Describe the use of atomic absorption spectroscopy (AAS) in detecting concentrations of metal ions in solutions and assess its impact on scientific understanding of the effects of trace elements
Hollow-cathode lamp selection
• The light source in the atomic absorption spectrometer is usually a hollow-cathode lamp of the element that is being measured. • So, if the concentration of lead in a water sample is to be determined, the AAS lamp uses a lead cathode. • Specific wavelengths of light characteristic of the element being analysed are generated from this lamp.
Standard solution preparation
• A standard solution of the metal to be analysed is prepared using standard volumetric techniques. • This solution is then diluted systematically to obtain diluted standard solutions.
Aspirating the solutions
• The dilution standards and the unknown solution are sprayed or aspirated in turn (using a nebuliser) into the flame. A flame AAS uses a slot type burner (~1000°C) to increase the total absorbance of light. • Alternatively, the sample can be heated in a graphite furnace. The graphite furnace (~3000°C) is more efficient than the flame method in that it can be used for smaller quantities of material, as well as providing a reducing environment for samples that are readily oxidised.
Measuring light absorption
• As the light beam passes through the vapourised sample, some of the light is absorbed by the hot atoms. A second reference beam bypasses the sample. • The emerging light beams pass through a monochromator which contains a diffraction grating and focussing mirrors. • The light then passes through a narrow slit to select only one of the wavelength bands to be measured. The light is now said to be monochromatic. • The intensity of this selected beam is then measured.
• Photomultiplier tubes are the most common detectors for AAS. They measure the light intensity and convert it into an electrical signal. • The amount of light absorbed relative to the reference beam (measured by the absorbance) is related to the concentration of element in the vapourised sample. The greater the concentration the greater the amount of light absorbed.
• Concentration measurements are usually determined from a calibration curve created with the standards of known concentration. • A control blank that contains only the solvent is also run. This blank should register zero absorbance.
The calcium ion concentration for two samples of water (X and Y) were determined using AAS. The absorbance of some standard solutions was first measured at a wavelength of 422.7 nm. The results are tabulated.
The two samples of water were measured and their absorbances were: AX = 0.2500 and AY = 0.1500
Use this information to:
(a) construct a calibration graph
(b) determine the calcium ion concentration in the water samples.
(a) Calibration graph
(b) Interpolate calcium ion concentration from the graph.
Cx = 45 ppm
Cy = 27 ppm
Trace elements are elements that are required by all living organisms in very small (trace) quantities. In high doses, trace elements can be toxic.
The development of sensitive analytical methods such as AAS allowed chemists to detect metal ions in very low (e.g. ppm or ppb) concentrations. This led to the discovery that living things required some ions at very low levels to maintain their biochemical functions. The old analytical techniques that relied on ‘wet’ methods (e.g. gravimetric or volumetric analysis) were too insensitive to detect such low levels of ions.
AAS allows chemists to conduct analyses very rapidly and in multiple batches. This was not possible with older ‘wet’ methods that were very time consuming. Reliability of results was therefore improved as many repetitions of the measurement could be...