Molecular Ion Formation and Further Fragmentation

In a mass spectrometer, electrons bombard the vaporised organic sample while going through the ionisation chamber, resulting in positive ions. These electrons have enough energy to knock one electron off an organic molecule. The molecular ion is written as M+ or m.. The dot in this second variant denotes the presence of a single unpaired electron within the ion. Molecular ion formation will give you the analyte’s molecular mass. It will be the initial clue in finding the mass spectrum. In this article, we will be talking about molecule ion formation and its importance in mass spectrometry. 

About Molecular Ion Formation

The mass of the most prevalent isotope of each element in the molecule will tell us about the mass of the molecular ion. The atomic mass is then rounded to the nearest whole integer, called nominal mass. It is the nominal mass because many mass spectrometers have a unit mass resolution.

For example, you can say that the CHBr3 molecular ion would be written as m/z 250. It wouldn’t be 253 because of its formula weight. 

The molecular ion would tell us the ion with the enormous mass to charge ratio in many mass spectra. However, the enormous mass to charge an ion could be an impurity or a molecular ion isotope. Also, several compounds fragment so quickly that no molecular ion in mass spectrometry would be visible in the EI spectrum at 70 eV. 

Features of Molecular Ion Formation 

• The m/z value must always match a reasonable molecular formula with sufficient isotope abundance.
• The molecular mass of various compounds is uniform. The “Nitrogen Rule” would be the only exception in the list. 
• As the nitrogen rule states, any compound containing an odd number of nitrogen atoms will have an odd molecular mass. The molecular mass of any compound would always be the same even number of nitrogen atoms (including zero).
• This is because nitrogen is the only common isotope that forms an odd valence with an even mass.
• In CH4, the molecular ion is m/z 16. The molecular ion for NH3 would be m/z 17, and the molecular ion for N2H3 is m/z 32.  
• Here, the peak would represent a molecular ion, and the loss of a probable neutral fragment will match the corresponding most excellent mass fragment. 

What Is Molecular Ion Fragmentation?

Fragmentation means that some molecular ions will break off into smaller bits if there is unstable energy. 

A molecular ion will distance itself into two components. For example, one will be a positive ion, and the other would be an uncharged free radical. An uncharged free radical will never form a line on the mass spectrum. The mass spectrometer will only accelerate, deflect, and detect charged particles. On the other hand, these uncharged particles will get lost in the machine, and the vacuum pump will finally remove them. 

The ion X+ will travel through the mass spectrometer like any other positive ion, leaving only the line on the stick diagram. There are various sorts of fragmentations in the starting molecule ion, which means that the mass spectrum can have a lot of lines. 

It is essential to learn these patterns formed in mass spectrometry to learn about organic compounds. They tell the difference between the pattern lines of an organic compound’s mass spectrum and an element’s mass spectrum. 

Conclusion 

In this article, we also talked about molecular ions in mass spectrometry and their various features. We also looked at molecular ion fragmentation, which is the breaking up of different unstable ions travelling in the ionisation chamber. 

All the reactions of molecular ion formation are recorded to determine the molar weight of different compounds or the structural formation of an unknown molecule.