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Atomic absorption spectroscopy is largely used for quantitative purposes often in the measurement of trace levels of metals in pollution studies, forensic studies and in alloy analysis. Quantitative measurements can be made employing either absorption peak heights or peak areas. At low concentrations both systems will give reasonable linear responses but at higher concentrations peaks areas are linear over a wider sample concentration range although still not completely linear. At still higher concentrations two analyses are often carried out the first identifying the approximate sample concentration followed by a second analysis employing an internal standard the concentration of which is close to that estimated for the sample. The types of response that can be expected are shown in figure 24.
Figure 24. Peak Height and Peak Area Response to Sample Concentration Compared with a Truly Linear Response
If the calibration graph is sensibly linear, a simple procedure can be employed for the analysis. Two identical volumes (VS) of the sample ((x) and (y)) are taken and a small volume (VSt) of a standard solution (concentration (CSt) in the solvent added to sample (x) and the same volume of solvent only added to sample (y). Then if the two samples are analysed giving absorbance values (Sx) and (Sy), then the concentration of the sample (CS) is given by,
A number of other procedures are available that can be employed, some more suitable for one particular spectrometer system than another but the manufacturer normally supplies a range of basic analytical routines appropriate for the instrument concerned.
There are also many sources of interference that can affect the quality of the analysis about which the chemist should be aware. Employing flame atomic absorption (Flame AA) techniques any difference between the viscosity, surface tension and density of the sample and the reference solution can affect the nebulization efficiency and, thus, the number of free atoms in the flame. The presence of certain volatile organic solvents can improve nebulization and increase the flame temperature. However, a major problem can arise from the presence of less volatile or involatile organic material as these can easily raise background emission and cause variations in the flame temperature. If a Graphite Furnace or one of its modifications is used, however, the sample is placed directly in the furnace without Atomization and the above interference sources are less important. Solvents such as xylene , isobutyl ketone and hexane have been shown to be sources of interference even in Graphite Furnace systems. In general, the physical nature of the sample and reference should be matched as close as possible.
In Flame AA the formation of thermally stable oxides, carbides and nitrides and the interaction of the analyte with anions and cations are common causes of chemical interferences. These effects can be reduced or avoided by raising the ionization temperature to dissociate the less volatile substances, inclusion of the interfering substances in both the reference and analyte solutions and the use of appropriate reagents to render the thermally stable materials more volatile. However, the use of electrothermal methods such as the Graphite Furnace to produce Atomization again reduces, and in most cases eliminates, most chemical sources of interference.
of the analyte will reduce the number of free
available for excitation. This effect can sometimes be reduced by
lowering the Atomization temperature or by adding an excess of
ionization suppressor (i.e.
an easily ionized element) to the standard reference and sample
solutions. Again, the use of electrothermal methods such as the
Graphite Furnace to produce Atomization reduces the effect of
ionization interference. The addition of easily ionized elements to
the sample and reference solutions to reduce ionization when
employing Electrothermal Atomization may well cause loss of
About the Author
RAYMOND PETER WILLIAM SCOTT was born on June 20 1924 in Erith, Kent, UK. He studied at the University of London, obtaining his B.Sc. degree in 1946 and his D.Sc. degree in 1960. After spending more than a decade at Benzole Producers, Ltd. Where he became head of the Physical Chemistry Laboratory, he moved to Unilever Research Laboratories as Manager of their Physical Chemistry department. In 1969 he became Director of Physical Chemistry at Hoffmann-La Roche, Nutley, NJ, U.S.A. and subsequently accepted the position of Director of the Applied Research Department at the Perkin-Elmer Corporation, Norwalk, CT, U.S.A.
In 1986 he became an independent consultant and was appointed Visiting Professor at Georgetown
University, Washington, DC, U.S.A. and at Berkbeck College of the University of London; in 1986 he retired but continues to write technical books dealing with various aspects of physical chemistry and physical chemical techniques. Dr. Scott has authored or co-authored over 200 peer reviewed scientific papers and authored, co-authored or edited over thirty books on various aspects of physical and analytical chemistry. Dr. Scott was a founding member of the British chromatography Society and received the American Chemical society Award in chromatography (1977), the M. S. Tswett chromatography Medal (1978), the Tswett chromatography Medal U.S.S.R., (1979), the A. J. P. Martin chromatography Award (1982) and the Royal Society of Chemistry Award in Analysis and Instrumentation (1988).
Dr. Scott’s activities in gas chromatography started at the inception of the technique, inventing the Heat of Combustion Detector (the precursor of the Flame Ionization Detector), pioneered work on high sensitivity detectors, high efficiency columns and presented fundamental treatments of the relationship between the theory and practice of the technique. He established the viability of the moving bed continuous preparative gas chromatography, examined both theoretically and experimentally those factors that controlled dispersion in packed beds and helped establish the gas chromatograph as a process monitoring instrument. Dr. Scott took and active part in the renaissance of liquid chromatography, was involved in the development of high performance liquid chromatography and invented the wire transport detector. He invented the liquid chromatography mass spectrometry transport interface, introduced micro-bore liquid chromatography columns and used them to provide columns of 750,000 theoretical plates and liquid chromatography separations in less than a second. Dr. Scott has always been a “hands-on” scientist with a remarkable record of accomplishments in chromatography ranging from hardware design to the development of fundamental theory. He has never shied away from questioning “conventional wisdom” and his original approach to problems has often produced significant breakthroughs.