Atomic Spectroscopy - Optical Spectroscopy

Optical Spectroscopy

Electrons exist in energy levels within an atom. These levels have well defined energies and electrons moving between them must absorb or emit an energy equal to the difference between them. In optical spectroscopy, the energy absorbed to move an electron to a more energetic level and/or the energy emitted as the electron moves to a lower energy level is in the form of a photon (a particle of light). Because this energy is well-defined, an atom's identity can be found by the energy of this transition. The wavelength of light can be related to its energy. It is usually easier to measure the wavelength of light than to directly measure its energy.

Optical spectroscopy can be further divided into absorption, emission, and fluorescence.

In atomic absorption spectroscopy, light is passed through a collection of atoms. If the wavelength of the light has energy corresponding to the energy difference between two energy levels in the atoms, a portion of the light will be absorbed. The relationship between the concentration of atoms, the distance the light travels through the collection of atoms, and the portion of the light absorbed is given by the Beer-Lambert law.

The energy stored in the atoms can be released in a variety of ways. When it is released as light, this is known as fluorescence. Atomic fluorescence spectroscopy measures this emitted light. Fluorescence is generally measured at a 90° angle from the excitation source to minimize collection of scattered light from the excitation source, often such a rotation is provided by a Pellin-Broca prism on a turntable which will also separate the light into its spectrum for closer analysis. The wavelength once again tells you the identity of the atoms. For low absorbances (and therefore low concentrations) the intensity of the fluoresced light is directly proportional to the concentration of atoms. Atomic fluorescence is generally more sensitive (i.e. it can detect lower concentrations) than atomic absorption.

Strictly speaking, any measurement of the emitted light is emission spectroscopy, but atomic emission spectroscopy usually does not include fluorescence and rather refers to emission after excitation by thermal means. The intensity of the emitted light is directly proportional to the concentration of atoms.