This technique is based on the fact that when atoms, ions or ion complexes of an element in the ground state are atomised in a flame, they absorb light at the characteristic wavelength of that element. If the absorption process takes place in the flame under reproducible conditions, the absorption is proportional to the number of absorbing atoms.
The measurement of the absorption of radiation by the atomic vapour of the element generated from a solution of that element is the basis of atomic absorption spectrometry. The determination is carried out at the wavelength of one of the absorption lines of the element concerned. The assay is done by comparing the absorbance of the test solution with that of the reference preparation.
An atomic absorption spectrophotometer consists of an emission source that provides the characteristic spectral line of the element such as a hollow-cathode discharge lamp, a monochromator to select the required resonance line, a system for introducing the sample solution into a flame and a detector system.
Since the radiation to be absorbed by the element in the test solution is usually of the same wavelength as that of its emission line, the element in the hollow-cathode lamp is the same as the element to be determined and usually a different lamp is used for each element.
The method of introducing the substance to be analysed depends on the type of atomic generator used. In flame atomic absorption, the sample is nebulised and water is the solvent of choice for preparing the test and reference solutions.
Organic solvents may also be used if precautions are taken to ensure that the solvent does not interfere with the stability of the flame. In furnace atomic absorption, the sample may be introduced as a solution in water or in an organic solvent. The atomic vapour may also be generated outside the spectrophotometer as in the case of mercury vapour generator or hydride vapour generator.
The manufacturer's instructions for the operation of the instrument should be strictly followed. Unless otherwise directed in the individual monograph, one or the other of the following methods may be used. In Method A, measurements are made by comparison with solutions containing a known amount of the element being analysed by means of a calibration graph and in Method B comparison is made by means of progressive addition of the reference solution of the element being analysed.
Method A - Prepare the solution of the substance under examination (test solution) as directed in the monograph. Prepare not fewer than three standard solutions of the element to be determined, covering the concentration range recommended by the manufacturer of the instrument for the element to be determined and including the expected value in the test solution. Any reagent used in the preparation of the test solution should be added to the standard solutions in the same concentration. After calibrating the instrument as directed above, introduce each standard solution into the flame three times, and record the steady reading, washing the apparatus thoroughly with water after each introduction. Between each measurement a blank solution should be aspirated and the reading should be allowed to return to zero level. If a furnace is used, it is fired between readings.
Prepare a calibration curve by plotting the mean of each group of three readings against the concentration of the reference solution and determine the concentration of the element to be determined from the calibration graph.
Method B - Place in each of not fewer than three similar volumetric flasks equal volumes of the test solution as directed in the monograph. Add to all but one of these flasks a measured amount of the specified standard solutions containing steadily increasing amounts of the element being determined. Dilute the contents of each flask to the required volume with water.
After calibrating the spectrometer as directed above, record the reading of each solution three times. Plot the mean of the readings against concentration of a graph the axes of which intersect at zero added element and zero reading. Extrapolate the straight line joining the points until it meets the extrapolated concentration axis. The distance between this point and the intersection of the axis represents the concentration of the element being determined in the solution of the substance under examination.
It is advisable to make a stock solution of higher concentration for the reference substance and then dilute it successively to get standard solutions of different concentrations. Care should be taken to avoid manual errors while making dilutions.
NOTE - For the purpose of this Appendix, water refers to deionised purified water distilled immediately before use.