As a technique for measuring the concentrations of metallic elements in different materials, atomic absorption spectroscopy (AAS) requires that you first vapourise samples. There are various techniques for doing this:
- Graphite furnace
Each of these methods lends itself to a form of AAS, and there are different specialist instruments for carrying them out.
How Does AAS Work?
During AAS, atoms absorb electromagnetic radiation at a defined wavelength. This produces a measurable signal.
This is because all elements exist on the electromagnetic spectrum, and the atoms in these elements will absorb wavelengths of light relating to their specific characteristics.
Therefore, you can use AAS to determine the parts per million (ppm) of specified metals in the material that you are testing.
AAS is a highly sensitive analysis method and there are various stages to it:
First the sample cell must be vapourised, to cause its atomisation
Next, it is prepared by weighing it then diluting it in a solution
A calibration curve determines the concentration of the sample
The light source is applied, bombarding the sample with ions in a beam passing through a quartz cell
A monochromator selects and transmits a specific wavelength, directing it onto a detector
The detector produces an electrical signal proportionate to the light’s intensity.
The flame aspiration method in AAS involves aspirating, or sucking, a solution into a flame, to vapourise most of it.
The thermal energy from the flame will cause the atom to undergo a transition, into its first excited state.
When atoms make this transition, they absorb some of the light in the beam used in AAS, and the more concentrated the sample solution, the more light it will absorb.
Flame aspiration occurs using a burner and spray chamber (nebuliser). A capillary tube connects the solution to the nebuliser. When this nebulises the solution, smaller drops vapourise in the flame, nebulising only around 1% of the solution.
In the graphite furnace technique for AAS, you vapourise the sample in a graphite-coated furnace.
To do this, you place the sample material in a graphite-coated tube. The coating can also be made of pyrolytic carbon.
Heating this tube will then vapourise and atomise the analyte.
This is a more sensitive method for AAS than flame aspiration, but it is less rapid.
A hydride is a class of chemical compound that combines hydrogen with another element.
In hydride generation atomic absorption spectrometry (HG-AAS), a hydride generator produces a volatile hydride by causing a reaction between a metalloid and sodium borohydride.
This is a vapourisation technique that requires no nebuliser.
As a detection method, HG-AAS is far more sensitive than flame detection and graphite furnace methods. By separating the hydride, the introduction of the analyte sample into the AAS process is made more efficient. And users can meet lower detection limits.
Typically, in HG-AAS instruments, you place the sample in a valve, where the sample mixes with sodium borohydride. This creates the hydride, to which the instrument adds argon, to carry the vapour containing the analyte to enter the quartz cell on the instrument for analysis.
Essentially, the HydrEA technique for AAS combines both the hydride and graphic furnace methods. This is an innovative AAS instrument from Analytik Jena, which integrates the two techniques.
It involves generating the metal hydride by conventional means, but then carrying it in a gas stream into a graphite tube, rather than a quartz cell.
The tube is pre-coated with iridium, to absorb the hydride flowing into it. The instrument then runs an optimised temperature programme, via the graphite furnace, and produces data for evaluation.
The advantages of this innovative AAS method are that it is largely immune to interference, and it enables greater automation of the process.
The HydrEA method provides a facility for enriching elements such as selenium and arsenic, unlike traditional hydride techniques.
This form of AAS is meeting increasing demands for accurate detection of ultra-trace levels of toxic elements in environmental testing.
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