Calibration A calibration curve is used to determine the unknown concentration of an element – eg lead – in a solution. The instrument is calibrated using several solutions of known concentrations. A calibration curve is produced which is continually rescaled as more concentrated solutions are used – the more concentrated solutions absorb more radiation up to a certain absorbance. The calibration curve shows the concentration against the amount of radiation absorbed (Fig. 8(a)). The sample solution is fed into the instrument and the unknown concentration of the element – eg lead – is then displayed on the calibration curve (Fig. 8(b)).
Other chemicals that are present in the sample may affect the atomisation process. For example, in flame atomic absorption, phosphate ions may react with calcium ions to form calcium pyrophosphate. This does not dissociate in the flame and therefore results in a low reading for calcium. This problem is avoided by adding different reagents to the sample that may react with the phosphate to give a more volatile compound that is dissociated easily. Lanthanum nitrate solution is added to samples containing calcium to tie up the phosphate and to allow the calcium to be atomised, making the calcium absorbance independent of the amount of phosphate. With electrothermal atomisation, chemical modifiers can be added which react with an interfering substance in the sample to make it more volatile than the analyte compound. This volatile component vaporises at a relatively low temperature and is removed during the low and medium temperature stages of electrothermal atomisation.
Atomic absorption spectrometry is sometimes used for investigating unusual problems. One such case was that of a seriously ill baby whose symptoms could not be explained. Lead is a toxic element that can cause poisoning in children. A baby was brought to a hospital suffering from vomiting and stomach pains, and was very drowsy. There were no obvious reasons or signs why the child should be ill. As part of the routine tests performed, the lead level in a blood sample from the child was measured using electrothermal atomisation AAS. The lead level was higher than normal and there was no known source for the lead. However, the parents explained that the child had been chewing the painted wood on its cot. The paint was also examined by dissolving it in nitric acid and then using flame AAS to find out the lead content. A very high level was found. Other paints in the baby’s bedroom were found to have low lead levels. This identified the cot paint as the source of lead in the baby. The baby’s cot was old and had been painted when leaded paint was very common. This type of paint is now banned from household use and by law all painted toys must be examined for lead and other toxic metals to make sure that they are safe for small children.
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