How do you interpret peaks in a mass spectrum?

Peaks in a mass spectrum represent different isotopes or fragments of the molecule being analysed.

In mass spectrometry, a sample is ionised to form positive ions, which are then accelerated and deflected by magnetic fields. The extent of deflection depends on the mass-to-charge ratio (m/z) of the ions. The ions are detected and a mass spectrum is produced. This spectrum is a graph with the relative abundance of ions (usually represented as a percentage) on the y-axis and the m/z ratio on the x-axis.

Each peak in the mass spectrum corresponds to an ion with a specific m/z ratio. The position of the peak on the x-axis gives the m/z ratio, which is usually equal to the relative atomic mass for ions with a single positive charge. The height of the peak indicates the relative abundance of that ion.

The highest m/z peak, known as the molecular ion peak or parent peak, represents the whole molecule and gives the molecular mass of the compound. This peak is often accompanied by a smaller peak at m/z+1, due to the presence of the isotope carbon-13 in organic molecules.

Other peaks in the mass spectrum are due to fragment ions, which are formed when the molecular ion breaks up. These peaks can provide valuable information about the structure of the molecule. For example, the presence of a peak at m/z 15 could suggest the presence of a methyl group (CH3) in the molecule, as the m/z ratio of a methyl cation is 15.

In summary, interpreting peaks in a mass spectrum involves identifying the molecular ion peak to determine the molecular mass, and analysing the fragment ion peaks to gain insights into the molecular structure. This process requires a good understanding of the principles of mass spectrometry and the properties of different atoms and groups of atoms.