- Define and explain Mass Spectrometry in your own words
- Use key terms such as molecular ion accurately
- Apply what you have learned to new examples and questions
- Avoid the common mistakes learners make with this topic
This lesson focuses on Mass Spectrometry: find relative masses and molecular formulae from m/z peaks.
Find relative masses and molecular formulae from m/z peaks.
Key ideas
Mass spectrometry weighs molecules
Molecules are ionised, usually by electron impact, and the molecular ion's m/z value gives the relative molecular mass. The molecular ion then fragments, and the pattern of fragment peaks identifies structural features — a peak at m/z 29, for example, suggests a C₂H₅⁺ fragment. Isotope abundances in the spectrum also give relative atomic masses: chlorine's 3:1 ratio of ³⁵Cl to ³⁷Cl produces the famous twin peaks.
Classic tests give fast answers
A glowing splint relights in oxygen, a burning splint pops in hydrogen, and limewater turns milky with carbon dioxide — three gas tests every chemist knows. For ions, flame tests colour the flame (lilac for potassium, brick red for calcium), while adding sodium hydroxide gives coloured precipitates with transition metal ions. These tests are quick, cheap and specific.
Key term — molecular ion: The ion formed when a molecule loses one electron in a mass spectrometer; its m/z gives the relative molecular mass.
A mass spectrum shows a molecular ion at m/z 74. Suggest a molecular formula containing C, H and O only.
C₃H₆O₂: (3 × 12) + (6 × 1) + (2 × 16) = 36 + 6 + 32 = 74 (e.g. propanoic acid or methyl ethanoate).
Answer: C₃H₆O₂: (3 × 12) + (6 × 1) + (2 × 16) = 36 + 6 + 32 = 74 (e.g. propanoic acid or methyl ethanoate).
- Reading the tallest peak as the molecular ion The tallest peak is the base peak, not necessarily the molecular ion. The molecular ion is the peak with the highest m/z (ignoring tiny isotope peaks just above it).
- Confusing the fingerprint region with functional-group peaks Useful functional-group absorptions appear above 1500 cm⁻¹; the fingerprint region below 1500 cm⁻¹ identifies a molecule by matching its whole pattern, not single peaks.
Practice
Oxygen.
Chloride — silver chloride is white and dissolves in dilute ammonia (bromide needs concentrated ammonia).
A carboxylic acid — the broad O–H stretch and the C=O stretch together are characteristic.
Three distinct environments: CH₃, CH₂ and OH. The CH₃ signal is split into a triplet by the two neighbouring CH₂ hydrogens (n+1 rule).
Quick check
Which of these best defines "molecular ion"?
Distinguish between propan-1-ol and propan-2-one using a chemical test.
- Mass Spectrometry: find relative masses and molecular formulae from m/z peaks.
- Mass spectrometry weighs molecules: Molecules are ionised, usually by electron impact, and the molecular ion's m/z value gives the relative molecular mass.
- fragmentation: The breaking of a molecular ion into smaller ions, giving a fingerprint pattern of peaks.
- Watch out for: reading the tallest peak as the molecular ion