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H3 Chemistry 9813 · Study focus: H3 Chemistry: Explain mass-spectrometric ionisation and fragmentation

H3 Chemistry: Explain mass-spectrometric ionisation and fragmentation

Start from the governing chemical model, test it against evidence, then transfer the reasoning to an unfamiliar case.

Your success criteria

  • Explain mass-spectrometric ionisation and fragmentation
  • Use named chemical evidence.
  • Transfer the governing reason to an unfamiliar case.
Diagnose this objective

Form the molecular radical cation

Electron impact removes one electron: CH4 + e− → CH4+• + 2e−.

The parent ion is both positively charged and odd-electron.

Explore this H3 topic and lesson sequence.

Separate ionisation from cleavage

Ionisation creates M+•; fragmentation is subsequent bond cleavage.

A spectrum may contain intact M+• and smaller charged fragments.

  1. Ethanol M+• can cleave to CH2OH+ m/z31 plus CH3•.

  2. Only charged products are accelerated/detected; neutral radicals are absent.

H3 Chemistry: Explain mass-spectrometric ionisation and fragmentation: move from the evidence or givens, through the governing Chemistry idea, to a conclusion that stays inside the selected course boundary.
H3 Chemistry: Explain mass-spectrometric ionisation and fragmentation evidence representation. Fixed equations distinguish electron removal, cleavage and detector visibility.
H3 Chemistry: Explain mass-spectrometric ionisation and fragmentation authored scientific diagramText states atoms, charge, radical and detection status.Electron ionisationCH₄e⁻CH₄⁺•; m/z 162e⁻ionisationCH₄ + e⁻ → CH₄⁺• + 2e⁻Ethanol fragmentation and detectionC₂H₆O⁺•CH₂OH⁺; m/z 31CH₃• neutraldetectorbond cleavageneutral radicalcharged fragment only

Text alternative: Text states atoms, charge, radical and detection status.

Track detected charge

Ethanol M+• can cleave to CH2OH+ m/z31 plus CH3•.

Only charged products are accelerated/detected; neutral radicals are absent.

Balance an EI equation

CH3Cl + e− → CH3Cl+• + 2e− conserves atoms and charge.

Do not write CH3Cl− or omit the second outgoing electron.

  • Balance the equation.
Open the feedback checkpoint after attempting
  • Credit parent radical cation and two product electrons.

Assign charge retention

Propanone+• → CH3CO+ m/z43 + CH3• places charge on the acylium fragment.

The neutral methyl radical makes no m/z43 peak.

  • Label both products.
Open the feedback checkpoint after attempting
  • Credit formulas, radical and detected charge.

Start the diagnostic and follow its feedback

Compare competing cleavage

Butan-2-one can yield CH3CO+ m/z43 or C2H5CO+ m/z57.

Relative stability/cleavage propensity changes abundance; both assignments conserve the parent atoms.

  • Check both channels.
Open the feedback checkpoint after attempting
  • Credit valid complementary neutral radicals.

Repair every-fragment detection

Neutral loss can be chemically essential yet invisible to the detector.

A peak represents an ion, not every product of bond cleavage.

  • Repair: ‘both fragments give peaks.’
Open the feedback checkpoint after attempting
  • Require charge tagging and neutral exclusion.

Cross-check a fixed spectrum

Ethanol peaks m/z46 and 31 can be M+• and CH2OH+ respectively.

Next: m/z at /learning/h3-mass-spectrometry-mass-to-charge-lesson.html.

  • Explain the two peak origins.
Open the feedback checkpoint after attempting
  • Credit ionisation before fragmentation and charge retention.