Mass spectrometry is based on slightly different principles to
the other spectroscopic methods.The physics behind mass spectrometry is
that a charged particle passing through a magnetic field is deflected
along a circular path on a radius that is proportional to the mass to
charge ratio, m/e. In an electron impact mass spectrometer, a high energy beam of electrons is used to displace an electron from the organic molecule to form a radical cation known as the molecular ion. If the molecular ion is too unstable then it can fragment to give other smaller ions. The collection of ions is then focused into a beam and accelerated into the magnetic field and deflected along circular paths according to the masses of the ions. By adjusting the magnetic field, the ions can be focused on the detector and recorded. |
- Probably the most useful information you should be able to obtain from a MS spectrum is the molecular weight of the sample.
- This will often be the heaviest ion observed from the sample provided this ion is stable enough to be observed.
Molecular ion | The ion obtained by the loss of an electron from the molecule |
Base peak | The most intense peak in the MS, assigned 100% intensity |
M+ | Symbol often given to the molecular ion |
Radical cation | +ve charged species with an odd number of electrons |
Fragment ions | Lighter cations formed by the decomposition of the molecular ion. These often correspond to stable carbcations. |
The MS of a typical hydrocarbon, n-decane is shown below. The molecular ion is
seen as a small peak at m/z = 142. Notice the series ions detected
that correspond to fragments that differ by 14 mass units, formed by the
cleaving of bonds at successive -CH2- units
The MS of benzyl alcohol is shown below. The molecular ion is
seen at m/z = 108. Fragmentation via loss of 17 (-OH) gives a common
fragment seen for alkyl benzenes at m/z = 91. Loss of 31 (-CH2OH)
from the molecular ion gives 77 corresponding to the phenyl cation.
Note the small peaks at 109 and 110 which correspond to the presence of
small amounts of 13C in the sample (which has about 1% natural
abundance).
- Mass spectrometers are capable of separating and detecting individual ions even those that only differ by a single atomic mass unit.
- As a result molecules containing different isotopes can be distinguished.
- This is most apparent when atoms such as bromine or chlorine are present (79Br : 81Br, intensity 1:1 and 35Cl : 37Cl, intensity 3:1) where peaks at "M" and "M+2" are obtained.
- The intensity ratios in the isotope patterns are due to the natural abundance of the isotopes.
- "M+1" peaks are seen due the the presence of 13C in the sample.
The first MS is of 2-chloropropane. Note the isotope pattern at 78 and 80 that represent the M amd M+2 in a 3:1 ratio.
Loss of 35Cl from 78 or 37Cl from 80 gives the base peak a m/z = 43, corresponding to the secondary propyl cation. Note that the peaks at m/z = 63 and 65 still contain Cl and therefore also show the 3:1 isotope pattern.
Loss of 35Cl from 78 or 37Cl from 80 gives the base peak a m/z = 43, corresponding to the secondary propyl cation. Note that the peaks at m/z = 63 and 65 still contain Cl and therefore also show the 3:1 isotope pattern.
The
second MS is of 1-bromopropane. Note the isotope pattern at 122 and 124
that represent the M amd M+2 in a 1:1 ratio. Loss of 79Br from 122 or 81Br
from 124 gives the base peak a m/z = 43, corresponding to the propyl
cation. Note that other peaks, such as those at m/z = 107 and 109 still
contain Br and therefore also show the 1:1 isotope pattern.
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