Chemisode Spectra LabVCE Chemistry learning simulation
Module 1

Mass spectrometry: from molecules to a peak pattern

A mass spectrometer separates gas-phase ions according to their . This page uses a simplified magnetic-sector model so you can clearly see vaporisation, ionisation, acceleration, deflection and detection.

Step-by-step instrument animation

Choose a step to loop that part of the process, or play the whole sequence from start to finish.

Simplified mass spectrometer animation Multiple molecules are vaporised, ionised, accelerated, separated by magnetic deflection and detected to produce a spectrum. 1 Vaporise sample enters gas phase M M M 2 Ionise electron hit → ion + fragments M F⁺ ·R M → M⁺· → fragments 3 Accelerate electric field speeds up ions M⁺· 43⁺ 78⁺ 4 Separate by m/z different m/z → different paths electromagnet B field adjusted 43⁺ 78⁺ 80⁺ 5 Detect ion current spectrum printout
1. Vaporisation: several molecules are heated so they enter the gas phase. This allows them to move into the ion source.

Ionisation and fragmentation, shown separately

This sequence now separates the two equations students need to recognise: first electron ionisation produces the molecular ion M⁺·, then the molecular ion fragments to give a positive ion that can be detected.

1 Neutral moleculeMnot detected while neutral
→
2 Ionisation equationM + e⁻ → M⁺· + 2e⁻electron ionisation knocks out one electron
→
3 Molecular ionM⁺·this intact radical cation gives the M peak
→
4 Fragmentation equationM⁺· → F⁺ + ·Ronly the positive fragment reaches the detector
Stage 1: the sample contains neutral molecules. They must first become ions before the machine can separate and detect them.
Teaching note: this matches the VCE-style story: molecule → electron ionisation → molecular ion M⁺· → fragmentation to smaller positive ions and neutral radicals.

Interactive spectrum: click a peak to see its fragment

The examples show major, representative peaks rather than every small measured peak.

Ethanol

Select a peak. The relevant part of the molecule will be highlighted. The formula shown is the positive ion reaching the detector.

Simplified electron-ionisation mass spectrum

How to read a mass spectrum

The horizontal axis is m/z. Most classroom fragment ions have charge z = +1, so m/z is numerically equal to the fragment’s nominal mass.

The tallest signal is the , assigned 100% relative abundance. It is the most abundant detected ion, not automatically the molecular ion.

The molecular ion M⁺· often gives the molecular mass, but it may be weak or absent if the molecule fragments very readily.

Isotope-pattern extension: chlorine

Chlorine has two common isotopes, Cl-35 and Cl-37, with an abundance ratio of about 3 : 1. A molecule containing one chlorine atom therefore often gives molecular-ion peaks two mass units apart, with the M peak about three times the height of the M+2 peak.

Choose 1-chloropropane above and compare the molecular-ion peaks at m/z 78 for the Cl-35 molecular ion and m/z 80 for the Cl-37 molecular ion.

Isotope pattern explorer

Compare the three isotope clues emphasised in the spectroscopy notes. Select a pattern and use the spacing and relative heights to decide what it means.

Worked example: propanone

Propanone has formula C₃H₆O and Mr ≈ 58. The molecular ion appears at m/z 58. Cleavage next to the carbonyl can form the resonance-stabilised acylium ion CH₃CO⁺ at m/z 43, which is the base peak in the simplified spectrum.

Reasoning pattern: peak mass → possible ion formula → locate a bond cleavage that produces that positive fragment → check whether the fragment is chemically plausible.

Question bank: 4 sets of 5

The bank is split into four mixed sets of five questions. Each set combines instrument and interpretation content. Answer choices are shuffled once when the attempt begins, then remain fixed while you work.