How are relative masses determined experimentally? A mass spectrometer is used to determine the relative masses of different atoms or molecules.
What is Mass Spectrometry?
Mass Spectrometry is a chemical analysis technique used to:
• Identify unknown samples, and
• Determine the amount of a chemical species in a sample.
Mass Spectrometry has an unparalleled combination of sensitivity, selectivity, and range of application (Astrophysics to Zoology). Most common areas of application are Chemistry and Biology.
Vaporisation and Ionisation
Most of the mass spectrometer is pumped out to give a high vacuum. This is to give the ions a reasonable chance of travelling from one end of the instrument to the other without any hindrance from air molecules. The sample under investigation is heated and vaporized to a gas. The sample molecules are then ionized, because ions are easier to manipulate than neutral molecules.
Ionisation
High energy electrons are fired at the gaseous sample. This knocks out electrons from the atoms or molecules of the sample to produce positive ions and high energy electron.
Acceleration of ions
Ions are accelerated by an electric field. Th en they go through a magnetic field where they are separated according to their mass (m)-to-charge (e) ratios (m/e). Lighter ions are deflected more than heavier ions with the same charge.
Compounds can also be analysed in a mass spectrometer. Th e molecules are vaporized, ionized and accelerated. Electron ionization causes fragmentation (break up) of the molecules in the sample. Th e lighter fragments are deflected more than the heavier ones.
