BIOL 380
Tom Huff, Instructor
thuff@gmu.edu

We describe the motion of macroscopic objects using Newton's laws of physics. We can easily measure velocity, acceleration, force, kinetic and potential energies as well as quantify gravitational effects. However, when we look at atomic particles (i.e., protons, electrons, neutrons, etc.), we can no longer describe motion and energy using Newtonian physics. The characteristics of atomic particles are described using the theorems of quantum mechanics.

Quantum mechanics--or quantum chemistry--describes the geometry of atoms and molecules in terms of complex mathematical expressions. It also describes the relative states of atomic matter. The atomic absorption spectrometer uses the principals of quantum chemistry to detect the presence of certain metals (i.e., iron, aluminum, copper, etc.) and determines the concentration of those metals in samples.

All atoms and their components have energy. The energy level at which an atom exists is referred to as its state. Under normal conditions, atoms exist in their most stable states. We refer to that most-stable level as the ground state. Al though we cannot measure the precise energy state for an atom, we can usually measure changes to its energy relative to its ground state.

Certain processes can change the energy state for an atom. For example, adding thermal energy (heat) can cause an atom to increase to a higher energy state. This change in energy is written as DE. We refer to energy states which are higher than the ground state as excited states. In theory, there are infinite excited states, however there are decreasing numbers of atoms from a population that reach higher excited states.

The laws of quantum mechanics tell us that atoms do not increase their energy levels gradually. An atom goes directly from one state to another without going through intermediates. We refer to these "quantum leaps" as transitions. The transition from the ground state (written as E_o) to the first excited state (E₁) requires some form of energy input. This energy is absorbed by the atom. That energy absorption is equal to DE_{0® 1}. When this energy absorption takes place in the presence of ultraviolet light, some of that light will be absorbed. This uv absorption occurs at a specific wavelength.

Each element in the periodic table will have a specific D E that will absorb a specific wavelength of uv light. The relationship between the energy transition and the wavelength (l) can be described by

where h is Planck's constant. Atomic absorption uses this relationship to determine the presence of a specific element based on absorption in a specific wavelength. For example, calcium absorbs light with a wavelength of 422.7 nm. Iron absorbs light at 248.3 nm.

Link to Instrument Design.

Last update: February 18, 1998