AAS is developed in the 1950s by Sir Alan Walsh and his team at CSIRO. Instrumental techniques can detect the concentrations of ions or molecules at very low levels and with great accuracy. It is used to detect and measure the concentration of metals in a sample. Atomic absorption spectroscopy (AAS) is a sensitive technique that can measure the concentrations down to parts per billion, ppb) of metals in various materials.
Different metals vaporised in a flame absorb light of specific frequencies.
Hollow cathode lamps emit light of specific frequencies for each metal that forms the cathode.
This light is absorbed by atoms of the element to be measured.
To measure the concentration of a specific metal in a sample, a hollow cathode lamp specific to -'pat metal is used (e.g. if copper is to be eared in a sample, then a hollow copper cathode lamp is used).
A series of diluted standard solutions are separately analysed by spraying them into the flame of the burner.
Light from the hollow cathode lamp is passed through the flame and is absorbed by the hot atoms in the flame. The degree of light absorption is proportional to the concentration of the metal in the flame.
Photomultiplier tubes detect the intensity of light transmitted for each sample and compare this with a control that contains no atoms of that element. The degree of absorption of light (absorbance, A) is then determined automatically.
A solution of the sample to be analysed is now sprayed into the flame of the burner and the intensity of light absorbed determined.
The concentration of metal in the sample can be determined from a calibration graph of the diluted standards.
A sample, thought to contain a metal ion, is heated in a flame. The species in the sample are converted into bases.
If light of a frequency known to be absorbed by this metal ion passes from the light source through the heated sample, the ion in the sample will absorb some of this light.
The proportion of the light energy absorbed by the sample (the absorbance) is proportional to the concentration of the metal ion in the sample. The instrument must be calibrated for each metal ion being tested, and a separate light source must be used for each metal ion.
Uses of atomic absorption spectroscopy
Micronutrients and trace elements
Before the development of AAS in the 1950s, it was difficult for scientists to determine the levels of micronutrients in plants and animals. Their standard chemical techniques were not sensitive enough to measure concentrations in the range 1-100 ppm.
Atomic absorption spectroscopy is an important technique in the determination of the levels of these micronutrients or essential trace elements in the soil and in living things.
A deficiency of essential trace elements in our diets leads to severe health problems. Blood and urine tests can quickly reveal the concentrations of these elements in our bodies.
There are about 15 trace elements that are essential for animal life. These include: Mn, Co, Cu, Zn, Mo, W and I. Zinc, for example, is needed for the metabolism of amino acids and in energy production. Manganese is required for blood clotting and is also involved in carbohydrate and fat metabolism, while copper is needed for the production of enzymes involved in oxidation reactions.
Atomic absorption spectroscopy is also very useful in monitoring the concentration of heavy metals in polluted water and soil as well as their levels in food.
Metals such as mercury, lead and cadmium are toxic to the body and their presence in the water or food we consume will lead to serious health problems.
Seafoods such as oysters are filter feeders and they tend to bioaccumulate (i.e. concentrate) heavy metals in their flesh. Higher order consumers in the ocean's food chain also accumulate heavy metals. Fish, for example, cannot contain more than 0.5 ppm of mercury. Consequently, an analysis of such foods will establish whether it is fit to consume. The analysis is also a first indication of the pollution of the waterway.