1.4- Mass Spectrometer

Question 1

What is ion drift

Answer 1

The 1+ ions will pass through a hole in the negatively charged plate and move into a flight tube

• The time of flight of each 1+ ion in this tube depends on their velocity

Question 2

What happens in detection

Answer 2

• the 1+ ions will hit a negatively charged ‘detector’ plate
• As they hit this electric plate, they gain an electron
• This gaining of an electron discharges the ion, and causes a current to be produced
  —> Thissize of the currentisproportional to the abundanceof those ions hitting the plate and gaining an electron
• The detector plate is connected to a computer, which produces the mass spectrum

Question 3

4 main stages of TOF mass spectrometer

Answer 3

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

Question 4

Interpreting mass spectra

Answer 4

Question 5

TOF calculations

Answer 5

Question 6

What happens in electron impact ionisation

Answer 6

• The sample is vaporised and then bombarded with high energy electrons
• The electrons are ‘fired’ from an electron gun
• The electron gun is a hot wire filament which emits electrons as a current runs through it
• As the sample is bombarded by these electrons, an electron is knocked off each particle, forming a 1+ ion

X (g) →X+(g) + e-

Question 7

Acceleration in TOF steps

Answer 7

• The 1+ ions formed from either ionisation method are accelerated using an electric field
• They are all accelerated to have thesame kinetic energy
• Since all 1+ ions will have the same kinetic energy, their velocity will depend on their mass
  —>Lighter ions will move faster and heavier ions will move slower

Question 8

Calculate the time of flight for a52Cr+ion with kinetic energy of 6.48 × 10-14 J through a flight tube 1.80 m long.

Answer 8

Question 9

What happens in Electron spray ionisation

Answer 9

• The solvent is injected into the mass spectrometer using a hypodermic needle
• This produces a fine mist or aerosol
• The needle is attached to a high voltage power supply, so as the sample is injected, the particles are ionised by gaining a proton from the solvent
  X (g) + H+→ XH+(g)
• The solvent evaporates and the XH+ions are attracted towards a negatively charged plate
• This accelerates them through the mass spectrometer

Question 10

What does a mass spectrometer do?

Answer 10

measures the mass to charge ratio of ions