Spectroscopy is the study of the interaction between electromagnetic radiation and matter. It is one of the most powerful tools in analytical chemistry, providing information about molecular structure, electronic configuration, and elemental composition across virtually every sample type. The electromagnetic spectrum spans from high-energy gamma rays (λ < 10 pm) through X-rays, ultraviolet (UV), visible (Vis), infrared (IR), microwaves, to low-energy radio waves (λ > 1 m). Each region probes different molecular or atomic transitions.
When radiation interacts with matter, three principal phenomena can occur: absorption (the molecule takes up photon energy and transitions to an excited state), emission (an excited species relaxes by releasing a photon), and scattering (radiation is deflected with or without energy change — Rayleigh and Raman scattering, respectively). Absorption spectroscopy is the most widely used, and the relationship between absorption and concentration is governed by the Beer-Lambert law: A = εbc, where A is absorbance, ε is the molar absorptivity, b is the path length, and c is the concentration.
Molecular spectroscopy involves transitions between quantized energy levels within molecules — electronic (UV-Vis), vibrational (IR, Raman), and rotational (microwave). Atomic spectroscopy involves transitions of electrons in free atoms, typically requiring atomization at high temperatures (flame, furnace, or plasma) to break molecular bonds. Atomic absorption (AAS), atomic emission (AES, ICP-OES), and atomic fluorescence (AFS) are the principal atomic techniques.
All spectroscopic instruments share common components: a radiation source (continuum or line source), a wavelength selector (monochromator or filter) that isolates the analytical wavelength, a sample holder (cuvette, flame, or plasma), and a detector (photomultiplier tube, photodiode, or charge-coupled device) that converts light intensity into an electrical signal. The signal-to-noise ratio (SNR) determines the smallest detectable signal and can be improved through signal averaging, modulation, and lock-in amplification. Modern instruments often incorporate autosamplers, computer-controlled data acquisition, and advanced chemometric software for spectral deconvolution.
