Spectroscopy Revision

Mass Spectrometry

Mass                                

                                             Spectroscopy is used to find out Relative Molecular Mass (Mr)
Electrons in the spectrometer bombarded the sample molecules and break electrons off, forming ions. A mass spectrum is produced, showing the relative amounts of ions with different mass-to-charge ratios.
The molecular ions M+ gives the peak in the spectrum with the second highest mass-to-charge ratio, so it’s the last peak but one on the spectrum.  This peak’s called the M peak. The M peak is the relative molecular mass if the molecule.
The Highest peak is called the base peak. Its relative abundance is 100%. It’s due to particularly stable fragment ions such as carbocations.
A spectrogram has a y axis which gives the abundance of ions, often as a percentage. The x-axis units are given as a ‘mass/charge’ ratio.

                              We can also use the M+1 Peak to work out the Number of Carbon Atoms in molecule.
The peak 1 unit to the right of the M peak is called the M+ 1 peak. It’s mostly due to the carbon-13 isotope which exists naturally. The equation to work out the number of Carbon atoms in a compound  is:         
      Height of M+1 peak
     ------------------------    x100
      Height of M peak 
 
For example: If a molecule has a molecular peak with relative abundance of 32.17% and a M+1 peak with a relative abundance of 2.34%, how many carbon atom does it have?

Answer : (2.34/32.17) x100 = 7.27 Therefore the molecule contains 7 Carbon atoms.

The Mass Spectrum of Ethanol is shown below.