UV-Vis Spectroscopy — What It Is, How It Works & Applications
What Is UV-Vis Spectroscopy?
Ultraviolet-visible (UV-Vis) spectroscopy — also written UV Vis spectroscopy — measures the absorption of light in the ultraviolet (190–400 nm) and visible (400–800 nm) regions of the electromagnetic spectrum. Unlike infrared spectroscopy, which probes molecular vibrations, UV-Vis spectroscopy detects electronic transitions — the promotion of electrons from lower-energy to higher-energy molecular orbitals.
UV-Vis is the most widely used quantitative spectroscopic technique. Its core application is concentration determination via the Beer-Lambert law, but it also provides information about electronic structure, conjugation, and the chemical environment of chromophores.
How Does UV-Vis Spectroscopy Work?
When a molecule absorbs UV or visible light, an electron is excited from a bonding or non-bonding orbital to an antibonding orbital. The main types of electronic transitions, in order of increasing energy:
| Transition | Wavelength Range | Intensity | Example |
|---|---|---|---|
| n → π* | 250–400 nm | Weak (ε 10–100) | Ketones, aldehydes, carboxylic acids |
| π → π* | 200–700 nm | Strong (ε 1,000–100,000) | Conjugated dienes, aromatics, dyes |
| n → σ* | 150–250 nm | Medium | Saturated compounds with lone pairs (amines, alcohols) |
| σ → σ* | <200 nm (vacuum UV) | Strong | Saturated hydrocarbons (not routinely measured) |
Chromophores and Auxochromes
A chromophore is the part of a molecule responsible for UV-Vis absorption. Common chromophores include C=C, C=O, N=N, aromatic rings, and extended conjugated systems. The absorption wavelength depends on the energy gap between the electronic ground state and excited state.
An auxochrome is a substituent that modifies the absorption of a chromophore without being a chromophore itself. Examples include -OH, -NH₂, and -OCH₃. These groups modify the absorption characteristics:
- Bathochromic shift (red shift) — absorption moves to longer wavelengths (lower energy). Caused by extended conjugation or electron-donating groups.
- Hypsochromic shift (blue shift) — absorption moves to shorter wavelengths (higher energy). Caused by electron-withdrawing groups or increased solvent polarity (for n → π* transitions).
- Hyperchromic effect — increase in absorption intensity (higher ε).
- Hypochromic effect — decrease in absorption intensity (lower ε).
The UV-Vis Spectrum
A UV-Vis spectrum plots absorbance (or transmittance) against wavelength in nanometers. Unlike FTIR spectra, which show many sharp peaks corresponding to individual vibrational modes, UV-Vis spectra typically show broad absorption bands because electronic transitions are accompanied by many simultaneous vibrational transitions (vibronic coupling).
The key parameters extracted from a UV-Vis spectrum are: λmax (the wavelength of maximum absorption, which identifies the chromophore) and εmax (the molar absorptivity at λmax, which indicates how strongly the substance absorbs). Together with path length and measured absorbance, the Beer-Lambert law converts these into concentration.
Instrumentation
A UV-Vis spectrophotometer consists of a light source (deuterium lamp for UV, tungsten-halogen lamp for visible), a monochromator or diffraction grating to select the wavelength, a sample holder (cuvette), and a detector (photomultiplier tube, photodiode, or CCD array).
Common configurations include single-beam (reference measured separately), double-beam (simultaneous reference and sample), and diode-array (all wavelengths measured simultaneously for fast kinetics). Standard cuvettes are 1 cm path length, made of quartz (UV-transparent) or glass/plastic (visible only). Microvolume instruments like the NanoDrop use 0.05–1 mm path lengths for small sample volumes.
UV-Vis Spectroscopy Applications
- Concentration determination. The most common use. Measure absorbance at λmax, apply Beer-Lambert law. Used daily in clinical diagnostics, environmental monitoring, and QC labs.
- Protein quantification. A280 method measures absorbance from tryptophan and tyrosine, with a minor contribution from disulfide bonds. The A280/A260 ratio assesses protein purity relative to nucleic acid contamination.
- Nucleic acid quantification. DNA absorbs at 260 nm (ε ≈ 50 μg/mL per A unit for dsDNA). The A260/A280 and A260/A230 ratios indicate purity.
