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THE SPECTRO-PAEDIA

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absorbance
adsorption
Atomization
bandwidth
Beamsplitters
bioluminescence
chemiluminescence
chromatography
electroluminescence
electromagnetic
emission
Emissivity
Fluorescence
luminescence
Michelson
monochromators
photo-multiplier
Phosphorescence
photodiodes
photoelectric
photoluminescence
Rayleigh
Raman
spectrofluorometer
spectrometer
spectrophotometer
Spectrum
Transmittance
ultraviolet
Visible
wavelength
Wavenumber
UV Light
Beer's Law
Diode Array
Deuterium Discharge Lamp
Low Pressure Zinc Discharge Lamp
High Pressure Mercury Discharge Lamp
Low Pressure Cadmium Lamp
Tungsten Halogen Lamp
Lens and Window Material in Spectrometers
Cuvettes
Luminescence
Photoluminescence
Fluorescence
Phosphorescence
Bioluminescence
Radioluminescence
Electroluminescence
Fluorescence Reagents
Spectrum
Diffraction Grating
Interferogram
Fourier Transform IR Spectrometer
FT-IR
Halide Disks
Mull Samples
Film Samples for IR Spectroscopy
Light Pipes
Attenuated Total Reflectance Spectroscopy
Multiple Internal Reflectance
External Reflectance
Specular Reflectance
Diffuse Reflectance
Photoacoustic Spectroscopy
Beam Splitter
Raman Scattering
Rayleigh Scattering
Raman Spectroscopy
Atomic Spectroscopy
Atomic Emission Spectroscopy
Atomic Absorption Spectroscopy
The Inductively Coupled Plasma Torch
The Helium Plasma Torch
Emission Spectrometer
Atomic Absorption Spectrometry
Flame Atomic Absorption Spectrometer
Flame AA
Hollow Cathode Lamp
Electrothermal Atomization
Graphite Furnace
L’vov Platform
Electron Paramagnetic Resonance
Zeeman Effect
Continuous Wave
Electron Paramagnetic Resonance
Pulsed EPR
Electron Spin Echo
Multple Resonance Spectroscopy
Magnetic Resonance Spectroscopy
NMR
Precessing
Nucleus Spin Decoupling in NMR
Superconducting Magnets
NMR Microcells
Electron Impact Ionisation
Chemical Ionization
Inductively Coupled Plasma Ionization
Secondary Ion Mass Spectrometry
Fast Atom Bombardment
Plasma Desorption Mass Spectrometry
Laser Desorption Mass Spectrometry
Matrix Assisted Desorption mass Spectrometry
Field Desorption Ionization
Thermospray Ionization
Electrospray Ionization
Atmospheric Pressure Ionization
Particle Beam Interface
Permeable Membrane Interface
Sector Mass Spectrometer
Quadrupole Mass Spectrometer
Ion Trap Mass Spectrometer
Time of Flight Mass Spectrometer
Optical RotationCircular Dichroism
Circularly Polarized Light
Verdet Constant
Faraday Effect
 

Atomic Absorption Spectra

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.

Ionization of the analyte will reduce the number of free atoms 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 sensitivity.

 

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.

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