Atomic Absorption Spectroscopy (AAS) is an analytical technique used to determine the concentration of metallic elements in a sample. It relies on the principle that free atoms in the ground state can absorb light at specific wavelengths characteristic of each element.
Principle of AAS
A sample is atomized, converting the analyte into free, neutral atoms in the gas phase. A hollow cathode lamp (HCL) emits light at a wavelength specific to the element being measured, and the free atoms absorb this light while the reduction in intensity is measured by a detector. The absorbance is proportional to the concentration of the element according to the Beer-Lambert Law.
Instrumentation
An AAS instrument consists of a hollow cathode lamp containing the same metal as the target analyte, emitting its characteristic line spectrum, an atomizer such as a flame (air-acetylene or nitrous oxide-acetylene) for liquid samples or a graphite furnace for trace analysis, a monochromator that isolates the specific wavelength of interest from other emission lines, and a photomultiplier detector that measures the intensity of transmitted light.
Atomization Techniques
Three atomization techniques are commonly used. Flame AAS (FAAS) aspirates the sample solution into a flame where it is desolvated, atomized, and analyzed, with detection limits from ppm to ppb. Graphite Furnace AAS (GFAAS) places a small sample volume in a graphite tube that is heated stepwise to atomization, achieving ppb to ppt detection limits. Hydride Generation AAS is used for elements like As, Se, and Hg, where volatile hydrides are formed and transported to the atomizer.
Interferences
Several types of interferences can affect AAS measurements. Spectral interferences arise from overlapping absorption lines from other elements and are corrected by using alternative wavelengths or background correction (Zeeman or deuterium lamp). Chemical interferences involve the formation of refractory compounds that resist atomization and are minimized by adding releasing agents such as lanthanum for calcium. Matrix effects arise from differences in viscosity and surface tension between standards and samples that affect nebulization efficiency.
Applications
AAS is used for environmental analysis of heavy metals (Pb, Cd, Hg, As) in water, soil, and air, clinical testing of trace elements (Fe, Cu, Zn, Mg) in blood and urine, food safety monitoring of toxic metals in agricultural products and seafood, and quality control of metals in alloys, ores, and pharmaceutical raw materials.
