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The heaviest significant peak in a mass spectrum is the molecular ion and gives the molecular mass; fragment peaks reveal the neutral pieces lost, the base peak is only the most abundant ion, and chlorine and bromine show M+2 peaks in 3 : 1 and 1 : 1 ratios.
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You will find an alkane's molecular ion, work out the mass a fragment lost, name common losses, identify a halogen from its M+2 pattern, and tell the base peak from the molecular ion.
Infrared tells you which bonds a molecule has but not how heavy it is. A mass spectrum supplies the mass, and its fragments say something about how the molecule is put together.
The molecular ion, M+, is the molecule with one electron removed; its m/z (mass over charge, here just the mass) is the molecular mass. Fragment ions are pieces broken off. The base peak is the tallest peak. The M+2 peak sits two units above M, from heavier isotopes.
In the spectrometer, each molecule loses an electron to become M+, and many M+ ions then break into a charged fragment and a neutral piece that goes unseen.
Common losses to know: 15 CH3, 17 OH, 18 H2O, 29 C2H5 or CHO, 43 C3H7 or CH3CO.
The base peak is simply the most abundant ion, often a stable fragment, and is not necessarily the molecular ion.
Another way: steps
To read a mass spectrum:
Most elements have one dominant isotope, but chlorine and bromine do not. Chlorine is about 75% 35Cl and 25% 37Cl, so a molecule with one chlorine shows M and M+2 in a ratio of about 3 : 1. Bromine is about 50% 79Br and 50% 81Br, so M and M+2 are nearly equal. Chloroethane shows peaks at 64 and 66, the second a third the height of the first; bromoethane shows 108 and 110 of equal height. The pattern alone tells you a halogen is present and which.
The tallest mass-spectral peak is always the molecular mass. It is often a fragment.
Fragments show both pieces. Only the charged piece is detected.
An M+2 peak means the formula has two extra hydrogens. It comes from a heavier isotope.
Every molecule's M+2 is tall. Only chlorine and bromine give a large one.
Heaviest significant peak at m/z 46: the molecular ion, C2H6O.
The molecular mass.
A strong peak at m/z 31: 46 − 31 = 15, a lost CH3, leaving CH2OH+.
Fragment and loss.
A peak at 45: loss of 1, an H atom.
Small losses count too.
What does the 1 : 1 pair mean?
One bromine atom.
Which peak is the molecular ion with 79Br?
m/z 122; the 124 peak carries 81Br.
At what m/z does the molecular ion of ethane, an unbranched alkane with $2$ carbons, appear? (C = 12, H = 1.)
Answer:
The mass spectrum of ethanol, CH3CH2OH shows its molecular ion at m/z $46$ and a strong fragment at m/z $31$. What mass was lost?
Answer:
Match each mass lost from a molecular ion to the neutral piece it most often means.
| CH3 | H2O | C2H5 or CHO | OH | |
|---|---|---|---|---|
| a loss of 15 | ||||
| a loss of 18 | ||||
| a loss of 29 | ||||
| a loss of 17 |
A mass spectrum shows a molecular ion peak M and a second peak at M+2, in the ratio about 3 : 1. Which halogen does the molecule contain?
The mass spectrum of 2-methylpropane, (CH3)3CH, molecular mass 58, has its tallest peak at m/z 43 and a small peak at m/z 58. A student concludes the molecular mass is 43. What is wrong?
Lesson test: one question per skill, one attempt each, no hints. Your answers are checked when you submit.
In the mass spectrum of chloroethane, CH3CH2Cl, the molecular ion peak at m/z 64 has a height of $57$ units. About how tall is the peak at m/z 66?
The M+2 peak is about a units tall.
You can read a molecular mass and fragments off a mass spectrum. Tell someone why the tallest peak is not always the molecular ion. Next: matching peaks of every kind to the structure they support.
8. Your turn: peaks at m/z 122 and 124, nearly equal, from a compound C3H7Br, step 3