2.1.4 Determining masses using mass spectrometry

2.1.4 Determining masses using mass spectrometry

1. What is mass spectrometry?

   • Mass spectrometer: piece of apparatus that is used to give info on molecules
     • Can identify an unknown compound
     • Find relative abundance of each isotope of an element
     • Determine structural info on molecules
   • Mass spectrometers determine mass of a molecule or isotope by measuring the mass-to-charge ratio of ions
     • It does this by causing substances to become cations which are then passed through the apparatus and separated according to mass and charge
   • A computer within the mass spectrometer analyses the data on ions and produces a mass spectrum
     • Masss spectrum is a complex bar graph which gives info on abundance of ions in the sample

2. Determining relative isotopic mass and abundance

   • One of the most important uses of a mass spectrometer is to determine isotopes present in an element
   • A mass spectrum shows relative or % abundance on y-axis and mass-to-charge ration on the x-axis
   • Mass-to-charge is shown as m/z, m= mass, z= charge
   • The number of peaks show how many isotopes are present
   • Heights of peaks give the abundance of each isotope
   • If there are no values in the acids, it can be worked out by measuring each line and following this method:
     • Total heights of all peaks: 2 cm + 8 cm = 10cm
       • Height of isotope A: 2cm
         • % abundance: (2/10) x100 = 20%
         • Height of isotope B: 8cm
           • % abundance: (8/10) x100 = 80%

3. Determining relative atomic mass

   • Once individual isotopic masses and relative abundances of the isotopes have been determined it can be used to find the relative atomic mass
   • EXAMPLE: Mg has 3 isotopes and their abundance is: Mg-24 79%, Mg-25 10% and Mg-26 11%
     • Ar= (24 x 79) + (25 x 10) + (26 x 11) /100 = 24.32