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How do chemists know that a new drug molecule has the structure they designed? How do astronomers know that the Sun contains helium, or that interstellar clouds contain water? How does a hospital scanner see inside your body without cutting it open? The answer to all three is spectroscopy: studying how matter interacts with electromagnetic radiation.
Spectroscopy is the single most important set of tools for identifying substances and working out their structures. This article is a map of the main techniques and how they fit together.
The core idea
Atoms and molecules can only have certain allowed energies: specific electron energy levels, vibrational energies and rotational energies. They’re quantised. When radiation of exactly the right energy arrives, a molecule can absorb it and jump to a higher energy level. When it drops back down, it may emit radiation.
The energy of radiation is linked to its frequency and wavelength:
E = hf = hc ÷ λ
where h is the Planck constant (6.626 × 10⁻³⁴ J s), f is frequency, c is the speed of light and λ is wavelength.
- Higher energy means higher frequency and shorter wavelength.
- Different regions of the electromagnetic spectrum have different energies, so they cause different changes in molecules.
By measuring which energies a substance absorbs or emits, and how strongly, we learn about its structure.
What each region does
| Region | Typical wavelength | Energy change caused | Technique |
|---|---|---|---|
| Gamma rays | below 0.01 nm | nuclear energy levels | Mössbauer spectroscopy (specialist) |
| X-rays | 0.01–10 nm | removing inner-shell electrons | X-ray fluorescence, X-ray crystallography |
| Ultraviolet | 10–400 nm | exciting outer (valence) electrons; can break bonds | UV–visible spectroscopy |
| Visible | 400–700 nm | exciting outer electrons (colour) | colorimetry, UV–visible spectroscopy |
| Infrared | 700 nm–1 mm | molecular vibrations (bond stretching and bending) | infrared (IR) spectroscopy |
| Microwaves | 1 mm–1 m | molecular rotations | rotational spectroscopy; microwave ovens heat water this way |
| Radio waves | over 1 m | flipping nuclear spins in a magnetic field | NMR spectroscopy, MRI |
Absorption and emission
- Absorption spectroscopy shines radiation through a sample and measures which wavelengths are absorbed. The result is an absorption spectrum. UV–visible, infrared and atomic absorption spectroscopy work this way.
- Emission spectroscopy gives energy to a sample (with heat, a flame, an electric discharge or a laser) and measures the radiation it emits. Flame tests and atomic emission spectroscopy work this way.
Both give information about the same energy levels, seen from opposite directions.
The main techniques
UV–visible spectroscopy
Measures absorption of ultraviolet and visible light, caused by electrons jumping to higher levels. Molecules with conjugated double bonds and transition metal complexes absorb strongly.
Used for: measuring concentrations (via the Beer–Lambert law), following reaction rates, checking DNA and protein samples.
More in UV–visible spectroscopy.
Infrared (IR) spectroscopy
Measures absorption of infrared radiation, which makes bonds vibrate. Each type of bond (O–H, C=O, C–H, N–H) absorbs at characteristic wavenumbers.
Used for: identifying functional groups in organic molecules, breathalysers, monitoring greenhouse gases, checking the purity of products.
More in infrared spectroscopy.
Nuclear magnetic resonance (NMR)
In a strong magnetic field, nuclei such as ¹H and ¹³C absorb radio waves at frequencies that depend on their chemical environment.
Used for: working out the detailed structure of organic molecules, including which atoms are connected to which. The same physics underlies MRI scanning in hospitals.
More in NMR explained.
Mass spectrometry (MS)
Strictly not spectroscopy (it doesn’t use light), but always grouped with it. Molecules are ionised and separated by their mass-to-charge ratio (m/z).
Used for: finding relative molecular mass, identifying compounds from their fragmentation patterns, measuring isotope ratios, and detecting tiny amounts of drugs or pollutants.
More in mass spectrometry.
Atomic spectroscopy
Measures light absorbed or emitted by individual atoms in a flame or plasma.
- Flame tests and atomic emission spectroscopy identify metals by the colours they emit.
- Atomic absorption spectroscopy (AAS) measures concentrations of specific metals.
- ICP-MS and ICP-OES measure many elements at once, down to trace levels.
Used for: metals in drinking water, soil and food; lead in blood; elemental composition of rocks and alloys.
X-ray techniques
- X-ray crystallography uses the diffraction of X-rays by crystals to reveal the exact 3D positions of atoms.
- X-ray fluorescence (XRF) identifies elements in a sample without destroying it, from paint layers in old masterpieces to metals in scrap.
A worked example: energy and wavelength
How much energy does one photon of green light (λ = 530 nm) carry, and how does that compare with a photon of infrared radiation at 10 μm?
- Green: E = hc ÷ λ = (6.626 × 10⁻³⁴ × 3.00 × 10⁸) ÷ (530 × 10⁻⁹) = 3.75 × 10⁻¹⁹ J
- Infrared: E = (6.626 × 10⁻³⁴ × 3.00 × 10⁸) ÷ (10 × 10⁻⁶) = 1.99 × 10⁻²⁰ J
The visible photon carries about 19 times more energy. That’s enough to move an electron to a higher energy level, while the infrared photon only has enough energy to make a bond vibrate faster. Per mole of photons, green light corresponds to about 226 kJ/mol, comparable to the strength of a weak chemical bond, which is why ultraviolet light, with even more energy per photon, can break bonds and damage skin.
Instruments have the same basic parts
Almost every spectrometer, whatever region it uses, has the same building blocks: a source of radiation, a way to select wavelengths (a filter, grating or interferometer), a sample holder made of a material that doesn’t absorb in that region (quartz for UV, salt plates or diamond for IR), and a detector that converts radiation into an electrical signal. Understanding this pattern makes it much easier to learn each new technique.
Putting techniques together
No single technique tells you everything. To identify an unknown organic compound, chemists usually combine several:
| Technique | Main question it answers |
|---|---|
| Mass spectrometry | What’s the molecular mass? What fragments does it break into? |
| Elemental analysis | What’s the empirical formula? |
| Infrared | Which functional groups are present? |
| ¹³C NMR | How many different carbon environments are there? |
| ¹H NMR | How many different hydrogen environments, and how are they connected? |
| UV–visible | Is there extended conjugation? How concentrated is it? |
Each technique narrows down the possibilities until only one structure fits all the data.
Spectroscopy beyond the lab
- Astronomy: the spectra of stars reveal their composition, temperature and motion. Helium was first discovered in the Sun’s spectrum in 1868, decades before it was found on Earth.
- Medicine: MRI scanners, pulse oximeters and blood gas analysers all rely on spectroscopy.
- Environment: satellites measure ozone, CO₂ and methane in the atmosphere by the radiation they absorb.
- Forensics: IR and mass spectrometry identify drugs, fibres and paints.
- Art and archaeology: non-destructive techniques identify pigments and detect forgeries.
Key takeaways
- Spectroscopy studies how matter absorbs or emits electromagnetic radiation.
- E = hf = hc/λ: shorter wavelengths carry more energy and cause bigger changes.
- UV–visible excites electrons; infrared causes bond vibrations; radio waves in NMR flip nuclear spins.
- Mass spectrometry separates ions by mass-to-charge ratio and is used alongside spectroscopy.
- Chemists combine several techniques to identify compounds and work out their structures.
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