Mass spectrometry and IR

A mass spectrometer turns molecules into ions, sorts them by mass divided by charge (m/z), and counts them. Most ions carry a charge of 1, so m/z is simply the mass. The ion of the whole molecule, the molecular ion, gives the molar mass: a peak at m/z 46 from an unknown alcohol points to ethanol, C₂H₆O.

The molecules also break into fragments, and the pattern is a fingerprint matched against libraries. A peak 15 below the molecular ion means a CH₃ fell off; 18 below, water.

Isotopes show up as extra peaks. Chlorine is about three-quarters chlorine-35 and a quarter chlorine-37, so a compound with one chlorine has two molecular-ion peaks 2 apart with heights about 3:1. Bromine's two isotopes are nearly 1:1, so one bromine gives two peaks of about equal height. Carbon-13 (1.1% of carbon) gives a small peak 1 above.

Infrared (IR) spectroscopy shines infrared light through a sample and records which wavenumbers (in cm⁻¹, waves per centimetre) are absorbed. Bonds vibrate at their own frequencies, so functional groups show up in set places: a broad O–H band at 3200–3550 cm⁻¹ (alcohols), a very broad O–H at 2500–3300 in carboxylic acids, C–H just under 3000, and a strong, sharp C=O at about 1700.

Put together they answer most "what is it?" questions: mass spec gives the molar mass and formula clues, IR the functional groups, and nuclear magnetic resonance (NMR, a third method using magnets and radio waves) shows how the carbons and hydrogens are joined.