Organic analysis

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).