spectroscopyabsorption

Absorption spectroscopy is a family of techniques that measure how a sample absorbs electromagnetic radiation as a function of wavelength. Absorbed photons can promote electronic transitions in atoms or molecules, typically in the ultraviolet to visible range, or vibrational and rotational transitions in the infrared and microwave regions. The resulting absorption spectrum serves as a molecular fingerprint and can be used to identify substances and quantify their concentration.

Quantitative use relies on the Beer–Lambert law: A = εlc = log10(I0/I), where A is absorbance, ε is molar absorptivity, l is the optical path length, and c is the concentration. Within a linear range, absorbance is proportional to concentration, enabling calibration and quantitative analysis.

Instrumentation generally includes a light source, a wavelength-selective element such as a monochromator, a sample holder, and a detector, with data processed to produce absorbance versus wavelength. Common configurations include UV–visible spectrophotometers for electronic transitions, infrared spectrometers for molecular vibrations, and atomic absorption spectrometers for elemental analysis.

Applications span chemical quantification, identification of compounds, purity assessment, and reaction monitoring in chemistry, biology, environmental science, and materials science. Data interpretation focuses on peak positions and shapes, which reflect electronic structure and functional groups, and on peak areas or heights for concentration estimates.

Limitations include the need for measurements within the instrument’s linear range, potential interference from scattering or fluorescence, solvent and temperature effects, and spectral overlap requiring calibration or data processing.