Analysis: spectroscopy and chromatography: every key term you need (+ practice quiz)

Infrared spectroscopy

Bonds absorb infrared at frequencies matching their vibrations, so absorption peaks identify which functional groups are present.

Fingerprint region

Below about 1500 wavenumbers, a complex pattern unique to each compound. Matched against a database to confirm identity.

Carbonyl absorption

A strong sharp peak near 1700 wavenumbers. Its presence with a broad hydroxyl peak points to a carboxylic acid.

Mass spectrum molecular ion

The peak at the highest mass-to-charge ratio, giving the relative molecular mass of the whole compound.

M+1 peak

A small peak one unit above the molecular ion, caused by the 1.1 per cent natural abundance of carbon-13. Its height indicates the carbon count.

Fragmentation pattern

The molecular ion breaks into smaller ions. Mass differences between peaks reveal which groups were lost.

Nuclear magnetic resonance

Nuclei with spin absorb radio frequency energy in a strong magnetic field, at frequencies depending on their chemical environment.

Chemical shift

The position of an NMR peak relative to a standard, measured in parts per million. It identifies the environment of the nucleus.

TMS reference

Tetramethylsilane gives a single sharp peak defined as zero. It is inert, volatile and absorbs well clear of most sample peaks.

Integration trace

In proton NMR, peak area is proportional to the number of protons in that environment, giving their whole-number ratio.

Spin-spin splitting

A signal splits into n plus one peaks, where n is the number of protons on adjacent carbons. Reveals connectivity.

Deuterated solvent

Used in NMR because deuterium produces no signal in the proton spectrum, so the solvent stays invisible.

Thin layer chromatography

Separates by relative affinity for a stationary phase and a mobile phase. Components are identified by retention factor.

Retention factor

Distance moved by the spot divided by distance moved by the solvent front. Constant for a compound in a given system.

Gas chromatography

A volatile sample is carried by an inert gas through a column. Retention time identifies components and peak area quantifies them.

Combined analysis

Chromatography separates a mixture, then mass spectrometry identifies each component — far more reliable than either technique alone.

More A-Level Chemistry guides

• Atomic structure and the periodic table
• Bonding, structure and intermolecular forces
• Amount of substance and quantitative chemistry
• Energetics and thermodynamics
• Kinetics, equilibria and acids