a partially evacuated glass eneveloppe with electrodes inserted. At appropriate gas pressure and voltage across electrodes a glowing light was observed - it was acc used at entertainment at the time!
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What did they then determine was present?
electrons- by experimenting with magnets and electric fields
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Who obtained MS of various gases in the 20th Cent.
Thompson. now the technique is improved and is used today for research, food, forensics and of course drug driscovery
whole process is carried out under vacuum to prevent sample combining with air. Sample is then volatilized and injected via hollow needle
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Ionisation
Electron beam of high speed high energy electrons knocks off an electron from sample molecules to produce positive ions
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Acceleration
positive ions are then accelerated by the electromagnetic field
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Deflection
+ve ions are deflected by curve and another electromagnetic field
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Detection
Ions hit detector , based on m/z value . Smaller ions reach the detector first
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Radius of flight
ions travel in a curved manner. inside the instrument
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Radius of path equation
r= mv/zh mass , velocity, z-charge h- strength of field
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what can we infer from the equation
essentially radius of path is dependent on molecule mass as vzh are constant `
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The molecular ion is always
a radical cation
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Fragments can be
either radical cations or carbocations
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what is a radical
Most stable coumpounds have an even no of e. If they lose 1 e to become odd no then this is a radical
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Length of bar in MS
relative abundance
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Most intense peak
base peak (peak with highest abundance)
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Furthest peak to the right
molecular ion peak (NB: the base peak isnt always the MI)
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Base peak is assigned value of
100%- so that all other peaks allocated are in comparison to the base peak
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Issues of MS
dont always see MI
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why dont we always see the MIP
1. may combine with water 2. alcohols readily eliminate water 3. alkyls are prone to being lose (CH3) 4. may combine wit hydrogen (MI+1)
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Why does the molecule fragment
the ionisation process produces ions with xs energy- this energy is dissipated via fragmentation
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C-C bonds Vs molecules with N and Os
C-C no particular weak link . N/O atoms weaker links exist within the molecule
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why is fragmentation a mixed blessing?
it can help us to work out the structure of the molecule. BUT if the MI lifetime is only a few seconds- not enough time to see it - without MI as ref - very hard to interpret structure
any molecule which contains C=O and at least 3C. ketones with 3C either side can occur on both sides
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MS and fingerprinting
because the fragmentation patterns are so complex we can use them to ID compounds. MS is a v.sensitive technique and can be used with v.small samples(<1mcg)`
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isotopes and MS
we can use D20 to give info on substitution of amines
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Other cards in this set
Card 2
Front
What did they then determine was present?
Back
electrons- by experimenting with magnets and electric fields
Card 3
Front
Who obtained MS of various gases in the 20th Cent.
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