How UV-Vis Spectroscopy Works
Ultraviolet–visible (UV-Vis) spectroscopy is one of the most widely used analytical techniques in the laboratory. At its core, it measures how much light a sample absorbs across the ultraviolet (190–400 nm) and visible (400–800 nm) regions, and uses that information to determine what is in the sample and how much of it is present.
Light and molecules
When light passes through a sample, molecules absorb specific wavelengths whose energy matches the gap between their electronic energy levels. A molecule promoted to a higher-energy electronic state removes that wavelength from the transmitted beam. Because each substance has a characteristic absorption pattern, the resulting spectrum acts as a fingerprint of the sample’s composition and chemical environment.
From transmittance to absorbance
The instrument compares the intensity of light before and after it passes through the sample. The ratio of transmitted to incident light is the transmittance (%T), and its logarithm gives absorbance (Abs). Absorbance is the more useful quantity because it is directly proportional to concentration, as described by the Beer–Lambert law.
Inside the instrument
A typical UV-Vis spectrophotometer combines a few key components working in sequence:
- Light source — a deuterium lamp for the UV region and a tungsten-halogen lamp for the visible/near-IR region.
- Monochromator — a diffraction grating that selects a single narrow band of wavelengths to pass through the sample.
- Sample compartment — holds the cuvette or cell containing the sample.
- Detector — a silicon photodiode (or array sensor) that converts transmitted light into an electrical signal.
What it is used for
UV-Vis spectroscopy is used for quantifying concentration, checking purity, monitoring reaction kinetics, and verifying the identity of compounds in fields ranging from environmental testing and biotechnology to pharmaceutical quality control. Its speed, simplicity, and reliability make it a foundational tool in nearly every analytical laboratory.