Infrared and Mass Spectrometry

In infrared spectroscopy, bonds absorb IR at characteristic wavenumbers: O–H in alcohols (3200–3600 cm⁻¹, broad), O–H in acids (2500–3300, very broad), C=O (1680–1750, sharp, strong), N–H (3300–3500), C≡N (≈2250), C–H (2850–3100). The fingerprint region (below 1500 cm⁻¹) identifies a compound by matching. In mass spectrometry, the molecular ion (M⁺, highest significant $m/z$ ) gives $M_r$ . Fragments give structural clues (15 = CH₃⁺, 29 = C₂H₅⁺ or CHO⁺, 43 = CH₃CO⁺ or C₃H₇⁺). The M+1 peak (from ¹³C) gives the number of carbons: $n=\frac{100\times\text{abundance of M+1}}{1.1\times\text{abundance of M}}$ . M+2 peaks show Cl (3 : 1) or Br (1 : 1).

Cambridge 9701 Paper 1Cambridge 9701 Paper 2Edexcel YCH11 WCH12AQA 7405 Paper 2≈ 45 minRevision 1 · updated 2026-10-30

Section 1 of 14

Learning objectives

Identify functional groups from IR absorptions.
Use the molecular ion to find $M_r$ .
Interpret fragmentation peaks.
Use M+1 and M+2 peaks.

Pause and think

How can an IR spectrum tell you whether an alcohol has been oxidised to an aldehyde or all the way to a carboxylic acid?

Section 2 of 14

Core idea

Start hereReading molecules with light and magnets

Infrared tells you which bonds are present; a mass spectrometer tells you how heavy the molecule is and how it breaks apart.

The big idea

Each bond absorbs IR at a characteristic wavenumber. The molecular ion gives $M_r$ ; fragments and M+1 / M+2 peaks reveal structure.

1. Infrared

Which compound class?

Drawing diagram…

Look for O–H and C=O first.

2. Mass spectrometry

n=\frac{100}{1.1}\times\frac{I_{M+1}}{I_M}

n: number of carbon atoms
I: peak height (abundance)

Worked exampleCounting carbons

M⁺ at $m/z$ 72 has height 40.0; M+1 has height 1.76. How many carbons?

Goal: $n$ .
Strategy: Use the M+1 formula.

$n=\frac{100}{1.1}\times\frac{1.76}{40.0}$
¹³C is 1.1% of carbon.
$n=4.0$
Round to a whole number.

Answer4 carbons (e.g. butanone, C₄H₈O).

✓ Check

4\times12+8+16=72

✓.

Key takeawayM+2 peaks Chlorine: M : M+2 = 3 : 1. Bromine: 1 : 1.

Summary

O–H broad; C=O sharp ~1700.
Fingerprint region: match the pattern.
M⁺ → $M_r$ ; M+1 → carbons; M+2 → Cl/Br.

Wavenumber: Number of waves per cm (cm⁻¹).
Molecular ion: Ion formed by losing one electron from the whole molecule.
Fragmentation: Breaking of the molecular ion into smaller ions and radicals.

HistoryJ. J. Thomson. J. J. Thomson built the first mass spectrometer around 1912 and used it to discover that neon has two isotopes.

Fun factGreenhouse gases. CO₂ and H₂O absorb infrared because their vibrations change the dipole, which is why they trap heat in the atmosphere while N₂ and O₂ don't.

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