UV–Visible Absorption Spectroscopy

UV–visible absorption spectroscopy measures the attenuation of ultraviolet or visible light by a sample to quantify analytes through electronic transitions.

Definition

UV–visible absorption spectroscopy is a molecular spectroscopic method that determines analyte concentration from the fraction of ultraviolet or visible radiation absorbed during electronic transitions.

Scope

This topic covers the instrumentation and practice of measuring molecular absorption between roughly 190 and 800 nm: continuum and line sources, monochromators and photodiode arrays, single- and double-beam spectrophotometers, and the application of the Beer–Lambert law to quantitative determination. It includes colorimetric methods in which a colour-forming reaction renders a non-absorbing analyte measurable.

Core questions

• How is absorbance related to concentration through the Beer–Lambert law?
• What chromophores and electronic transitions give rise to UV–visible absorption?
• How are colorimetric reagents used to make non-absorbing analytes detectable?
• What sources of error—stray light, deviation from Beer's law, instrumental noise—limit accuracy?

Key theories

Beer–Lambert law

Absorbance equals the product of molar absorptivity, path length, and concentration, allowing direct quantitation from a single measurement against a calibration; the law assumes monochromatic radiation, dilute solutions, and no chemical or instrumental deviations.

Mechanisms

Absorption arises when a photon promotes an electron from a ground-state molecular orbital to a higher-energy orbital; the energy gap sets the wavelength absorbed, and the molar absorptivity reflects the transition probability. A spectrophotometer measures the ratio of transmitted to incident intensity, reports it as transmittance, and converts it logarithmically to absorbance, which is proportional to concentration over the linear range.

Clinical relevance

UV–visible spectrophotometry underlies countless routine assays: protein and nucleic-acid quantitation, enzymatic clinical-chemistry tests, dissolved-species measurement in water analysis, and pharmaceutical assay and dissolution testing.

History

The quantitative basis of the method traces to Bouguer and Lambert's work on light attenuation in the 18th century and Beer's 1852 demonstration that absorption scales with concentration. Practical photoelectric spectrophotometers emerged in the 1940s, and diode-array instruments later enabled rapid full-spectrum acquisition.

Key figures

• August Beer
• Pierre Bouguer
• Johann Heinrich Lambert

Seminal works

• skoog2017
• harris2020

Frequently asked questions

What is the difference between transmittance and absorbance?

Transmittance is the fraction of incident light that passes through the sample; absorbance is the negative logarithm of transmittance and is directly proportional to concentration, which is why quantitative work uses absorbance.

Why is a blank measured in UV–visible spectroscopy?

A blank containing everything but the analyte corrects for absorption and reflection by the solvent, cuvette, and reagents, so the measured absorbance reflects only the analyte.