Mass spectrometry for structure determinationEdexcel A-Level Chemistry: Revision notes
Section 1
The molecular ion peak
In a mass spectrometer a molecule loses one electron to form a positive molecular ion, M⁺. The peak at the highest m/z (apart from small isotope peaks) is the molecular ion peak and its m/z value equals the relative molecular mass of the compound.
A low-resolution spectrometer gives m/z to the nearest whole number (the nominal mass). A high-resolution spectrometer measures m/z to four decimal places, which is far more precise.
Section 2
Why accurate masses differ
On the scale where ¹²C is exactly 12.0000, the masses of other isotopes are not whole numbers:
¹H = 1.0078; ¹⁴N = 14.0031; ¹⁶O = 15.9949
So two different molecular formulae with the same nominal mass add up to slightly different accurate masses. For example, nitrogen N₂ (28.0062), carbon monoxide CO (27.9949) and ethene C₂H₄ (28.0312) all have nominal mass 28 but are easily distinguished by a high-resolution spectrometer.
Do not explain the differences by electron loss or by different isotopes of carbon. The cause is that isotope masses are not whole numbers.
Section 3
Calculating an accurate relative molecular mass
Add the accurate atomic masses for every atom in the formula, using the data given. Show the working and keep four decimal places.
Worked example: C₃H₆O 3 × 12.0000 = 36.0000 6 × 1.0078 = 6.0468 1 × 15.9949 = 15.9949 Total = 58.0417
Other examples: C₂H₆O = 46.0417; CH₂O₂ = 46.0054; NO₂ = 45.9929.
Multiply first, add after, and check that your total starts with the nominal mass (58.04 for C₃H₆O).
Section 4
Identifying a compound from its accurate mass
- List the candidate formulae that have the correct nominal mass.
- Calculate the accurate Mᵣ of each to four decimal places.
- Choose the formula that matches the measured molecular ion peak.
Example: a peak at 58.0417 could be C₂H₂O₂ (58.0054), C₂H₆N₂ (58.0530), C₃H₆O (58.0417) or C₄H₁₀ (58.0780). Only C₃H₆O matches, so the others are rejected.
A peak at 46.0054 matches CH₂O₂ (methanoic acid), not C₂H₆O (46.0417) or NO₂ (45.9929).
Section 5
Suggesting possible structures
The accurate mass gives the molecular formula only. Once the formula is known, you can suggest possible structures, using the groups the formula allows. For C₃H₆O, possible structures are propanal, propanone or prop-2-en-1-ol.
Isomers have identical accurate masses, so accurate Mᵣ cannot tell them apart. Fragment ions in the spectrum, and other evidence such as infrared and NMR spectra, are used to decide between them. Accurate mass is a first step: it fixes the formula and removes impossible ones.
Do not claim that an accurate mass identifies a structure; it identifies a formula. Isomers share the same accurate mass.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Mass spectrometry for structure determination
- Carbon monoxide (CO), nitrogen (N₂) and ethene (C₂H₄) all have a nominal relative molecular mass of 28. An environmental chemist uses a high-resolution mass spectrometer to analyse a sample of exhaust gas. Accurate relative atomic masses: ¹H = 1.0078, ¹²C = 12.0000, ¹⁴N = 14.0031, ¹⁶O = 15.9949.A second sample gives a molecular ion peak at m/z 28.0312. Show by calculation which of the three gases it contains.2 marks
- A compound gives a molecular ion peak at m/z 58.0417 in a high-resolution mass spectrum. Four possible molecular formulae with a nominal relative molecular mass of 58 are C₂H₂O₂, C₂H₆N₂, C₃H₆O and C₄H₁₀. Accurate relative atomic masses: ¹H = 1.0078, ¹²C = 12.0000, ¹⁴N = 14.0031, ¹⁶O = 15.9949.Show by calculation that the accurate relative molecular mass of C₃H₆O matches the measured peak, and state why a low-resolution mass spectrum could not distinguish C₃H₆O from C₄H₁₀.2 marks
- A gas sample is known to be ethanol vapour (C₂H₆O), methanoic acid vapour (CH₂O₂) or nitrogen dioxide (NO₂). All three have a nominal relative molecular mass of 46. A high-resolution mass spectrum of the sample shows a molecular ion peak at m/z 46.0054. Accurate relative atomic masses: ¹H = 1.0078, ¹²C = 12.0000, ¹⁴N = 14.0031, ¹⁶O = 15.9949.Calculate the accurate relative molecular mass of each of the three possible compounds to four decimal places, and use your values to identify the compound in the sample.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).