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Background interference is caused by either, non-specific absorption arising from light scattering caused by solid particles or liquid droplets in the atomizing cell or, by light absorption caused by molecules or radicals originating in the sample matrix. To compensate for background absorption the background absorption is usually measured by separate experiment and subtracted from the absorption of the sample solution. One method for eliminating background absorption is by exploiting the Zeeman Effect.
The Zeeman effect is the splitting of spectral lines into several polarized components as a result of the effect of an applied magnetic field. In fact, it is analogous to the Stark effect where a spectral line is split into a number of separate lines as a result of the application of an electrostatic field. On the application of the magnetic field a central line appears at the same wavelength as the original line (the π line) having half the intensity of the original line. On either side of the π line appears two other lines (the σ± lines) having one quarter of the intensity of the original line. The π line is linearly polarized with the electric vector parallel to the magnetic field and the σ± lines are circularly polarized (rotating in opposite directions) about the lines of force with their electric vector linearly polarised at right angles to the direction of the magnetic field. In certain elements (e.g. sodium and silver ) the π and σ± are further split into a number of lines; the sodium D1 line splits into four lines (2 π lines and 2 σ± lines) and the D2 line of the sodium into 6 lines (2 π lines and 4 σ± lines). The use of the Zeeman Effect in Atomic Spectroscopy is based on the fact that background absorption due largely to molecular scattering is not affected by the presence of a magnetic field.
A magnetic field can be applied to either the radiation source (source-shift Zeeman background correction) or to the atomizing cell (analyte-shift Zeeman background correction). The field can be applied transverse or longitudinal to the optical path. The polarizer can be placed before or after the atomizing cell. In source-shift Zeeman background correction the spectra; source line is split into π lines and σ± lines and on passing through the atomized sample the π line is absorbed by both sample and back ground whereas the σ± components are only absorbed by the background. The technique can be employed with either flame or electrothermal ionization techniques.
In analyte-shift Zeeman background correction the atomizing cell is placed in the magnetic field. A diagram of an analyte-shift Zeeman background correction instrument is shown in figure 24.
Light from the source passes through a polarizer that splits the radiation into two linearly polarized beams, parallel and perpendicular to the direction of the magnetic field. These beams pass alternately through the atomic vapour in the atomizing cell. During the first cycle the polarized light parallel to the magnetic field is absorbed by the sample and attenuated by the background. In the second cycle the polarized light perpendicular to the magnetic field passes through the sample cell and only the background attenuation is measured. Thus, from the two sets of data the true exclusive absorption by the sample can be determined.
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.