IB Chemistry Mass Spectrometry: FAQs, Worked Examples and Exam Tips

For IB Chemistry you need two separate skills: calculating relative atomic mass from isotopic abundances (Structure 1.2, SL and HL), and — at HL only — reading the molecular ion peak and fragmentation pattern to deduce an organic molecule's structure (Structure 3.2). Everything else is exam technique built on those two ideas.

A mass spectrometer separates charged particles by their mass-to-charge ratio (m/z) and records their relative abundance as a spectrum. In IB Chemistry you use that spectrum for three things: calculating relative atomic mass from isotope peaks, reading relative molecular mass (Mr) from the molecular ion peak, and — at HL — identifying structural fragments.

Multiply each isotope's m/z value by its percentage abundance, add those together, then divide by 100. For chlorine — 75% abundance at m/z 35, 25% at m/z 37 — the calculation gives (35 × 75 + 37 × 25) ÷ 100 = 35.5, matching the value on the IB data booklet.

The peak furthest to the right on the spectrum with meaningful abundance is the molecular ion peak, M+, formed when the whole molecule loses a single electron. Its m/z value equals the compound's relative molecular mass directly — no calculation needed, just correct identification of which peak it is.

Isotopes share the same proton number but have different neutron numbers, so they have different masses and register at different m/z values. Peak height reflects each isotope's natural abundance, which is exactly why the spectrum lets you calculate a weighted relative atomic mass rather than a simple average.

When high-energy electrons bombard a molecule, the molecular ion can shatter into smaller charged fragments, each producing its own peak. At HL you're expected to work out which bond broke by finding the mass difference between the molecular ion peak and a fragment peak — a loss of 15 means a methyl group left, a loss of 29 means an ethyl group did.

Look for the peak sitting at the highest m/z value on the spectrum — this corresponds to the unfragmented molecule that has simply lost one electron. Its m/z equals the compound's Mr directly. Common mistake: students confuse this with the tallest peak, which is usually a different, more abundant fragment called the base peak.

No — the IB groups these three under structural elucidation, but each reveals something different. Mass spectrometry gives molecular mass and fragment identity, IR spectroscopy identifies functional groups from bond vibration frequencies, and ¹H NMR spectroscopy reveals hydrogen environments and how many hydrogens sit in each one.

SL students only need to interpret isotope peaks for relative atomic mass and read the molecular ion peak for Mr. HL students go further, using fragmentation patterns in Structure 3.2 to deduce structural features of unknown organic compounds — usually combined with IR and NMR data in the same question.

Yes, it appears at SL too — but only isotope calculations for relative atomic mass and reading Mr off the molecular ion peak. Fragmentation-pattern analysis to deduce an organic structure is HL-only content, typically appearing in longer Paper 2 questions alongside IR and NMR data.

Paper 1 multiple-choice questions usually test isotope calculations and reading Mr straight off a given spectrum. HL Paper 2 typically gives a mass spectrum alongside IR and NMR data and asks you to identify an unknown organic compound, justifying your answer using specific fragment losses and the molecular ion peak.

The error I mark most often is forgetting to divide by 100 after multiplying isotope mass by percentage abundance, giving an answer roughly a hundred times too large. Students also confuse the molecular ion peak with the tallest peak — the base peak — which is a different, more abundant fragment.

Drill isotope calculations until the arithmetic is automatic, then move to HL Paper 2 structural-elucidation questions that combine mass spec with IR and NMR data — that combination is where most marks get lost. On RevisionPrep, the Topical Worksheets and Revision Notes covering Structure 1.2 and Structure 3.2 group these calculations with full mark schemes.