mass spectrometer

overview of mass spectrometer

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  • mass spectrometer
    • four key stages
      • ionisation- an electron gun fires electrons at the atoms of the sample to knock one of the outershell elctrons out. this makes them +1 positive ions. (5% become +2 ions)
      • acceleration- the positive ions are attracted to the negatively charged plates. this speeds up the ions, their speed depends on their size. lighter ions travel faster.
      • deflection- ions move into a magnetic field. the deflection depends on the size of the ions, heavier ones are deflected less than lighter ones, it also depends on the strength of the field.
        • the magnetic field must be steadily increased so that ions of increasing mass enter the detector one after another.
      • detection-  when ions hit the detector they lose their charge and create a current which is proportional to the abundance of each ion
    • what is mass spectra used for?
      • to find the Mr of a sample
      • to find the mass of different isotopes that make up an element
      • to identify a substance
    • using mass spectra information
      • (mass number of isotope 1 x relative abundance%) +  (mass number of isotope 2 x relative abundance%)   all divided by 100 = average relative atomic mass
      • be careful of double charged ions, these will be measured as half as light as they should be because they have traveled at twice the speed so there will be small peaks at half the mass of the other peak, they should be ignored in calculations.


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