IB Chemistry›Structure 3›3.2›3.2.8
HL

Mass Spectrometry (MS)

Using fragmentation patterns to deduce molecular structure.

1. Index of Hydrogen Deficiency (IHD)

Calculates the "Degree of Unsaturation". First step in any analysis.

\[ IHD = \frac{2C + 2 + N - H - X}{2} \]
  • IHD = 1: One double bond OR one ring.
  • IHD = 4: Suggests Benzene Ring (3 \(\pi\) bonds + 1 ring).

2. Fragmentation Patterns

High energy electrons bombard the molecule, forming the molecular ion \(M^+\) which then fragments.

m/z Lost Fragment Lost Indicates
M − 15 \(CH_3\) Loss of Methyl group
M − 17 \(OH\) Loss of Hydroxyl (alcohols)
M − 29 \(C_2H_5\) or \(CHO\) Loss of Ethyl or Aldehyde
M − 31 \(CH_2OH\) Primary Alcohol loss
M − 45 \(COOH\) Loss of Carboxyl
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