The mass spectrometer

Question 1

1. Ionisation (Electron impact)

Answer 1

• used for elements and low Mr compounds
• high energy electrons are fired at the sample from an electron gun
• this knocks off one electron from each atom/ molecule to form 1+ ion: x (g) —> x^+ (g) + e^-

Question 2

1. Ionisation (Electrospray ionisation)

Answer 2

• used for high Mr compounds (e.g. proteins)
• the sample is dissolved in a volatile solvent (e.g. methanol, water) and injected through a fine hypodermic needle as a fine spray into a vacuum in the ionisation chamber
• a very high voltage is applied to the end of the needle where the spray emerges
  (The needle is positively charged)
• the particles gain a proton and become ions as fine mist:
  x (g) + H+ (g) —> XH+ (g)
• the solvent evaporates leaving 1+ ions

Question 3

2. Acceleration of Ions

Answer 3

• the ions are accelerated using an electric field so that all the ions have the same kinetic energy (where kinetic energy = 1/2 mv^2)

Question 4

3. Separated of charged ions

Answer 4

• ion drift - the ions then enter a flight tube (length d)
• ions with different masses (mass m) have a different time of flight
• the lighter ions travel faster and take less time to reach the detector where
  _________
  / m / 2KE
  t = d/

Question 5

4. Detection

Answer 5

• the detector is a negatively charged plate
• a current is produced when the ions hit the plate as electrons flow from the plate to the positive ions
• the size of the current is proportional to the number of ions