Flame Atomic Absorption Spectrometry

The technique of flame atomic absorption spectroscopy (FAAS) requires a liquid sample to be aspirated, aerosolized, and mixed with combustible gases, such as acetylene and air or acetylene and nitrous oxide. The mixture is ignited in a flame whose temperature ranges from 2100 to 2800 ^oC.

During combustion, atoms of the element of interest in the sample are reduced to free, unexcited ground state atoms, which absorb light at characteristic wavelengths, as shown in figure 3.

The characteristic wavelengths are element specific and accurate to 0.01-0.1nm. To provide element specific wavelengths, a light beam from a lamp whose cathode is made of the element being determined is passed through the flame. A device such as photonmultiplier can detect the amount of reduction of the light intensity due to absorption by the analyte, and this can be directly related to the amount of the element in the sample.

Features of Atomic Absorption Instrumentation Hardware

Figure 1. Flame absorbrion spectrometer with attached graphite furnace.

Figure 1 : an atomic absorption spectrometer. This instrument in particular is designed to operate either with a flame or with a graphite furnace.

The graphite furnace is additionally equipped with an auto sampler.

Flame atomic absorption hardware is divided into six fundamental groups that have two major functions: generating atomic signals and signal processing. Signal processing is a growing additional feature to be integrated or externally fitted to the instrument. The instrument parts are shown in figure 2

Figure 2. schematic of basic instrumental parts of atomic absorption spectrometer

Figure 3. Cathode lamp.

A cathode lamp (1), shown in figure 3, is a stable light source, which is necessary to emit the sharp characteristic spectrum of the element to be determined. A different cathode lamp is needed for each element, although there are some lamps that can be used to determine three or four different elements if the cathode contains all of them. Each time a lamp is changed, proper alignment is needed in order to get as much light as possible through the flame, where the analyte is being atomized, and into the monochromator.

Figure 4. Atom cell.

The atom cell (2), shown in figure 4, is the part with two major functions: nebulization of sample solution into a fine aerosol solution, and dissociation of the analyte elements into free gaseous ground state form. Not all the analyte goes through the flame, part of it is disposed as shown in the figure.

As the sample passes through the flame, the beam of light passes through it into the monochromator (3). The monochromator isolates the specific spectrum line emitted by the light source through spectral dispersion, and focuses it upon a photomultiplier detector (4), whose function is to convert the light signal into an electrical signal.

The processing of electrical signal is fulfilled by a signal amplifier (5). The signal could be displayed for readout (6), or further fed into a data station (7) for printout by the requested format.