. This causes the molecule to fragment into two or more pieces. When a molecule or part thereof fragments, one portion would be positively charged while the second portion will be uncharged. All uncharged fragments are ultimately lost in the machine as mass spectrometers only work with positive ions.
Stage 2: Acceleration
The ions are accelerated so that they all have the same kinetic energy.
Stage 3: Deflection
Generally only univalent ions are present, so the only variable would be their masses and thus deflection is based on their mass on charge ratio (m/z) ratio.
Stage 4: Detection
The beam of ions passing through the machine is detected electrically.
What the mass spectrometer output looks like
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. The stick diagram for molybdenum looks like this:
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. NB relative abundance = percentage
The origin of fragmentation patterns
The formation of molecular ions
When the vaporised organic sample passes into the ionisation chamber of a mass spectrometer, it is bombarded by a stream of electrons. These electrons have a high enough energy to knock an electron off an organic molecule to form a positive ion. This ion is called the molecular ion - or sometimes the parent ion. The molecular ion is often given the symbol M+
The molecular ions are...
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