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Mass Spectroscopy (MS)

What should you be able to do with MS ?


Mass spectrometry is based on slightly different principles to the other spectroscopic methods. 

The physics behind mass spectrometry is that a charged particle passing through a magnetic field is deflected along a circular path on a radius that is proportional to the mass to charge ratio, m/e. 
In an electron impact mass spectrometer, a high energy beam of electrons is used to displace an electron from the organic molecule to form a radical cation known as the molecular ion. If the molecular ion is too unstable then it can fragment to give other smaller ions. 
The collection of ions is then focused into a beam and accelerated into the magnetic field and deflected along circular paths according to the masses of the ions. By adjusting the magnetic field, the ions can be focused on the detector and recorded.

Formations of ions in MS

schematic of an electron impact mass spectrometer

Molecular ion The ion obtained by the loss of an electron from the molecule
Base peak The most intense peak in the MS, assigned 100% intensity
M+ Symbol often given to the molecular ion
Radical cation +ve charged species with an odd number of electrons
Fragment ions Lighter cations formed by the decomposition of the molecular ion. 
These often correspond to stable carbcations.

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organic chemistry © Dr. Ian Hunt, Department of Chemistry University of Calgary