Mass spectrometry in organic analysisAQA A-Level Chemistry: Revision notes
Section 1
The molecular ion peak
In a mass spectrometer, a sample is ionised and the positive ions are separated according to their mass-to-charge ratio, m/z. For a molecule M, the ion M⁺ formed by losing one electron is the molecular ion.
The molecular ion gives the peak with the highest m/z in the spectrum (ignoring very small peaks due to heavier isotopes such as carbon-13). Its m/z value equals the relative molecular mass, Mᵣ, of the compound. Peaks at lower m/z are fragment ions.
The molecular ion peak is the one with the highest m/z, not necessarily the tallest peak. The tallest peak is called the base peak.
Section 2
Low and high resolution
A low-resolution spectrometer gives m/z to the nearest whole number. Different compounds can have the same whole-number mass. For example, C₄H₁₀O and C₃H₆O₂ both have Mᵣ = 74.
A high-resolution (precise) spectrometer measures the mass to several decimal places. Because the precise masses of these formulae differ, the molecular formula can be found.
Section 3
Why precise masses differ
Only a carbon-12 atom has a mass that is exactly a whole number (12.0000). The precise masses of the other atoms are not whole numbers:
- H = 1.0078
- N = 14.0031
- O = 15.9949
So different combinations of atoms with the same whole-number mass add up to slightly different precise masses. For example, CO = 27.9949, N₂ = 28.0062 and C₂H₄ = 28.0312.
Section 4
Finding a molecular formula
- Use the whole-number mass to list possible formulae.
- Calculate the precise Mᵣ for each, using the precise atomic masses given.
- The formula whose precise mass matches the measured value is the molecular formula.
Worked example: a compound of C, H and O has precise Mᵣ 88.0524.
- C₄H₈O₂ = 4 × 12.0000 + 8 × 1.0078 + 2 × 15.9949 = 88.0522
- C₅H₁₂O = 5 × 12.0000 + 12 × 1.0078 + 15.9949 = 88.0885
The measured value matches C₄H₈O₂.
Show every calculation in full with the numbers substituted, and give the answer to the same number of decimal places as the data.
Section 5
Limitations
A precise mass gives the molecular formula, not the structure. Isomers have the same molecular formula, so they have the same precise Mᵣ. For example, butanal and butanone are both C₄H₈O.
To identify the compound, combine mass spectrometry with other evidence, such as infrared spectra or test-tube reactions.
Must Know
- The molecular ion peak has the highest m/z and gives Mᵣ
- Low resolution gives whole-number masses; high resolution gives precise masses
- Calculate precise Mᵣ for each candidate formula and match to the measured value
- Isomers cannot be separated by precise mass
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Mass spectrometry in organic analysis
- A pure organic compound is analysed by mass spectrometry. In the mass spectrum, the peak with the highest mass-to-charge ratio (ignoring any very small peaks above it) is at m/z = 74.The compound is known to contain only carbon, hydrogen and oxygen. Explain why the m/z value of 74 alone cannot be used to find its molecular formula.2 marks
- High-resolution mass spectrometry measures relative molecular masses to four decimal places. Precise relative atomic masses: H = 1.0078, C = 12.0000, N = 14.0031, O = 15.9949.A compound has a precise relative molecular mass of 46.0054. Calculate the precise relative molecular masses of C₂H₆O and CH₂O₂ and decide which is the molecular formula of the compound.2 marks
- A pharmaceutical analyst finds that a compound containing only carbon, hydrogen and oxygen has a precise relative molecular mass of 88.0524 by high-resolution mass spectrometry. Precise relative atomic masses: H = 1.0078, C = 12.0000, O = 15.9949.The compound has a nominal mass of 88. Calculate the precise relative molecular masses of C₄H₈O₂ and C₅H₁₂O and deduce the molecular formula of the compound.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).