Principles of Spectrophotometry and Radiant Energy Interaction

Color and the Visible Spectrum

The eye can discern the visible spectrum.

Spectrometry and Spectrophotometry

In spectrometry and spectrophotometry, the energy incident on a sample is monochromatic radiation (radiant energy of a single wavelength or, for practical reasons, a very narrow band of wavelengths). The transmitted radiation is measured by highly sensitive devices such as photomultiplier tubes, photocells, and thermocouples.

Interaction Between Radiant Energy and Matter

Molecules, ions, or atoms can absorb radiation if the energy of the photons matches the natural frequency of vibration of electrons and/or atoms in molecules. Photons of different energies produce distinct effects on the absorbing material:

• Gamma and Cosmic Rays: Very short wavelengths can lead to nuclear transformations. Energies are on the order of millions of electron volts.
• X-rays: Cause transitions of electrons from the inner layer of the atom. X-ray absorption is independent of the chemical state of the atoms.
• Ultraviolet and Visible Spectrum: Radiation absorption affects outer-shell electrons.
• Infrared Absorption: Alters the vibration of molecules.
• Far-Infrared and Microwave: Changes occur in the rotation of molecules, resulting in energy losses.

Definitions

• Absorbance (A): The base-ten logarithm of the reciprocal transmittance (T). In pure material, the solvent is the reference. A = log(1/T).
• Absorptivity (a): The relationship between absorbance and the product of optical path length (b) and concentration (c). a = A / bc or A = abc. The specific absorbance is the absorbance per unit of optical path (usually in cm) and concentration (g/L).
• Molar Absorptivity (ε): The absorptivity when the concentration is expressed in moles per liter and the light path in cm.
• Power of Radiation (P): Also called radiation flux, this is the amount of energy carried by a beam of radiant energy, measured with photocells or thermocouple detectors.
• Transmittance (T): The ratio between the power of radiation transmitted through a sample (P) and the power of radiation incident on the sample (Po), measured at the same position in the spectrum and with the same slit. T = P / Po.

It is assumed that the radiation beam is incident on flat, parallel sample surfaces at right angles.