Spectroscopy Topic 6 Flashcards
What does mass spectrometry measures?
The mass-to-charge (m/z) ratio of gas-phase ions.
What are four basic regions in mass spectroscopy?
Sample inlet, ion source, mass analyser and detector
What happens in the sample inlet?
The sample is introduced into the instrument
What happens in the ion source?
The sample molecules are ionised
What happens in the mass analyser?
It measures the mass to charge (m/z) ratio* of the ions produced
What happens in detector?
It detects the ions.
What happens in electron ionisation of mass spectroscopy?
The sample is introduced into the high vacuum ion source of the mass spectrometer in vapour form. Molecules pass through a beam of high energy (70 eV) electrons, which transfer their kinetic energy into internal energy of the molecules.
If the internal energy of the excited molecules exceeds their ionisation energy, an electron is ejected producing a molecular ion M+. which often has excess energy and can fragment.
Ions are accelerated out of the ion source by a high voltage and passed into the mass analyser, which measures their m/z vlaues.
What is a mass spectrum?
A plot of m/z on the horizontal axis and ion intensity on the vertical axis
What is the forst step in the interpretation of MS?
The first step in interpretation is working out the most likely charge site, i.e. where the +. is found in the ion.
Since the highest energy electrons are most likely to be removed, this means that a non-bonding (lone pair) electron is preferentially lost during ionisation, and hence the charge will be located on an O, N, S, halogen etc. This is followed in preference by a π-bonding electron and finally a σ-bonding electron.
What is heteroatom cleavage?
The first mechanism of fragmentation. It occurs when the bond between the heteroatom X (which carries the charge) and the rest of the molecule (R) breaks heterolytically. This results in X being lost as a radical and R+ being detected in the spectrum.
What is heterolytical breaking?
Is the process of cleaving/breaking a covalent bond where one previously bonded species takes both original bonding electrons from the other species.
What is alpha cleavage?
The second mechanism of fragmentation. It occurs when the bond next but one to the heteroatom X (which carries the charge) breaks homolytically. This results in an R group being lost as a radical and the remainder of the molecule being detected in the spectrum.
Alhpa cleavage can occur at saturated (e.g. in alkyl halides, alcohols, ethers, amines, etc.) and unsaturated (e.g. carbonyl compounds, imines etc.) sites.
What is homolysis?
Homolysis or homolytic fission is chemical bond dissociation of a molecular bond by a process where each of the fragments retains one of the originally bonded electrons. During homolytic fission of a neutral molecule with an even number of electrons, two free radicals will be generated.
What is the third mechanism of fragmentation?
It involves rearrangement of H radicals preceding fragmentation. This can occur at saturated and unsaturated sites.
At saturated sites, this can result in both loss of XH from the precursor ion and production of +.XH as an ion.
At unsaturated sites, charge can remain on the portion of the molecule with X, which is called the McLafferty rearrangement, or with this portion being lost as a neatural.
Describe mass spectroscopy of alkanes.
MS of alkanes is dominated by direct σ bond cleavage during EI. An electron is lost from C-C σ bond and that induces direct cleavage of that σ bond.
This gives a statistical distribution of fragments 14 Da apart.
Describe MS of aromatics.
Alpha cleavage at the benzylic position dominates. This produces a characteristic ion at m/z 91.
Explain what is happening in the MS of the diagram shown.
First electron ionisation has removed an electron to give us a radical cation in the π system. And then we get an alpha cleavage process by which we loose R group. And that gives rise to first product directly. But that can show different resonance products. And it also can expand the ring into a trope hellium ion (last resonance form) by taking CH2 into the ring where it can be delocalised by the π system.
