1.4 the mass spectrometer

Cards (12)

  • 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.