ІІ.4.2.6. Atomic Absorption Spectroscopy (AAS) :
Atomic absorption spectrometry (AAS) refers to an analytical technique for measuring concentration of chemical elements in a sample based on absorption of radiation energy by a particular chemical element. The method is highly sensitive since it can be employed to determine up to parts per billion in a gram of a sample. The technique relies on the light wavelength absorbed by atoms of specific element within the ultraviolet to visible light region, which corresponds to the amount of energies required to excite electrons from lower energetic state to higher energetic state.
Atomic absorption spectroscopy determines the quantity of an element based on the photons of energy absorbed by the atoms of the test sample. A detector is used to determine the light wavelengths that the sample transmits, which is then compared to the original wavelengths absorbed by the sample. This is followed by integration of the changes in absorbed wavelength to generate an output of peaks that correspond to the absorbed energy at discrete wavelengths. The amount of energy required to emit an electron from the atom is called ionization energy, and it is unique for different chemical element. During the transition of an electron from one state of energy to another, a photon of energy, E is emitted. Atoms of a given element have characteristic spectral line due to emission produced. This implies that each atom has unique wavelength pattern at which energy absorption takes place depending on the unique electronic configuration in the outermost energy level. While this allows for qualitative determination of individual element in a sample, quantitative analysis is determined by calculation using Beer-Lambert law.
According to Sun et al. , atomic absorption spectrophotometry is one of the most frequently used methods in determination of heavy metals in herbal materials. That is, the quantity of absorbing atoms of the sample element is passed through light ray from the source (hollow cathode lamp) for the element analyzed, through numerous atoms of the sample analysed. It is worth noting that the only atoms that absorb transmitted are those similar to ones in the lamp. As pointed out by Beaut and Kerber , reduction in light quantity reaching the detector corresponds to the concentration of sample element.