- Reaction kinetics. Monitoring absorbance change over time at a diagnostic wavelength gives real-time concentration profiles. Essential for enzyme kinetics and photodegradation studies.
- Color measurement. Transmission or reflection spectra are converted to CIE color coordinates for quality control in dyes, coatings, and food products.
- Nanoparticle characterization. Metal nanoparticles (gold, silver) exhibit localized surface plasmon resonance (LSPR) peaks in the visible region. Peak position and width indicate particle size, shape, and aggregation state.
- Pharmaceutical QC and dissolution testing. UV-Vis is used to verify drug identity, assay active ingredient content, and monitor dissolution profiles in quality control laboratories. Pharmacopoeial methods (USP, EP) frequently specify UV absorbance as the analytical endpoint.
- Environmental water analysis. UV absorbance at 254 nm (a proxy for dissolved organic carbon), nitrate (220 nm), and various metal-indicator complexes are routinely measured by UV-Vis in water treatment plants and environmental monitoring. The technique is fast enough for field-deployable instruments.
Common Solvent Cutoffs
Every solvent absorbs UV light below a certain wavelength (the cutoff). You cannot measure sample absorption below the solvent cutoff. Common cutoffs:
| Solvent | UV Cutoff (nm) |
|---|---|
| Water | 190 |
| Acetonitrile | 190 |
| Methanol | 205 |
| Ethanol | 210 |
| Dichloromethane | 230 |
| Chloroform | 245 |
| Acetone | 330 |
Related Resources
- Beer-Lambert Law Explained — the equation behind UV-Vis quantitative analysis
- FTIR vs Raman Spectroscopy — Comparison Guide — compare UV-Vis with FTIR, Raman, and NIR
- Absorbance, Transmittance & Reflectance — understanding measurement modes
- Beer-Lambert Calculator — solve for concentration, absorbance, or path length
- Unit Converter — convert between wavelength and energy units
Frequently Asked Questions
What is UV-Vis spectroscopy?
UV-Vis (ultraviolet-visible) spectroscopy is an analytical technique that measures the absorption of ultraviolet (190–400 nm) and visible (400–800 nm) light by molecules. When a molecule absorbs a photon, electrons are promoted from lower-energy to higher-energy molecular orbitals. UV-Vis spectroscopy is the most widely used quantitative spectroscopic technique, applied daily in clinical diagnostics, environmental monitoring, and research laboratories for concentration determination via the Beer-Lambert law.
How does UV-Vis spectroscopy work?
A UV-Vis spectrophotometer passes ultraviolet and visible light through a sample and measures how much light is absorbed at each wavelength. The instrument uses a deuterium lamp (UV) and tungsten-halogen lamp (visible) as light sources, a monochromator to select wavelengths, and a detector to measure transmitted light. Absorption occurs when photon energy matches the energy gap for an electronic transition in the molecule. The resulting spectrum plots absorbance vs. wavelength, where peaks indicate chromophores and their electronic environment.
What does UV-Vis spectroscopy measure?
UV-Vis spectroscopy measures the absorption of ultraviolet (190–400 nm) and visible (400–800 nm) light by molecules. The absorbed energy promotes electrons from lower-energy to higher-energy molecular orbitals (electronic transitions). The wavelength of maximum absorption (λmax) identifies the chromophore, and the absorbance value relates to concentration via the Beer-Lambert law.
What is a chromophore?
A chromophore is the part of a molecule responsible for absorbing UV or visible light. It contains electrons that undergo electronic transitions when excited by photons. Common chromophores include C=C double bonds, C=O carbonyl groups, N=N azo groups, aromatic rings, and extended conjugated systems. The degree of conjugation determines the absorption wavelength — more conjugation shifts absorption to longer wavelengths (red shift).
Why are UV-Vis absorption bands broad?
UV-Vis bands are broad because each electronic transition is accompanied by many simultaneous vibrational transitions (vibronic coupling). A single electronic excitation can land the molecule in any of many vibrational levels of the excited electronic state, creating a broad envelope of closely spaced transitions that merge into one broad band. In contrast, IR and Raman peaks are narrow because they involve only vibrational transitions.