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OCR (A) A-Level
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*Revision Materials and Past Papers* 2.1.1 Atomic structure and isotopes 2.1.2 Compounds, formulae and equations 2.1.3 Amount of substance 2.1.4 Acids 2.1.5 Redox 2.2.1 Electron structure 2.2.2 Bonding and structure 3.1.1 Periodicity 3.1.2 Group 2 3.1.3 The halogens 3.1.4 Qualitative analysis 3.2.1 Enthalpy 3.2.2 Reaction Rates 3.2.3 Chemical equilibrium 4.1 Basic concepts and hydrocarbons 4.1.2 Alkanes 4.1.3 Alkenes 4.2.1 Alcohols 4.2.2 Haloalkanes 4.2.3 Organic synthesis 4.2.4 Analytical techniques 5.1.1 How fast? 5.1.2 How far? 5.1.3 Acids, bases and buffers 5.2.1 Lattice enthalpy 5.2.2 Enthalpy and entropy 5.2.3 Redox and electrode potentials 5.3.1 Transition elements 5.3.2 Qualitative analysis 6.1.1 Aromatic compounds 6.1.2 Carbonyl compounds 6.1.3 Carboxylic acids and esters 6.2.1 Amines 6.2.2 Amino acids, amides and chirality 6.2.3 Polyesters and polyamides 6.2.4 Carbon–carbon bond formation 6.2.5 Organic synthesis 6.3.1 Chromatography and qualitative analysis 6.3.2 Spectroscopy Required Practicals

4.2.4 Analytical techniques

Combined techniquesInfrared spectroscopyMass spectrometry

Mass spectrometry

Specification Reference 4.2.4 (f)–(g)

Quick Notes

  • Mass spectrometry can be used to identify organic compounds using mass-to-charge ratio (m/z) of molecular ions.
  • The molecular ion peak (M⁺) gives the relative molecular mass (Mr).
  • Smaller peaks are caused by fragmentation, the breakdown of the molecular ion.
  • [M+1]⁺ peaks indicate the presence of carbon-13 isotopes.

Full Notes

Mass Spectrometry and how it works is covered in more detail here.
This page is just what you need to know for OCR (A) A-level :)

The Basics

Mass spectrum with labelled molecular ion peak and fragment peaks.

The molecular ion peak (M⁺) represents the relative molecular mass (Mr) of the compound.

Identifying Molecular Mass and Formula

The molecular ion peak (M⁺) in a spectra has the highest m/z value.
We can use precise atomic masses to confirm the molecular formula based on the M+ peak.

For Example: Determining the molecular formula of a compound with M⁺ = 58.12

Worked example using M+ = 58.12 to suggest a molecular formula, e.g. C4H10.

Fragmentation

Inside a mass spectrometer, molecular ions can break into fragments.

Each fragment forms a smaller ion and shows up as a peak at a lower m/z.
These peaks help identify parts of the molecule.

Worked Example: Fragmentation patterns of isomers

For example, Hydrocarbons A and B both have a molecular formula of C4H10 (same molecular ion peak), however they have different fragment patterns in their spectra, showing different structures,

Fragmentation patterns in mass spectra for C4H10 isomers highlighting m/z 15, 29 and 43.

Fragment peaks at 15 and 43 show a CH3 fragment and C3H7 fragment. However, no fragment at 29 means no C2H5 group. This means the likely possible structure is CH3CH(CH3)CH3

Fragmentation example showing peaks confirming a branched alkane structure.

The extra peak at m/z 29 for Hydrocarbon B means it has a C2H5 group in its structure (as well as a CH3 and C3H7 group). This would indicate CH3CH2CH2CH3 as its structure.

The [M+1]⁺ Peak

The M+1 peak is a small peak 1 unit higher than the main molecular ion peak, M⁺, caused by molecules containing one carbon-13 atom.

Illustration of an M+1 peak one unit above the molecular ion peak caused by 13C.

Summary