Mass Spec

• Mass spectrometer- gives you info about the relative atomic mass of an element and the relative abundance of its isotopes, and can be used to identify elements.
• Mass spec:

  • Ionisation
  • Acceleration
  • Ion drift
  • Detection
• Ionisation:

  • Electrospray- sample is dissolved in solvent, pushed through a small nozzle at high pressure, high voltage is applied, each particles gains an H+ ion.
  • Electron Impact- sample is vaporised, electron gun is used to fire high energy electrons at it, knocks one electron off of each particle
• Acceleration- accelerated by an electric field, gives the same kinetic energy to all the ions, lighter ions experience a greater acceleration.
• Ion drift- lighter ions will be drifting at higher speeds.
• Detection- detector detects the current created due to electrons transferring to positive ions, when ions hit it and records how long they took to pass through the spectrometer, used to calculate the mass/charge values needed to produce a mass spectrum.
• 1. Ionisation

  • The first step is ionisation of the sample. The main techniques are:
  • Electrospray ionisation.
  • Electron impact ionisation.
  • Electrospray ionisation is a gentler technique and prevents fragmentation.
  • It’s typically used for polymers and biological materials like DNA.
  • The sample is dissolved in a solvent and a high voltage is applied.
  • The high voltage rips a proton off the solvent and attaches it to the sample molecules.
  • The sample molecules are now positively charged ions.
• In electron impact ionisation, the sample is first vaporised and then hit with electrons from an electron gun.

  • The electrons knock off electrons from the molecule
  • The molecules are now positively charged ions.
  • This method often causes the sample to fragment.
• 2) Acceleration

  • Molecules are accelerated to all have the same kinetic energy, so the speed is dependent on the mass of the molecule.
  • Lighter particles move faster and are detected before heavier particles.
• 3) Ion drift​​

  • Lighter ions take less time as the time is dependent on the square root of the mass.
• 4) Detection

  • The ions hit a negatively charged plate.
  • This causes a current and the size of this current is proportional to the number of molecules hitting the plate.
  • This gives the abundance of the molecule.
• • When the sample has passed through the mass spectrometer, a spectrum is produced by the spectrometer.
  • On this spectrum:
  • The x-axis is mass/charge ratio.
  • The y-axis is % abundance.
• • The spectrum produces lots of peaks, but the most important is the molecular ion peak.
  • This is the peak of the greatest mass/charge ratio.
  • This represents the mass/charge value of the molecule we are analyzing.
• • Smaller peaks will cluster around the molecular ion peak.
  • These are from the same molecules but with different isotopes in them.
  • The isotopic molecules have different masses and so different mass/charge ratio values.