Principle

The principle used in atomic absorption spectroscopy was discovered in 1802 by Wollaston when he observed the "Fraunhofer lines" or absorption lines in the spectrum of the sun, yet this principle was only applied in 1955 by an Australian physicist, Alan Walsh. The principle states that "Matter absorbs light at the same wavelength at which it emits light". Basically this means that atoms in the ground state absorb the same radiation as they emit in the excited state. An atom in the ground state will absorb an amount of energy equal to the energy difference between the energy level of the electron in the excited state and the energy level that the electron occupies in the excited state. 

In Atomic Absorption Spectrometry, the sample solution is first vaporised and atomized in a flame, transforming it to unexcited ground state atoms, which absorb light at specific wavelengths. A light beam from a lamp whose cathode is made of the element in question is passed through the flame. Radiation is absorbed, transforming the ground state atoms to an excited state. The amount of radiation absorbed depends on the amount of the sample element present. Absorption at a selected wavelength is measured by the change in light intensity striking the detector and is directly related to the amount of the element in the sample. 

Process

An unknown sample in a solution is dissolved and sprayed finely, in the presence of suitable conditions, into the flame burner of the atomic absorption spectrometer. An example would be zinc solution. The sample is then converted to atoms in ground, unexcited state by a burner e.g., graphite furnace. A cathode lamp will emit light to reach these electrons. The lamp must contain a cathode of the same element within the sample (e.g. zinc). This is because of the energy required to excite the similar electrons in the sample, hence enabling concentration to be determined. Most spectrometers contain a number of different cathode lamps suitable for various solution samples. 

In the lamp, taking in energy excites electrons. They jump to higher energy levels by taking in a fixed quantum amount of energy. As they fall back down, they emit a fixed amount of light. This light radiates to the ground atoms in the sample solution, under specific conditions. These unexcited electrons absorb the light. It is of a fixed wavelength. As the amount required to excite the electrons in the atoms is fixed, according to the radiated light, the spectrometer can detect the measure of light absorbed. In this way, the concentration of the elements can be calculated, as it is directly proportional to the amount of element present. This is calculated in parts per million (ppm). 

The element is detected by an atomic absorption spectrum, by the light intensity emitted by the sample. This is a series of coloured lines on a dark background, depending on the element, at differing wavelengths. Each element has a unique spectrum, see figure below. 

Applications

This process is employed in both qualitative and quantitative use. AAS is a rapid method for the former, if only a few elements are being tested. However if many elements are of interest the process can be too time consuming and uneconomical. The usual quantitative method brackets the sample's absorption spectrum with that of standard concentrations to produce a linear calibration curve. 

Examples of the applications of AAS include: 

• Analysis of water for metals like lead, mercury and cadmium 

• Drug testing

• Identification of unknown compositions

• Analysis of rocks on space missions   

Photograph of atomic absorption spectrometer, with thanks to http://csep10.phys.utk.edu/astr162/lect/light/absorption.html   

Continuous spectrum, emission spectrum and atomic absorption spectrum (lines should be black). With thanks to http://csep10.phys.utk.edu/astr162/lect/light/absorption.html

By Emma Smith and Ita Shanahan