1.4 the mass spectrometer

• the relative atomic mass is the average mass of an atom of an element compared to 1/12th the mass of a carbon 12 atom
• the mass spectrometer determines the mass of separate atoms
• in a time of flight mass spectrometer, substances are converted to positive ions, accelerated to high speeds, and arrive at a detector
• TOF mass spectrometer:

  1. vacuum
  2. ionisation
  3. acceleration
  4. ion drift
  5. detection
  6. data analysis
• a TOF mass spectrometer has a vacuum so that there's no collisions between particles
• In electrospray ionisation, the sample is dissolved in a volatile solvent and forced through a fine hollow needle that is connected to the positive terminal of a high voltage supply. This produces tiny positively charged droplets that have gained a proton from the solvent. The solvent evaporates from the droplets into the vacuum and the droplets get smaller until they contain a single positively charged ion
• In electron impact, a sample is vaporised and high energy electrons are fired at it from an electron gun which usually knocks off one electron from each particle, forming a +1 ion
• the positive ions are attracted towards a negatively charged plate and accelerated towards it
• the ions pass through a hole in the negatively charged plate, forming a beam and travel along a tube, called the flight tube to a detector
• the flight times are recorded and the positive ions pick up an electron from the detector, which causes current to flow
• the signal from the detector is passed to a computer which generates a mass spectrum
• the mass spectrometer can be used to identify the different isotopes that make up an element.