Atomic Structure: Mass Spectrometry

Question 1

What can mass spectrometry tell us?

Answer 1

• The mass spectrometer gives accurate information about relative isotopic mass and also about the relative abundance of isotopes

• Mass spectrometry can be used to identify elements
• Mass spectrometry can be used to determine relative molecular mass

Question 2

Time of Flight Mass Spectrometer Sections

Answer 2

1. Ionisation
2. Acceleration
3. Ion drift
4. Detection

Question 3

Ionisation

Answer 3

• The sample needs to be ionised before it enters the mass spectrometer

• Ions, not molecules will interact with an electrical field to be accelerated
• Only ions will create a current

• There are two methods of doing this:

• Electrospray ionisation
• Electron impact ionisation

• Apparatus under vacuum

• This excludes air
• Prevents the ions from colliding with air

Question 4

Ionisation: Electrospray

Answer 4

• Sample is dissolved in a solvent and pushed through a hypodermic needle at a high pressure
• Tip of the needle is attached to a positive terminal of a high-voltage power supply
• High voltage is supplied, causing each particle to become ionised by gaining a H+ ion
• They leave the needle producing XH+ ions (ions with a single positive charge and a mass or Mr +1)
• X(g) + H+ —> XH+ (g)

Question 5

Ionisation: Electron Impact

Answer 5

• Sample is vapourised
• Then high energy electrons are fired at it
• The high energy electrons come from an ‘electron gun’ which is a hot wire filament with a current running through it that emits electrons
• These electrons knock off one electron from each particle, so they become +1 ions
• X(g) —> X+(g) + e-

Question 6

Acceleration

Answer 6

• Ions must be accelerated to ensure they have the same kinetic energy

• Ions are accelerated by an electric field
• The positive ions are attracted to a negatively charged electric plate
• Lighter ions experience a greater acceleration as they have a lower mass

• KE = (1/2)mv^2

• KE = kinetic energy
• m = mass (kg)
• v = velocity (ms^-1)

• v = √(2KE/m)

Question 7

Ion Drift

Answer 7

• The positive ions travel through a hole in the negatively charged plate into a tube (flight tube)
• The time of flight of each particle through this flight tube depends on its velocity which in turn depends on its mass
• Heavier ions travel slower
• Distance travelled is usually 3-4m
• t = d/v
• t = d/√(2KE/m)
• t = d √(m/2KE)

Question 8

Detection

Answer 8

• The positive ions hit a negatively charged electric plate
• When they hit the detector plate, the positive ions are discharged by gaining electrons from the plate
• This generates a movement of electrons and hence an electric current that is measured
• The size of the current is proportional to the number of ions hitting the plate
• Detector also records how long the ions took to pass through the spectrometer

• m / z

• m = mass
• z = charge