Atomic Absorption (Graphite Furnace)

Atomic absorption spectroscopy (AAS) is the summary of spectroscopic processes, which can be used for the quantitative and qualitative analyses of elements in watery solutions and in solids. Basically, atomic absorption spectroscopy represents a part of analytic chemistry.

Classification of atomic absorption spectrometry

Atomic absorption spectrometry can be classified as follows: flame atomic absorption spectrometry (F-AAS), graphite furnace atomic absorption spectrometry with electrothermal heating (GF-AAS), cold vapour atomic absorption spectrometry (CV-AAS) and atomic absorption spectrometry with continuum source and high-resolution Echelle double monochromator (HR-CS-AAS).

Functionality of an atomic absorption spectrometer

For AAS, a light source emits light of different wavelengths and a defined intensity. An atomisation device in which the components of an analyte are atomised, i. e., converted into the state of single, excitable atoms, is in the optical path of the atomic absorption spectrometer. This atomisation can be achieved either by a flame of gas (a mixture of acetylene/air or acetylene/nitrogen oxide), into which the analyte solution will be sprayed, or by strong heating in a graphite tube. The intensity of the attenuated beam of light is measured behind the atomisation device and compared to that of the unattenuated light. Detection follows, observing how much of the irradiated light of a defined wavelength has been absorbed by the element of the analyte. The Lambert-Beer law applies here, stating that the concentration of the analyte is directly proportional to the attenuation of the irradiated light. The light energy that was absorbed during this process is emitted again by the atom that had been excited this way. This is the so-called fluorescence.

Light source of an atomic absorption spectrometer

Different light sources can be applied in atomic absorption spectrometry. The hollow-cathode lamp consists of a cathode which is composed of the element of the analyte. Superlamps dispose of another, additional cathode. The electrodeless discharging lamp is based on the principle of gas discharge.

Atomisation with graphite tube atomic absorption spectrometry

Graphite, being conductive, is heated when applying voltage, due to the electric resistance of the graphite. The sample solution is carried bit by bit into the graphite tube and is heated in several steps. The nature of the analyte determines the programme. The standard programme is as follows. In drying 1, the oven is heated within 30 seconds from 90 to 130 °C. The sample is squeezed and dried. Drying 2 applies 20 seconds of heating up to 400 °C. At this stage, the sample is completely dried, and the crystal water is removed. Pyrolysis removes organic components, when the oven is heated within 30 seconds from 400 to 1500 °C. During atomisation, the sample is atomised for about five seconds at a temperature from 1500 to 2500 °C. The last step of this standard programme is designated cleaning. The oven is for three seconds heated up to about 2500 °C, to atomise the residuals of the sample.

Literature

• http://de.wikipedia.org/w/index.php?title=Atomspektroskopie&oldid=85044962 (called: 30.03.11).

• http://de.wikipedia.org/w/index.php?title=Atomabsorptionsspektrometrie&oldid=86725842 (called: 30.03.11).

• http://www.chemgapedia.de/vsengine/vlu/vsc/de/ch/16/anac/aaseinf.vlu.html (called: 30.03.11).

• http://www.chemgapedia.de/vsengine/vlu/vsc/de/ch/16/anac/aasanwend.vlu.html (called: 30.03.11).