The Pharmaceutics and Compounding Laboratory
Spectrophotometry as an
Analytical Tool

The Absorption Process

Matter is composed of particles (neutrons, protons, electrons, atoms, molecules, etc.) that are held together by a variety of forces. Neutrons and protons are held together in the nucleus of an atom by very strong nuclear binding forces. Electrostatic forces hold electrons in specific energy levels around the nucleus. Atoms are bonded to each other by forces of attraction within certain allowed energy levels, depending on the nature of the atoms. All of these forces help to define a world in which different particles are held in specific energy levels relative to each other. Each level has a single energy value and is, therefore, quantized.

The energy associated with each of these levels depends on the type of particles being held together. The energy levels of nuclear forces are higher than those of the electrostatic forces that hold electrons in orbit around the nucleus. Even lower energy levels are associated with the bonds between atoms and molecules, while very low energy levels are required to rotate molecules in space.

For most of these particles, there is more than one quantized energy level in which the particle can exist. For example, the outermost electron in a sodium atom normally resides in a 3' orbital, its lowest possible energy level (or ground state). This electron can, however, be excited to an empty 4p orbital, if the atom absorbs the amount of energy exactly equal to the energy difference between the two orbitals. It is this phenomenon that forms the basis of absorption spectroscopy

When matter interacts with an energy source (heat, sound, electricity, light, etc.) some of the energy can be absorbed, causing the particles to be elevated to different energy levels. In some cases, the particles can be excited totally out of the original chemical system holding them in place. This is what happens when sugar absorbs heat energy, which causes the sugar molecules to decompose into carbon dioxide and water. Under certain conditions, though, the amount of energy absorbed can be controlled, and information about the chemical system can be obtained. This is the case in absorption spectroscopy. The table below lists the types of changes that occur during the light absroption process in regions of the electromagnetic spectrum.

Wavelength Region Wavelength Limits Types of Transitions in Chemical Systems with
Similar Energies
Gamma ray 0.01 - 1 A Nuclear proton/neutron arrangements
Ultraviolet 10 - 380 nm Outer-shell electrons in atoms
Bonding electrons in molecules
Visible 380 - 720 nm Same as ultraviolet
Infrared 0.72 - 1000 µm Vibrational position of atoms in molecular bonds
Microwave 0.1 - 100 cm Rotational position in molecules
Orientation of unpaired electrons in an applied
    magnetic field
Radiowave 1 - 1000 m Orientation of nuclei in an applied magnetic field