Topic 10 · page 25 of 27

Mass spectra

The peak at the highest m/z in a mass spectrum is the molecular ion, and its m/z is the relative molecular mass. The smaller peaks are fragments, and the way a molecule breaks up helps to show its structure.

14 min read10.21

The molecular ion 10.21

In a mass spectrometer a molecule, M, has an electron knocked off. This gives a positive ion that is also a radical, the molecular ion:

M→MX+ ∙ +eX− \ce{M -> M^{+.} + e-}

The spectrum plots abundance (height) against m/z, the mass-to-charge ratio. For a singly charged ion the m/z equals the mass of the ion.

A mass spectrum of ethanol drawn as vertical lines. m/z 46 is labelled M plus, the molecular ion, at about 40 percent. A line at 45 is about 30 percent. The tallest line, the base peak at 100 percent, is at 31. Lines at 29 and 15 are about 32 and 10 percent. The horizontal axis runs from 0 to 50.
A schematic mass spectrum of ethanol, C₂H₅OH, M_r = 46. The tallest line is the base peak at m/z 31. The molecular ion is at 46.

The tallest peak is the base peak. It is the most abundant ion, which is often a fragment and not the molecular ion. The molecular ion is not always the tallest.

Worked exampleMolecular formula from the molecular ion

A compound containing only C, H and O has a molecular ion at m/z 74. Show that two different formulae fit.

  1. MrM_\text{r} of C₄H₁₀O = 4(12.0) + 10(1.0) + 16.0 = 74.0.
  2. MrM_\text{r} of C₃H₆O₂ = 3(12.0) + 6(1.0) + 2(16.0) = 74.0.
  3. Both fit, so the molecular ion alone does not give the formula. Fragments, the infrared spectrum or other data are needed to choose.

Fragmentation 10.21

The molecular ion is unstable and many of the ions break apart. A bond breaks, and the ion splits into a positive fragment ion and a neutral radical:

MX+ ∙ →XX++YX ∙  \ce{M^{+.} -> X+ + Y^.}

Only the positive ion, X⁺, is detected. The radical is not charged, so it does not appear in the spectrum. The peak for each fragment shows its mass.

The difference between the molecular ion and a fragment shows what was lost:

Mass lost from M⁺ Neutral species lost
15 CH₃
17 OH
18 H₂O
29 C₂H₅ or CHO

Learn the common fragment ions:

m/z Fragment ion
15 CH₃⁺
29 C₂H₅⁺ or CHO⁺
31 CH₂OH⁺
43 CH₃CO⁺ or C₃H₇⁺
45 COOH+\text{COOH}^{+} or CH₃CHOH⁺
57 C₄H₉⁺ or C₂H₅CO⁺

The fragment masses are sums of atomic masses. For CH₂OH⁺: 12 + 2(1) + 16 + 1 = 31. For CH₃CO⁺: 12 + 3(1) + 12 + 16 = 43.

Same molecular ion, different fragments 10.21

Propanal, CH₃CH₂CHO, and propanone, CH₃COCH₃, are isomers. Both have MrM_\text{r} = 58.0, so the molecular ion at 58 cannot tell them apart. The fragments can.

Two schematic mass spectra side by side, each from 0 to 70. Propanal: molecular ion at 58 of about 62 percent, a line at 57 of about 40 percent, and a tall base peak at 29. Propanone: molecular ion at 58 of about 48 percent, a base peak at 43 of 100 percent, and a line at 15 of about 24 percent.
Schematic spectra of two isomers of C₃H₆O. Propanal has its base peak at m/z 29, propanone at m/z 43.
Propanal Propanone
Molecular ion 58 58
Key fragments 29 (CH₃CH₂⁺ or CHO⁺), 57 43 (CH₃CO⁺), 15 (CH₃⁺)
Why breaking either C–C bond next to the C=O gives CH₃CH₂⁺ or CHO⁺, both at m/z 29 breaking the C–C bond next to the C=O leaves CH₃CO⁺ and a CH₃ radical

Check yourself

  1. State what the peak at the highest m/z in a mass spectrum shows.
  2. Explain why only some of the fragments appear as peaks.
  3. A compound has a molecular ion at m/z 60 and fragments at 45 and 43. Suggest the fragment ions for 45 and 43.
  4. Calculate the m/z of CH₃CH₂⁺ using ArA_\text{r} values.
  5. Propanal and propanone both have a molecular ion at m/z 58. Explain how their spectra differ.
  6. Calculate the MrM_\text{r} of C₃H₆O and say what you could not tell from the molecular ion alone.

Answers cover these until you have tried the questions

  1. The molecular ion: its m/z is the relative molecular mass of the compound.
  2. Only positive ions are detected. In each break the molecular ion gives a positive ion and a neutral radical, and the radical is not seen.
  3. 45: COOH+\text{COOH}^{+}. 43: CH₃CO⁺. (The compound is ethanoic acid, CH₃COOH.)
  4. 2(12.0) + 5(1.0) = 29.0, so the ion is at m/z 29.
  5. Propanal has its base peak at 29 (and a peak at 57). Propanone has its base peak at 43 (CH₃CO⁺) and a peak at 15 (CH₃⁺).
  6. 3(12.0) + 6(1.0) + 16.0 = 58.0. The molecular ion cannot say whether the compound is an aldehyde (propanal) or a ketone (propanone), or another isomer such as prop-2-en-1-ol.