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Lecture 3

CHEM 2OA3 Lecture 3: Podcast 3 Mass Spectrometry

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Paul Harrison

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Introduction to Mass Spectrometry
- Gives molecular weight of the compound, but does not give the functional
- In solving chemical structures, we can almost never rely on one technique
- Used to determine molar mass and formula for a compound.
A compound is vaporized and then ionized. The masses of the ions are detected and
- We make molecules charged by taking away electrons, adding electrons, or
adding a proton to it. This makes the molecule unstable. If all bonding MOs
are full, added e will move to antibonding orbital, making it less stable.
- The most common method of ionizing molecules is by electron impact, in
which the sample is bombarded with a with of high energy electrons, ejecting
an e from the molecule.
- Radical cation: uneven number of electrons, the species we make my ionizing
a molecule. Mass is essentially the same as the original molecule.
- If the radical cation remains intact, it is known as the parent ion or molecular
ion. It undergoes fragmentation because there is a weak C-C bond with only
one electron holding the carbons together.
If radical cation breaks apart, this results in a radical with neutral charge and a
carbocation with a positive charge.
- The ions are created by electron impact, accelerated towards a negatively
charged plate, and then pass through a magnetic field where they will be bent
over to one side and arrive at a detector. Positively charged ions go one way,
negatively charged ions go a different way.
- Smaller mass and higher charge fragments are affected more by a magnetic
field. The fragments are sorted according to mass.
Mass spectrum: the number of fragments arriving at the detector as a function of
- X axis: mass to charge ratio (but charge stays constant for us)
- Y axis: relative abundance, number of ions with that particular mass
- The smaller peaks are fragment charges, created by parent ion fragmenting
losing (’s, breaking up of radical cation
- If the fragmentation of the radical cation is too great, the radical peak might
not show up on the graph, so we must use softer ionization techniques.
- To determine molecular mass, we must look to the peak that is the furthest to
the right, not necessarily the highest peak.
Mass spectrum of methane
- 15: one of the CH bonds only has one e holding it together. If this bond is
broken, the molecule will only weigh 15.
- Subsequent H radicals can be fragmented to give the ions with a mass/charge
of 12 (lost all H, has a positive charge), 13, 14
- 17: result of isotopes. 1% of all C are naturally occurring isotope C-13.
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