7.1 - Mass Spectra of Organic Compounds

• what's mass spectometry used for?

  to identify unknown compounds by helping determine relative molecular masses, structures and relative atomic masses
• why is mass spectometry done in a vacuum?

  air particles would collide with unstable charged species so would affect results
• what does mass spectometry produce?

  a mass spectogram
• main processes in a mass spectometer

  ionisation

  fragmentation

  acceleration

  deflection

  detection
• ionisation and fragmentation

  vaporised sample is bombarded by a beam of high energy electrons to knock out electrons forming cations and split them into fragments

  molecules break up at weak bonds or where more stable fragments are formed

  so mass spectrum consists of a fragmentation pattern
• acceleration

  charged species are accelerated through a drift region by the same current so their resulting speed is only dependent on their size - velocity is inversely proportional to mass
• deflection

  charged species are deflected by an electric/magnetic field depending on their m/z ratio

  greater m/z ratio=less deflection

  smaller m/z ratio=more deflection
• detection

  charged species reach a detector causing a small current which is amplified and the signal fed to a computer
• molecular ion peak

  highest m/z ratio

  mass gives the relative molecular mass of the molecule
• molecular ion (parent ion)

  positive ion formed when a molecule loses one electron

  a radical
• how to work out the structural formula of a compound from its mass spectogram

  work out what ion could produce a certain m/z ratio using the mass numbers of the atoms that make up that ion
• how to differentiate between similar molecules in mass spectometry

  even if they have the same atoms they will have different structures so will produce different fragments