The Major Modes of Various Fragmentations

Mass spectrometry provides tools and methods to determine the ratio between the total molecular weight of a mixture and the charge in the molecules in the mixture. Various fragmentation modes in mass spectrometry achieve the mass-to-charge ratio of molecules. Modes of fragmentation of various compounds can be determined by following rules for fragmentation of molecules. This article explores the modes of fragmentation in mass spectrometry and addresses questions like what is mode of fragmentation and its factors.

Mass Spectrometry

In a chemical solution, mass spectrometry is defined as a tool used to determine the ratio between molecular mass and the charge carried by the molecules of the solution. The determined ratio is called the mass-to-charge ratio of the solution. 

Generally, mass spectrometry tools are used to determine the molecular weight of a compound or a chemical mixture. However, these tools can also identify and determine unknown molecules/compounds in a chemical mixture. These tools also help understand the various physical and chemical properties of different chemical mixtures.

Tools of mass spectrometry consist of three components:

• Ionisation source
• Mass analyser
• Ion detection system

Ionisation source

• Molecules of a chemical mixture are converted into gas ions.
• This allows the ions to move about and be manipulated by an external electric charge.
• This method results in molecules of chemical mixtures being separated into positively or negatively charged ions.

Mass analyser

• Based on their mass-to-charge ratios, the ionised molecules are separated.
• A different variety of mass analysers are used, depending on the requirements of the mixture.

Ion detection system

• The ion detection system gives a mass spectrum for the components of the chemical mixture.
• A mass spectrum is a collection of mass-to-charge (m/z) ratios with respect to its intensities.

Fragmentations

In general terms, fragmentation in chemistry can be defined as the formation of ions from a molecule in a mixture.

When a molecule is added to a mass spectrometer, it undergoes a bombardment of electrons; these electrons are composed of high energy and can knock out one electron from the molecules. This process results in the formation of positive ions; the ion that is ejected from the molecule is called the molecular ion or parent ion.

The parent ions are unstable, and some of these parent ions break down into smaller pieces. Generally, a parent ion breaks down into two parts, forming one positive ion and a neutral component.

Upon plotting a mass spectrum for the parent ion, only the positive charge is visible on the spectrum and not the neutral component after the fragmentation of the ion.

Modes of fragmentations

Let us now understand what is the mode of fragmentation?

Modes of fragmentation in mass spectrometry define the process through which a molecule in a mixture produces ions.

Mass spectrometry tools provide the procedure for fragmentation of molecules into ions; this usually results in the formation of an ion and an uncharged component.

The following three types give modes of fragmentation in mass spectrometry:

• Cleavage of σ bond
• Cleavage of 2 σ bonds
• Cleavage of complex rearrangements 

Rules of fragmentations

Modes of fragmentation in mass spectrometry have to follow specific rules, some of which are mentioned below.

• For the mode of fragmentation of various compounds, the relative height of molecular ions is greatest for the straight chain-forming compounds.
• The relative height of the molecular ion spectrum appears to be decreasing in the compounds with homologous chain structures.
• Cleavage depends on the substituted carbon atoms in alkyl compounds, which means more substitution results in more cleavage.
• The parent ion or the molecular ion is stabilised by the presence of factors like double bonds, cyclic structures, and aromatic rings.
• Saturation rings of a chemical compound are likely to lose the alkyl side chain at the α bond. However, this rule does not apply to all compounds.
• Cleavage is formed at the β bond for alkyl-substituted aromatic compounds. This process gives the ions of benzene.
• The carbon-carbon bond, or simply the C-C bond, frequently forms a cleavage. The non-bonding electron provides stabilisation to the molecule. 
• Cleavage is generally attributed to the removal of a neutral component of the molecule.

Conclusion

Mass spectrometry is a method to define the ratio of molecular weight to the total charge in a molecule. Mass spectrometry also gives tools for determining the m/z ratios and modes of fragmentation of various compounds.

The ion formation in a molecule is achieved by the process of fragmentation. Mass spectrometry tools provide the procedure for fragmentation of molecules into ions; this usually results in the formation of an ion and an uncharged component. The above article defines what is mode of fragmentation and its rules for various compounds.