• Mass spectroscopy?is an analytical technique used to identify unknown compounds
• The molecules in the small sample are?bombarded?with high energy electrons which can cause the molecule to lose an electron
• This results in the formation of a positively charged?molecular?ion?with one unpaired electron
  • One of the electrons in the pair has been?removed?by the beam of electrons

• The molecular ion can further?fragment?to form new ions, molecules, and radicals

Fragmentation of a molecule in mass spectroscopy

• These?fragmentation?ions are?accelerated?by an electric field
• Based on their mass (m) to charge (e) ratio, the fragments of ions are then separated by deflecting them into the?detector
  • For example, an ion with mass 16 and charge 2+ will have a?m/e?value of 8
• The smaller and more positively charged fragment ions will be?detected?first as they will get?deflected?the most and are more attracted to the?negative?pole?of the magnet
• Each fragment corresponds to a specific?peak?with a particular?m/e?value in the mass spectrum
• The?base peak?is the peak corresponding to the most?abundant?ion

Isotopes

• Isotopes are different atoms of the?same?element?that contain the same number of?protons?and?electrons?but a different number of?neutrons.
  • These are atoms of the same?elements?but with different mass number
  • For example, Cl-35 and Cl-37 are isotopes as they are both atoms of the same element (chlorine, Cl) but have a different mass number (35 and 37 respectively)
• Mass spectroscopy can be used to find the?relative abundance?of the isotopes experimentally
• The?relative abundance?of an isotope is the proportion of one particular isotope in a mixture of isotopes found in nature
  • For example, the relative abundance of Cl-35 and Cl-37 is 75% and 25% respectively
  • This means that in nature, 75% of the chlorine atoms is the Cl-35 isotope and 25% is the Cl-37 isotope
• The?heights?of the peaks in mass spectroscopy show the proportion of each isotope present

The peak heights show the relative abundance of the boron isotopes: boron-10 has a relative abundance of 19.9% and boron-11 has a relative abundance of 80.1%

Worked example: Calculating m/e ratio

Answer

⁵⁶Fe³⁺?has a smaller?m/e?ratio and will therefore be deflected more.

It also has the largest positive charge and will be more attracted to the negative pole of the magnet within the mass spectrometer.