Mass Spectrometry

• Use of mass spectrometer.


  To separate ions with different mass.

  Help identify complex molecules.
• Processes in mass spectrometer.


  Electrospray / Electro Impact ionisation, acceleration, ion drift, detection
• Electrospray ionisation.


  Sample is dissolved in a solvent.

  Forced through a high voltage nozzle to produce droplets containing positively charged ions.

  Solvent evaporates from droplets to produce gaseous ions.
• Electron impact ionisation 


  Sample is vaporized.

  High energy electrons fired at sample from an electron gun,

  Knocking an electron off atom / molecule.

  Creating an ion with a +1 charge.

  E.G M(g ) -> M+ (g) + e- MUST INCLUDE STATE SYMBOLS.
• Acceleration.


  Constant electric field applied to ions.

  Ions with the same charge will all have the same kinetic energy.

  Velocity of ions determined by m/z ratio.
• Ion drift


  Ions with different m/z have different velocity.

  Therefore flight time depends on m/z ratio.

  Smaller m/z ratio = faster flight time
• Detection


  Ion hits detector, accept electrons and create a small electric current. E.G M+ (g) + e= -> M(g)

  The bigger the current the greater the abundance of the ion.

  Current amplified and signal sent to computer to create spectrum.
• Spectrum of a monotomic element.


  Number of peaks = number of isotopes.

  M/Z of peaks = relative mass of ions assuming charge is 1+

  Relative height of peaks = relative abundance of each isotope - Fraction.
• Relative atomic mass from mass spectrum data.


  Ar = the sum of relative mass of isotope x fractional abundance of isotope.
• Relative molecular mass from mass spectrum for a molecule. 


  Peak with highest value of m/z generally gives value for Mr.
• Relative atomic mass of an element.


  AR = Mean mass of one atom of an element / 1/12th mass of an atom of . carbon-12.
• Relative formula or molecular mass of an entity.

  Mr = Mean mass of an entity / 1/12th mass of an atom of carbon-12

  Entity means molecule.
• Mr from formula

  Sum of relative atomic masses of all atoms inn the formula.
• Conversions for Ideal Gas Equations

  Pressure= Pa

  1kPa = 1000 Pa.

  Volume = m^3

  1 cm^3 = DIVIDE BY 1 x 10^-6

  1dm^3 = DIVIDE BY 1000

  Temperature = Kelvin. +273