Mass Spectrometry

• Mass spectrometry is an accurate instrumental technique used to determine the relative atomic mass and the molecular mass.
• The mass spectrum shows the abundance of different masses present in the sample, with the most abundant species being at the top of the graph.
• Uses of mass spectrometry include detecting illegal drugs, forensic science, space exploration and carbon-13 dating
• Requirement: the whole system is under high vacuum to prevent ions produced colliding with molecules in the air.
• Stages:

  1. ionisation
  2. acceleration
  3. ion drift
  4. ion detection
  5. data analysis
• There are 2 ways to ionise a sample: electron impact/bombardment, electrospray ionisation
• Electron impact: the sample is vaporised and the gaseous molecules are then bombarded with high-speed electrons from an electron gun. These knock out an electron from some of the molecules, creating molecular ions. This technique is used for elememts and substances with low molecular mass.
• Electrospray ionisation: The sample is dissolved in a solvent. This is injected through a very narrow, hollow needle. A high voltage is applied to the end of the needle where the spray emerges. Particles are ionised by gaining a proton from the solvent. This technique is used for many substances with higher molecular mass.
• Acceleration - by electric field.
• Acceleration: the positive ions are accelerated towards a negatively charged plate. All ions gain the same kinetic energy.
• The velocity of each particle depends on mass. Lighter particles have a faster velocity, and heavier particles have a slower velocity.
• Ion drift: Now the ions pass through a narrow hole in the negatively charged plate. They are no longer accelerated but 'drift' in a narrow tube towards the detector. The TOF depends on the particle's velocity which depends on its mass.
• Ion detection: The positive ions hit a negatively charged electric plate. The positive ions are discharged by gaining electrons from the plate. This generates a movement of electrons and hence an electric current. The size of the current gives a measure of the number of ions hitting the plate. Greater current = greater abundance.
• Data analysis: The signal generated by the ions is passed to a computer. The ions are reported as a mass-charge ratio (m/z). The peak heights give a relative abundance of that ion.