Question

How do you read mass spectroscopy graphs?

Answer 1

The mass spectroscopy graph gives the value of mass to charge ratio for one or more component molecules present in the compound and this value can then be used to find the molecular weight and structure of the interested compound.

Complete answer:
Mass spectroscopy is an analytical technique that makes use of the magnetic field to detect a chemical substance by measuring the deflection caused by them according to their mass to charge ratio.
Before going into details of the mass spectroscopy graph, let us discuss in brief how mass spectrometers work.
The mass spectrometer is made up of three components: an ion source, a mass analyzer, and a detector. Firstly, a vaporized organic molecule is passed through an ionization chamber where it is bombarded with an electron beam. This beam contains high-energy electrons which knocks off an electron from the organic molecule to form a positive molecule ion. This ionic species is called a molecular ion or radical cation.
The molecular ions are short-lived species and energetically unstable, so they break into a charged species and an uncharged free radical as shown below:
$\text{M}_{\bullet }^{+}\to {{\text{X}}^{+}}+{{\text{Y}}^{\bullet }}$
These fragments are then passed through the magnetic field where only charged species will be deflected and detected by the detector. The different fragments of the compound will travel through a mass spectrometer as a molecular ion and each possible fragment will produce a line on the spectroscopy graph.
The spectroscopy graph looks like this:
It is a spectroscopy graph of propane. The X-axis on the graph represents the mass to charge ratio of the fragments produced and the Y-axis represents the relative abundance or intensity of each fragment.
The line corresponding to the heaviest ion is known as the molecular ion peak and it is always produced by the parent molecular ion. In this case, the heaviest ion has a mass-to-charge ratio of 44, which corresponds to the molar mass of propane.
The other small lines in the graph are due to the presence of smaller fragments in the molecule, such as methyl group or ethyl group.
The tallest line in the graph is known as the base peak and it represents the most abundant fragment ion that can be produced in many ways or maybe it is a very stable fragment ion. In the case of propane, the base peak is at 29 which corresponds to the ethyl cation. Thus, the ethyl cation would be produced frequently.
Thus, this is how we can read the mass spectroscopy graph and if we know the proposed structure of a compound, we can find out its actual structure or its isomers and their mass-to-charge ratio.

Note: The mass spectroscopy of an element is different than that of an organic compound. In the case of the element, each line on the mass spectroscopy graph represents a different isotope of an element and not a molecular ion.