Atomic absorption spectroscopy(AAS)
Author: Hans-Jürgen Schwarz
English Translation by Sandra Leithäuser
back to Analysis of Salts
Abstract[edit]
Atomic absorption spectroscopy[1] is a method that allows rapid and accurate quantitative determinations of many elements. Since the method relies on the specific energy absorption by free atoms, it can usually be used without prior separation or isolation steps of other elements present in the sample. The quantity of sample needed for analysis is low. Element concentrations of a millionth (ppm) or one billionth part (ppb) of the sample can be detected.
Introduction[edit]
Atomic absorption spectroscopy (AAS)is a spectroanalytical procedure that delivers a rapid and very accurate quantitative determination of nearly all elements. Since the method is based on energy absorption by free atoms, it can be used without previous separation or isolation from other elements present in the sample. The required sample size is small and concentrations ranging from one part per million (ppm) to one part per billion (ppb) can be detected in the sample.
Basic principle[2]
Atoms can be excited to emit their characteristic radiation. Equally, an atom is able to absorb light of a similar spectrum. If a cloud of atoms is irradiated, the light beam’s intensity decreases in proportion to the density of the cloud. AAS utilizes absorption spectrometry to assess the concentration of an analyte in a sample. Hollow cathode lamps (HCL) are the most common radiation sources in AAS. Inside the spectrometer, the hollow cathode lamp excites the radiation spectrum of the element of interest. The quantity of an element can be determined by selecting a suitable absorption line and measuring the radiation intensity with a photomultiplier tube.
In order to make a quantitative analysis possible, the elements to be determined have to be atomized. For this purpose the sample is often dissolved. The solute is transformed into an aerosol and introduced into a spray chamber. Then a defined amount of the solution is atomized, for example in a stream of combustible gas under standardized conditions. First the solvent evaporates (desolvation), then the substance is transferred to the gaseous phase (vaporization), the molecules are dissociated into free atoms (atomization), then they possibly convert into ions/ gaseous ions (ionization). The wavelength of the light beam (guided through the flame) is selected to allow for the observation of the maximum absorption. Furthermore, the most appropriate concentration range of the of the aerosol has to be determined experimentally (linearity of the calibration curve). In order to achieve this, standard solutions of known concentrations can be measured. AAS is a relative method, i.e. the quantitative analysis of the element of interest in the sample, is carried out by comparison with standard solutions.
Instead of the excitation in the flame, the electro-thermal atomization of the sample (heating to about 2000°C), inside a graphite tube is also possible (electrothermal/ ET AAS).
In atomic absorption spectroscopy the atoms are detected in their ground state not in their excited state, like in atomic emission analysis (AES), . Because far more atoms are configured in the ground state (of every detectable material?), this method is considerably more sensitive in detecting elements than AES .
The graphite tube technique, in contrast to the flame technique, has a better sensitivity and better detection limits by two to three orders of magnitude. Also, it is possible to analyze micro-samples and even solid micro-samples.
Advantages:
In the analysis of salt forming ions, sodium, potassium, calcium and magnesium can be determined quantitively.
Disadvantage:
Anions cannot be detected. The method is more time consuming than comparable methods such as ICP. Not a non-destructive method.
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