Ultraviolet–visible spectroscopy

UV-Vis spectroscopy relies on the absorption of photons that promote electrons from a lower to a higher molecular orbital, typically π→π* or n→π* transitions. The Beer–Lambert law, A = εcl, relates absorbance (A) to molar absorptivity (ε), path length (l), and concentration (c), enabling quantitative analysis.¹

Instruments consist of a light source (deuterium lamp for UV, tungsten-halogen for visible), a monochromator, sample holder, and detector (photomultiplier or diode array). Modern instruments often use double-beam designs to correct for source fluctuations and solvent effects. Samples are typically in solution, but solid samples can be measured using diffuse reflectance or integrating spheres.

UV-Vis is a workhorse in analytical laboratories for determining concentrations of analytes in pharmaceuticals, environmental monitoring, and food quality control. It is also used to study reaction kinetics by monitoring absorbance changes over time, and to determine acid–base equilibria via pH-dependent spectra.²

In biochemistry, UV-Vis is used to quantify proteins and nucleic acids (e.g., A260/A280 ratio for protein purity), and to assay enzyme activities. In materials science, it characterizes band gaps of semiconductors and the optical properties of nanoparticles, such as the surface plasmon resonance of gold nanoparticles.

UV-Vis is relatively insensitive compared to techniques like fluorescence or mass spectrometry, with detection limits typically in the micromolar range. It requires a chromophore; colorless compounds without UV absorption need derivatization or alternative methods. Scattering from turbid samples can distort spectra, and stray light can cause deviations from Beer's law at high absorbance.

Solvent choice is critical: many solvents absorb in the UV region, limiting the usable wavelength range. For example, acetone and benzene absorb strongly below 300 nm, so quartz cuvettes and UV-transparent solvents like water or methanol are preferred. Temperature and pH can also affect spectra, requiring careful control.

Beyond routine analysis, UV-Vis has niche applications such as in forensic science for ink and fiber analysis, and in art conservation to identify pigments and dyes. It is also used in astronomy to study the composition of celestial objects, though Earth's atmosphere blocks most UV, necessitating space-based observatories.

Historically, the technique evolved from early colorimetry; the first commercial UV-Vis spectrophotometer was introduced by Beckman in 1941 (the DU model), which became a standard in laboratories. A lesser-known fact is that UV-Vis can be used to determine the pKa of compounds by measuring absorbance as a function of pH, a method known as spectrophotometric titration. Additionally, the technique is used in high-performance liquid chromatography (HPLC) as a detection method, and in the pharmaceutical industry for dissolution testing of tablets.

Chromophore

A part of a molecule responsible for its color, typically containing conjugated π-electron systems.

Molar absorptivity

A measure of how strongly a chemical species absorbs light at a given wavelength, expressed in L·mol⁻¹·cm⁻¹.

Beer–Lambert law

A linear relationship between absorbance and concentration of an absorbing species, A = εcl.