ATOMIC EMISSION SPECTROMETRY

The technique is used to determine the concentration of certain metallic ions by measuring the intensity of emission of light at a particular wavelength by the vapour of the element generated from the substance. The measurement of the intensity of one of the emission lines of the atomic vapour of the element generated from a solution of that element is the basis of atomic emission spectrometry. The determination is carried out at the
wavelength corresponding to this emission line of the element concerned. The assay is done by comparing the intensity of emission from the test solution with that from reference preparations with known concentrations of the element to be determined.

Apparatus

An atomic emission spectrophotometer consists of an atomic generator of the element to be determined (such as flame, plasma, arc etc), a monochromator and a detector. If a flame is used for generating the vapour, water is the usual solvent 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.

Methods

The manufacturer’s instructions for the operation of the instrument should be strictly followed. The spectrometer should be operated at the prescribed wavelength setting. Introduce  a blank solution into the atomic generator and adjust the instrument reading to zero. Introduce the most concentrated reference solution and adjust the sensitivity to obtain a suitable reading.

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 include 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 flask equal volume 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 amount of the element being determined. Dilute the content 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.