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3.6.2Physical Chemistry

Mass Spectrometry — AQA AS Chemistry Revision Notes & Topical Questions

5 min read 6 parts Lesson 88 of 102

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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Part 1 of 6How the Spectrum Is Produced

1
A small sample is vaporised and bombarded with high-energy electrons, which knock an electron off each molecule.
2
This forms a positive molecular ion with one unpaired electron, written M⁺ or [M]⁺•.
3
The molecular ion can break apart into smaller positive fragment ions and uncharged radicals or molecules.
4
An electric field accelerates the positive ions, and a magnetic field deflects them by an amount that depends on their mass-to-charge ratio, m/z.
5
A detector records each ion, giving a spectrum of relative abundance against m/z, with the tallest peak called the base peak.

Part 2 of 6Reading the Spectrum

m/z
  • m/z For ions with a 1+ charge, the m/z value equals the relative mass of the ion.
Molecular ion peak
  • Molecular ion peak The peak at the highest m/z gives the relative molecular mass of the compound.
M+1 peak
  • M+1 peak A very small peak one unit above M⁺ is caused by carbon-13, which is about 1% of natural carbon.
Isotope peaks
  • Isotope peaks In an element, the height of each peak is proportional to the relative abundance of that isotope.
Doubly charged ions
  • Doubly charged ions An ion with a 2+ charge appears at half its mass, so ⁵⁴Fe²⁺ appears at m/z 27.

Part 3 of 6Fragmentation Patterns

Fragmentation Patterns
Alkyl fragments
  • Alkyl fragments Common peaks are CH₃⁺ at 15, C₂H₅⁺ at 29, C₃H₇⁺ at 43 and C₄H₉⁺ at 57.
Alcohols
  • Alcohols Alcohols often lose water, giving a peak 18 units below M⁺, and CH₂OH⁺ gives a peak at m/z 31.
Small molecules
  • Small molecules Losses of 28 (CO) and 44 (CO₂) are common in carbonyl and acid fragments.
Isomers
  • Isomers Isomers have the same M⁺ but different fragment peaks, so fragments help decide the structure.

Part 4 of 6Halogen Isotope Patterns

Halogen Isotope Patterns
Bromine
  • Bromine Bromine has ⁷⁹Br and ⁸¹Br in about equal amounts, so a bromoalkane shows M and M+2 peaks of similar height.
Chlorine
  • Chlorine Chlorine has ³⁵Cl and ³⁷Cl in a ratio of about 3:1, so a chloroalkane shows M and M+2 peaks in that ratio.
Counting peaks
  • Counting peaks Two peaks two units apart near the top of a spectrum suggest one Br or Cl atom is present.
Loss of halogen
  • Loss of halogen A fragment peak from loss of the halogen, such as C₂H₅⁺ at 29 from bromoethane, is also common.
HalogenIsotopesM : M+2 ratio
Chlorine³⁵Cl, ³⁷Cl3 : 1
Bromine⁷⁹Br, ⁸¹Br1 : 1

Part 5 of 6Worked Example

Question

  • The mass spectrum of bromomethane, CH₃Br, shows two molecular ion peaks.
  • Calculate their m/z values and state their relative heights (Ar: C = 12, H = 1, ⁷⁹Br and ⁸¹Br).

  1. The ion containing ⁷⁹Br has m/z = 12 + (3 × 1) + 79 = 94.
  2. The ion containing ⁸¹Br has m/z = 12 + (3 × 1) + 81 = 96.
  3. Bromine's two isotopes are present in roughly equal amounts, so the peaks are almost equal in height.

Answer Peaks appear at m/z 94 and 96, of about equal height.

Part 6 of 6Exam Checklist & Tips

3.6.2 – Key Points for the Exam

  • Molecular ion
    • The ion formed when a molecule loses one electron;
      • its m/z gives the relative molecular mass.
  • m/z The mass-to-charge ratio of an ion.
  • Base peak The tallest peak in a mass spectrum, from the most abundant ion.
  • Fragmentation Breaking of the molecular ion into smaller ions and neutral species.
  • Isotopes Atoms of the same element with the same number of protons but different numbers of neutrons.

Common Mistakes

  • Taking the base peak as the molecular ion;
    • the molecular ion is the peak at the highest m/z, apart from the small M+1 peak.
  • Counting the M+1 carbon-13 peak as a separate molecular ion.
  • Forgetting that a 2+ ion appears at half its mass number.
  • Expecting a single molecular ion peak for a bromoalkane or chloroalkane.

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