Atomic Absorption Spectrometry (AAS)

 

An Atomic Absorption Spectrophotometer measures electromagnetic radiation released or absorbed by atoms.  Every atom emits and absorbs energy at particular wavelengths.  The quantity of atoms present has a direct effect on how much energy is absorbed and released. A spectrophotometer measures the output and analyzes the elemental composition of compounds.

 

When energy passes through atoms, in the case of AAS in a gaseous state, they absorb photons from the energy, generally in the form of light, which causes electrons in the atoms to jump to a higher valence shell – this is known as the shift from ground state to an excited state.  Photons absorbed correspond directly to the atomic absorption spectrum of the compound analyzed.  The resultant electromagnetic radiation is measured in the machine.

 

The source is the light source that emits light at a narrow line source (?) – this narrow line source is important because the width of the source line must be less than the width of the absorption line.

 

The atom cell is a flame run by acetylene or nitrous oxide which excites the sample which is in solution into a gaseous state – the high temperature of the flame, around 3000 C, forces the sample into separate compounds and then into individual atoms.

 

Since it is likely many atoms of different elements are present in a sample, a monochromater is used to isolate emitted EMR into different resonant lines.  The monochromator disperses wavelengths using a fine grating constructed at the sub-nanometer scale.

 

The detector is made from a photomultiplier tube which amplifies the EMR wavelength into an electric signal which is then fed to a computer for analysis.

 

10 to 12 grams of sample is sufficient.  Metals, plant mateirals, biological samples, and chemical products are all fair game. Sub-samples taken from the sample usually are less than one gram.

 

Examples

 

Donais, Mary Kate, Holly Jakubowski, Cindy Lebel

Determination of Tin in Ancient Bronze Coins by Flame Atomic Absorption Spectroscopy. Department of Chemistry, Saint Anselm College.

 

In this article, Donais et al. describe analysis of ancient bronze coins from Crete using Flame Atomic Absorption Spectroscopy.  Since coins were made at unique mints using unique formulas for bronze, it is possible to source and identify coins based on their elemental composition. This method is particularly advantageous because of environmental corrosion of coins making them visually unrecognizable. The authors used FAAS because they had used it previously to look for lead in the same sample – here they look for tin. The tin content of the coins ranged from below the detection limit to 5.49% – they analyzed 2-3 subsamples for each coin to ensure homogeneity of the sample, which can vary, since some Roman mints produced coin of varying quality. If the sub-samples vary above 10%, then they are above their detection limit. 5 of 11 coins showed less than 10% sub-sample variation. However, tin alone cannot be used to source coins, other metals must also be analyzed, and they are currently searching for other analytical methods.

 

 

 

Omolo, Ouma J., Sumesh C. Chhabra, Gathu Nyagah

1997  Determination of Iron Content in different parts of herbs used traditionally for Anaemia treatment in East Africa. Journal of Ethnopharmacology 58:97-102.

 

AAS was used to test the iron content of various parts of eight different plant used in traditional pharmacopeia for the treatment of anemia. Roots, bark, stems, and leaves of certain plants have often been used to treat anemia – this study tests their iron concentrations using AAS. Omolo et al. choose 8 plants and test the roots, stems, and leaves of each of the plants.  Samples were prepared both wet and dry., with significant agreement in the results. They find that while some plants had relatively high levels of iron,  they did not have significantly higher quantities than certain commonly consumed vegetables. Furthermore, the manner in which the plants are commonly prepared, in decoctions, results in relatively low iron concentrations in the beverage.

 

 

 

 

 

 

 

 

Seelenfreund, A., E. Fonseca, F. Llona, L. Lera, C. Sinclaire, and C. Rees

2009  Geochemical Analysis of Vitreous Rocks Exploited during the Formative Period in the Atacama Region, Northern Chile. Archaeometry 51(1): 1- 25.

 

Seelenfreund et al. use a variety of methods to study the geochemical makeup of certain lithic artifacts from Formative period sites in the Atacama Desert of northern Chile. Source samples were collected from around Chile and compared to archaeological specimens using petrography, AAS, ICP-MS and ICP-OES. They find the Linzor lithic source was most commonly used during the Formative.  Their research was focused on finding the source of vitreous dacite and rhyodacite.

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