Atomic Absorption Spectroscopy (AAS)

The following is general background for AAS and for specific operational details on the protocol for the P-E 3030B.

Introduction -- Atomic-absorption spectroscopy (AAS) uses the absorption of light to measure the concentration of gas-phase atoms. Since samples are usually liquids or solids, the analyte atoms or ions must be vaporized in a flame or graphite furnace. The atoms absorb ultraviolet or visible light and make transitions to higher electronic energy levels. The analyte concentration is determined from the amount of absorption. Applying the Beer-Lambert law directly in AAS is difficult due to variations in the atomization efficiency from the sample matrix, and nonuniformity of concentration and path length of analyte atoms (in graphite furnace AAS). Concentration measurements are usually determined from a working curve after calibrating the instrument with standards of known concentration.

Light source -- The light source is usually a hollow-cathode lamp of the element that is beingmeasured. Lasers are also used in research instruments. Since lasers are intense enough to excite atoms to higher energy levels, they allow AAS and atomic fluorescence measurements in a single instrument. The disadvantage of these narrow-band light sources is that only one element is measurable at a time. A typical instrument is shown.

Atomizer -- AAS requires that the analyte atoms be in the gas phase. Ions or atoms in a sample must undergo desolvation and vaporization in a high-temperature source such as a flame or graphite furnace. Flame AAS can only analyze solutions, while graphite furnace AA can accept solutions, slurries, or solid samples.

Flame AAS uses a slot type burner to increase the path length, and therefore to increase the total absorbance ( Beer-Lambert law). Sample solutions are usually aspirated with the gas flow into a nebulizing/mixing chamber to form small droplets before entering the flame.

The graphite furnace has several advantages over a flame. It is a much more efficient atomizer than a flame and it can directly accept very small absolute quantities of sample. It also provides a reducing environment for easily oxidized elements. Samples are placed directly in the graphite furnace and the furnace is electrically heated in several steps to dry the sample, ash organic matter, and vaporize the analyte atoms.

Light separation and detection -- AAS use monochromators and detectors for uv and visible light. The main purpose of the monochromator is to isolate the absorption line from background light due to interferences. Simple dedicated AAS instruments often replace the monochromator with a bandpass interference filter. Photomultiplier tubes are the most common detectors for AAS.

Excitation -- A flame provides a high-temperature source for desolvating and vaporizing a sample to obtain free atoms for spectroscopic analysis. In atomic absorption spectroscopy ground state atoms are desired. For atomic emission spectroscopy the flame must also excite the atoms to higher energy levels. The following table lists temperatures that can be achieved in some commonly used flames.

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