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3.2.8Additional lessons

Mass Spectrometry (MS) Fragmentation Patterns (HL) — IB Chemistry HL Revision Notes & Topical Questions

5 min read 5 parts Lesson 65 of 172

📚 IB Chemistry HL Hub / Topical Revision
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Part 1 of 5Reading a Spectrum

3.2.8 Mass Spectrometry (MS) Fragmentation Patterns (HL)
Molecular ion
  • The peak with the highest m/z (apart from small isotope peaks) is the M⁺ ion and gives the relative molecular mass.
Fragmentation
  • The energetic molecular ion breaks into a positive fragment and a neutral radical, and only the positive species are detected.
Base peak
  • The tallest peak is the base peak and represents the most abundant ion.
M+1 peak
  • A small peak one unit above M⁺ arises from molecules containing a ¹³C atom.
Halogens
  • Chlorine gives M and M+2 peaks in a 3 : 1 ratio and bromine gives them in a 1 : 1 ratio.

Part 2 of 5Common Fragments

1
m/z 15 corresponds to CH₃⁺, formed by loss of a larger fragment from the molecular ion.
2
m/z 29 is C₂H₅⁺ in an ethyl compound or CHO⁺ in an aldehyde.
3
m/z 43 is C₃H₇⁺ or CH₃CO⁺, and m/z 57 is C₄H₉⁺ or C₂H₅CO⁺.
4
A loss of 17 from M⁺ suggests –OH, and a loss of 18 suggests loss of water, common in alcohols.
5
m/z 77 is C₆H₅⁺ and suggests a benzene ring.
m/zLikely ion
15CH₃⁺
29C₂H₅⁺ or CHO⁺
43C₃H₇⁺ or CH₃CO⁺
77C₆H₅⁺

Part 3 of 5Strategy

Strategy
Find M
  • Locate the molecular ion to give the molar mass and then compare with candidate molecular formulas.
Differences
  • Calculate the mass differences between M⁺ and fragment peaks to find the neutral piece lost.
Stability
  • Stable cations such as tertiary carbocations and acylium ions often give the strongest peaks.
Combine data
  • Use IR and NMR with MS to decide between possible structures.

Part 4 of 5Worked Example

Question

  • The mass spectrum of propanone, CH₃COCH₃, has peaks at m/z 58, 43 and 15.
  • Assign the peaks.

  1. The peak at 58 is the molecular ion, C₃H₆O⁺, since Mᵣ = 58.
  2. The mass difference 58 − 43 = 15 corresponds to the loss of a CH₃• radical.
  3. The ion at 43 is therefore CH₃CO⁺, the acylium ion and base peak.
  4. The peak at 15 is CH₃⁺ from cleavage on the other side of the carbonyl group.

Answer 58: M⁺; 43: CH₃CO⁺; 15: CH₃⁺

Part 5 of 5Exam Checklist & Tips

3.2.8 – Key Points for the Exam

  • Molecular ion The ion formed when a molecule loses one electron, with m/z equal to its relative molecular mass.
  • Fragment ion A smaller positively charged piece formed when the molecular ion breaks.
  • Base peak The most intense peak in the spectrum.
  • m/z The mass-to-charge ratio of an ion.

Common Mistakes

  • Treating the highest peak as the molecular ion when it is the base peak.
  • Forgetting that neutral fragments are not detected.
  • Writing fragment ions without their positive charge.
  • Ignoring the M+2 peak in compounds that contain chlorine or bromine.

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