Absorption, atomic

Analysis for silicon in the larger laboratories has been revolutionized by the atomic adsorption method. Although equipment represents a large investment, the method can be used for a wide variety of elements and, once samples have been prepared in solution, permits dozens of samples to be run in a few hours. Usually the instrument manufacturers can recommend suitable methods for preparing solutions for analysis. 

Bowman and Wills (285) have recommended specific procedures for silicon. Dissolution of solid samples and preparation of suitable solutions have been described by Terashima (286). Dissolution of mineral samples in H3PO is a convenient method, according to Hofton and Baines (287), especially since it eliminates the background correction when the silicon is dissolved after alkali fusion. Spectral interference of vanadium can be a problem (288). The method is ideal for determining total silicon. 

A wide range of chemical methods and procedures is found in the treatise of Kolthoff and Elving (289). They also give a summary of the chemistry and solubility of silica. Preparation of solutions for analysis, the silicomolybdic acid yellow and blue colorimetric methods including interferences, gravimetric procedures, and special procedures for biological materials are discussed in concise detail. 

Procedures especially suited to silicate rocks, minerals, and refractory silicates and aluminosilicates have been described by Bennett and Reed (290). A history of analytical methods was published by Andersson (291), and three new spectro-photometric procedures were developed. Available chemical methods listed by Meites (292), in addition to the conventional gravimetric and colorimetric methods, also include precipitation of the silicate ion as the cobalt salt, which is then determined by chelometric titration, and as a nitrogen base salt which is titrated with perchloric acid. 

It has been my experience that for research purposes, in addition to the atomic absorption method for total silica, the alkali titration of silica as SiF is most useful for concentrations greater than 0.1%, and the yellow and blue silicomolybdate methods for concentrations down to 1 and 0.1 ppm, respectively. 

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As shown in Table 2.4, atomic absorption is extremely sensitive. It is particularly suited to the analyses of arsenic and lead in gasolines, for sodium in fuel oils (where it is the only reliable method) and for mercury in gas condensates. 

Taking into account the range of wavelength and the intensity of emission beams, certain elements cannot be determined by atomic absorption, such as C, H, 0, N, S, and the halogens. 

With the exception of alkalis, the sensitivity is generally higher than that of atomic absorption (at least flame atomic absorption). Refer to Table 2.4. 

The choice between X-ray fluorescence and the two other methods will be guided by the concentration levels and by the duration of the analytical procedure X-ray fluorescence is usually less sensitive than atomic absorption, but, at least for petroleum products, it requires less preparation after obtaining the calibration curve. Table 2.4 shows the detectable limits and accuracies of the three methods given above for the most commonly analyzed metals in petroleum products. For atomic absorption and plasma, the figures are given for analysis in an organic medium without mineralization. 

This same principle, as indicated earlier, is used in atomic absorption spectroscopy and UV absorption. 

Elemental Analysis Atomic absorption spectrometry X-Ray fluorescence spectrometry Plasma emission spectrometry... 

FLAME ATOMIC EMISSION, FLAME ATOMIC ABSORPTION,... 

ELECTROTHERMAL (FURNACE) ATOMIC ABSORPTION, ARGON INDUCTION COUPLED PLASMA, AND PLASMA ATOMIC FLUORESCENCE... 

Element Wavelength, nm Flame emission Flame atomic absorption Electrothermal atomic absorption Argon ICP Plasma atomic fluorescence... 

From J. A. Dean and T. C. Rains, Standard Solutions for Flame Spectrometry, in Flame Emission and Atomic Absorption Spectrometry, J. A. Dean and T. C. Rains (Eds.), Vol. 2, Chap. 13, Marcel Dekker, New York, 1971. 

The section on Spectroscopy has been retained but with some revisions and expansion. The section includes ultraviolet-visible spectroscopy, fluorescence, infrared and Raman spectroscopy, and X-ray spectrometry. Detection limits are listed for the elements when using flame emission, flame atomic absorption, electrothermal atomic absorption, argon induction coupled plasma, and flame atomic fluorescence. Nuclear magnetic resonance embraces tables for the nuclear properties of the elements, proton chemical shifts and coupling constants, and similar material for carbon-13, boron-11, nitrogen-15, fluorine-19, silicon-19, and phosphoms-31. 

Van Loon, J. C. Analytical Atomic Absorption Spectroscopy. Academic Press New York, 1980. 

A technique is any chemical or physical principle that can be used to study an analyte. Many techniques have been used to determine lead levels. For example, in graphite furnace atomic absorption spectroscopy lead is atomized, and the ability of the free atoms to absorb light is measured thus, both a chemical principle (atomization) and a physical principle (absorption of light) are used in this technique. Chapters 8-13 of this text cover techniques commonly used to analyze samples. 

Finally, analytical methods can be compared in terms of their need for equipment, the time required to complete an analysis, and the cost per sample. Methods relying on instrumentation are equipment-intensive and may require significant operator training. For example, the graphite furnace atomic absorption spectroscopic method for determining lead levels in water requires a significant capital investment in the instrument and an experienced operator to obtain reliable results. Other methods, such as titrimetry, require only simple equipment and reagents and can be learned quickly. 

UV molecular absorption Vis molecular absorption molecular fluorescence IR molecular absorption IR molecular absorption IR molecular absorption atomic absorption molecular fluorescence... 

The atomic absorption spectrum for Na is shown in Figure 10.19 and is typical of that found for most atoms. The most obvious feature of this spectrum is that it consists of a few, discrete absorption lines corresponding to transitions between the ground state (the 3s atomic orbital) and the 3p and 4p atomic orbitals. Absorption from excited states, such as that from the 3p atomic orbital to the 4s or 3d atomic orbital, which are included in the energy level diagram in Figure 10.18, are too weak to detect. Since the... 

Another feature of the spectrum shown in Figure 10.19 is the narrow width of the absorption lines, which is a consequence of the fixed difference in energy between the ground and excited states. Natural line widths for atomic absorption, which are governed by the uncertainty principle, are approximately 10 nm. Other contributions to broadening increase this line width to approximately 10 nm. 

Equation 10.1 has an important consequence for atomic absorption. Because of the narrow line width for atomic absorption, a continuum source of radiation cannot be used. Even with a high-quality monochromator, the effective bandwidth for a continuum source is 100-1000 times greater than that for an atomic absorption line. As a result, little of the radiation from a continuum source is absorbed (Pq Pr), and the measured absorbance is effectively zero. Eor this reason, atomic absorption requires a line source. 

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