Atomic Structure - TOF mass spectrometer Flashcards
electron impact
Electron impact:
- a vaporised (heated) sample is injected at a low pressure
-an electron gun fires high energy electrons at the sample
-This knowcks out an outer electron
-forming +1 ions - loses an electron.
What is the half equation for electron impact?
X (g) → X+ (g) + e-
Electrospray Ionisation
- sample is dissolved in a volatile solvent
- it is injected though a fine needle giving a fine mist
- the tip has a high voltage - this is where it gains a proton, H+, from the solvent forming MH+
-the solvent evaporated away towards the negative plate.
Electrospray half equation
X (g) + H+ → XH+ (g)
The detector will present the current as +1 because it gains a proton so the Mr will be the number -1.
Acceleration
- The 1+ ions are accelerated by an electric field
- they are all accelerated to have the same KE
- since all the ions will have the same KE, their velocity will depend on their mass (lighter = travel faster, heavier = travel slower.
KE equation
KE= 1/2 x m x V^2
Ion drift (flight tube)
- ions will pass through the hole in the negatively charged plate and move into the FT
- The TOF depends on their veolicty (heavier or lighter)
- the ions are distinguished by different flight times.
equation linking distance, velocity, time
V (ms^-1) = D (m) / T (s)
Detection
-the ions will hit a negatively charged ‘detector’ plate .
- the electrons are attracted to the positive ions and so move towards them meaning the ions will gain an electron
- this causes a current
- The detector plate is connected to a computer which produces mass spectrum
Relationship between the current and the abundance
The size of the current is proportional to the abundance of the ions
What is the claculation for comparing isotopes?
m/t^2 = m/t^2
