A mass spectrometer consists of three components: an ion source, a mass analyzer, and a detector.
The ionizer converts a portion of the sample into ions. There is a wide variety of ionization techniques, depending on the phase (solid, liquid, gas) of the sample and the efficiency of various ionization mechanisms for the unknown species. An extraction system removes ions from the sample, which are then targeted through the mass analyzer and into the detector. The differences in masses of the fragments allows the mass analyzer to sort the ions by their mass-to-charge ratio. The detector measures the value of an indicator quantity and thus provides data for calculating the abundances of each ion present. Some detectors also give spatial information, e.g., a multichannel plate.
A mass spectrum is a plot of the ion signal as a function of the mass-to-charge ratio. These spectra are used to determine the elemental or isotopic signature of a sample, the masses of particles and of molecules, and to elucidate the chemical identity or structure of molecules and other chemical compounds.
In a typical MS procedure, a sample, which may be solid, liquid, or gaseous, is ionized, for example by bombarding it with electrons. This may cause some of the sample's molecules to break into charged fragments or simply become charged without fragmenting. These ions are then separated according to their mass-to-charge ratio, for example by accelerating them and subjecting them to an electric or magnetic field: ions of the same mass-to-charge ratio will undergo the same amount of deflection.[1] The ions are detected by a mechanism capable of detecting charged particles, such as an electron multiplier. Results are displayed as spectra of the signal intensity of detected ions as a function of the mass-to-charge ratio. The atoms or molecules in the sample can be identified by correlating known masses (e.g. an entire molecule) to the identified masses or through a characteristic fragmentation pattern.
Fragmentation is a type of chemical dissociation, in which removal of the electron from molecule result in ionization. Removal of electrons from either sigma bond, pi bond or nonbonding orbitals causes the ionization.[2]That can take place by a process of homolytic cleavage/ homolysis or heterolytic cleavage/ heterolysis of the bond. Relative bond energy and the ability to undergo favorable cyclic transition states affect the fragmentation process. Rules for the basic fragmentation processes are given by Stevenson’s Rule.
.png){kind=small}
.png)
Two major categories of bond cleavage patterns are simple bond cleavage reactions and rearrangement reactions.[2]
Majority of organic compounds undergo simple bond cleavage reactions, in which, direct cleavage of bond take place. Sigma bond cleavage, radical site-initiated fragmentation, and charge site-initiated fragmentation are few types of simple bond cleavage reactions.[2]
Sigma bond cleavage is most commonly observed in molecules, which can produce stable cations such as saturated alkanes, secondary and tertiary carbocations. This occurs when an alpha electron is removed. The C-C bond elongates and weakens causing fragmentation. Fragmentation at this site produces a charged and a neutral fragment.[2]
Sigma bond cleavage also occurs on radical cations remote from the site of ionization. This is commonly observed in alcohols, ethers, ketones, esters, amines, alkenes and aromatic compounds with a carbon attached to ring. The cation has a radical on a heteroatom or an unsaturated functional group. The driving force of fragmentation is the strong tendency of the radical ion for electron pairing. Cleavage occurs when the radical and an odd electron from the bonds adjacent to the radical migrate to form a bond between the alpha carbon and either the heteroatom or the unsaturated functional group. The sigma bond breaks; hence this cleavage is also known as homolytic bond cleavage or α-cleavage.[2]
The driving force of charge site-initiated fragmentation is the inductive effect of the charge site in radical cations. The electrons from the bond adjacent to the charged-bearing atom migrate to that atom, neutralizing the original charge and causing it to move to a different site. This term is also called inductive cleavage and is an example of heterolytic bond cleavage.[2]
Rearrangement reactions are fragmentation reactions that form new bonds producing an intermediate structure before cleavage. One of the most studied rearrangement reaction is the McLafferty rearrangement / γ-hydrogen rearrangement. This occurs in the radical cations with unsaturated functional groups, like ketones, aldehydes, carboxylic acids, esters, amides, olefins, phenylalkanes. During this reaction, γ-hydrogen will transfer to the functional group at first and then subsequent α, β-bond cleavage of the intermediate will take place. [2] Other rearrangement reactions include heterocyclic ring fission (HRF), benzofuran forming fission (BFF), quinone methide (QM) fission or Retro Diels-Alder(RDA).[6]
Comments
Post a Comment