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*Revision Materials and Past Papers* 1 Atomic Structure 2 Amounts of Substance 3 Bonding 4 Energetics 5 Kinetics 6 Chemical Equilibria & Kc 7 Redox Equations 8 Thermodynamics 9 Rate Equations 10 Kp (Equilibrium Constant) 11 Electrode Potentials & Cells 12 Acids and Bases 13 Periodicity 14 Group 2: Alkaline Earth Metals 15 Group 7: The Halogens 16 Period 3 Elements & Oxides 17 Transition Metals 18 Reactions of Ions in Aqueous Solution 19 Intro to Organic Chemistry 20 Alkanes 21 Halogenoalkanes 22 Alkenes 23 Alcohols 24 Organic Analysis 25 Optical Isomerism 26 Aldehydes & Ketones 27 Carboxylic Acids & Derivatives 28 Aromatic Chemistry 29 Amines 30 Polymers 31 Amino Acids, Proteins & DNA 32 Organic Synthesis 33 NMR Spectroscopy 34 Chromatography RP1–RP12 Required Practicals

3.6 Organic Analysis

3.6.1 Identification of Functional Groups By Test-Tube Reactions 3.6.2 Mass Spectrometry 3.6.3 Infrared Spectroscopy

Mass Spectrometry

Specification Reference Organic chemistry, Organic analysis 3.3.6.2

Quick Notes

  • Mass spectrometry determines the molecular formula of a compound.
  • The molecular ion peak (M+) represents the molecular mass of the compound.
  • Precise atomic masses can be used to calculate the exact molecular formula.

Full Notes

How Mass Spectrometry Works

A sample is ionised to form positive ions.

Ions are accelerated and separated based on their mass-to-charge ratio (m/z).

AQA A-Level Chemistry annotated mass spectrum showing the molecular ion peak M+ and fragment peaks at lower m/z values

The detector records the ions, producing a mass spectrum.

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

Identifying the Molecular Formula

The molecular ion peak (M+) has the highest m/z value (peak furthest to the right on a spectra).

We can use precise atomic masses to confirm the molecular formula as only a specific combination will give the exact value of the molecular ion peak.

Worked Example

Determine the molecular formula of a compound with an M+ peak of 58.12

AQA A-Level Chemistry example calculation showing that M+ 58.12 corresponds to butane C4H10 using precise atomic masses

Summary