Topic 2 · page 5 of 15

Molecules and 2+ ions

For a molecule, the peak with the largest m/z is the molecular ion, and its m/z is the relative molecular mass. Ions with a 2+ charge give extra peaks at half the mass.

13 min read2.6(iii)2.6(iv)

Read first: The mass spectrometer, Relative atomic mass from mass spectra

The molecular ion and relative molecular mass 2.6(iii)

A mass spectrometer also works on molecules. The electron beam knocks one electron off a whole molecule, M, to make a molecular ion:

M(g)+eX−→MX+(g)+2 eX− \ce{M(g) + e- -> M+(g) + 2e-}

The molecular ion has a 1+ charge, so its m/z equals the relative molecular mass of the molecule. The peak for the molecular ion is written M⁺.

That is the largest m/z, not the tallest peak. The impact of the electron beam is violent enough to break many molecules into smaller pieces, called fragments. Fragment ions are lighter than the molecule, so they appear at smaller m/z, and they are often more abundant.

Mass spectrum of an unknown liquid X with peaks at m/z 27, 29, 31, 45 and 46. The tallest peak is at 31 but the molecular ion peak is the one with the largest m/z, at 46.
A schematic spectrum of an unknown liquid. The molecular ion is the last peak on the right.

Worked exampleIdentify an unknown liquid

Liquid X is methanol, ethanol or propan-1-ol. Its spectrum is above.

  1. Work out the candidates' MrM_\text{r} values: CH₃OH = 12.0 + 4(1.0) + 16.0 = 32.0; C₂H₅OH = 24.0 + 6(1.0) + 16.0 = 46.0; C₃H₇OH = 36.0 + 8(1.0) + 16.0 = 60.0.
  2. The highest m/z is 46, so the molecular ion is at 46 and MrM_\text{r} = 46.
  3. X is ethanol, C₂H₅OH.

The same idea identifies molecules in a sample. Each molecule in a mixture gives its own molecular ion peak.

Worked exampleTwo gases in a mixture

A gas sample contains two of CH₄, NH₃, CO₂, SO₂ and Cl₂. It gives molecular ion peaks at m/z 16 and m/z 64.

  1. Work out the MrM_\text{r} values: CH₄ = 16.0, NH₃ = 17.0, CO₂ = 44.0, SO₂ = 64.1.
  2. Chlorine is ruled out because Cl₂ would show peaks at 70, 72 and 74 (see Predicting spectra), and none are given.
  3. m/z 16 matches CH₄ and m/z 64 matches SO₂. The sample contains CH₄ and SO₂.

Ions with a 2+ charge 2.6(iv)

Sometimes the electron beam removes two electrons from one particle:

Mg(g)+eX−→MgX2+(g)+3 eX− \ce{Mg(g) + e- -> Mg^2+(g) + 3e-}

A 2+ ion has twice the charge, so its m/z is half the mass:

mz=mass number2 \frac{m}{z} = \frac{\text{mass number}}{2}

So ²⁴Mg²⁺ gives a peak at m/z 12, and ²⁵Mg²⁺ gives one at m/z 12.5. The peak at 12.5 shows there is a 2+ ion, because no isotope has a mass number of 12.5.

Mass spectrum of magnesium with its 1+ peaks at m/z 24, 25 and 26, and three small peaks at m/z 12, 12.5 and 13 from the 2+ ions of the same isotopes.
Magnesium with its 2+ peaks (illustrative heights). Each 2+ peak is at half the m/z of its 1+ peak.

The 2+ peaks are small. Removing a second electron needs much more energy than removing the first, so only a few particles lose two. You will meet the reason in Ionisation energy.

Worked exampleNaming the ions in a spectrum

An element gives peaks at m/z 20 and 22, and smaller peaks at m/z 10 and 11. Name each ion.

  1. m/z 20 and 22 are 1+ ions of mass 20 and 22: ²⁰Ne⁺ and ²²Ne⁺ (the element is neon).
  2. m/z 10 is half of 20, and m/z 11 is half of 22. These are 2+ ions: ²⁰Ne²⁺ and ²²Ne²⁺.

Check yourself

  1. The molecular ion peak of an alkane with the general formula CnH2n+2\text{C}_n\text{H}_{2n+2} is at m/z 58. Which alkane is it?
  2. What is the m/z of the molecular ion of ethanoic acid, CH₃COOH?
  3. A sample contains two of CH₄, NH₃, CO₂, SO₂ and Cl₂ and gives molecular ion peaks at m/z 17 and 44. Name the two gases.
  4. Why is the tallest peak in a mass spectrum of a molecule not always the molecular ion?
  5. Give the m/z of the 2+ ions ⁴⁰Ar²⁺ and ⁶⁴Zn²⁺.
  6. Explain why a spectrum of magnesium has a small peak at m/z 12.5.

Answers cover these until you have tried the questions

  1. Each CH₂ unit has a mass of 14.0 and the two end hydrogens add 2.0, so 14n + 2 = 58 and n = 4. The alkane is butane, C₄H₁₀ (MrM_\text{r} = 4(12.0) + 10(1.0) = 58.0).
  2. MrM_\text{r} = 2(12.0) + 4(1.0) + 2(16.0) = 60.0, so the peak is at m/z 60.
  3. MrM_\text{r} values: NH₃ = 17.0 and CO₂ = 44.0. The gases are ammonia and carbon dioxide.
  4. Molecules break into fragments when they are ionised. Fragment ions can be more abundant than the molecular ion, so they can make taller peaks. The molecular ion is the peak with the largest m/z.
  5. ⁴⁰Ar²⁺: 40 ÷ 2 = 20. ⁶⁴Zn²⁺: 64 ÷ 2 = 32.
  6. It comes from ²⁵Mg²⁺: mass number 25, charge 2+, so m/z = 25 ÷ 2 = 12.5. It is small because few particles lose two electrons.