Spectrophotometry atomic absorption

Atomic absorption (AA) spectrophotometry is used widely in clinical laboratories to measure elements such as aluminum, calcium, copper, lead, lithium, magnesium, zinc, and other metals. 

A technique closely related to flame emission spectrometry is atomic absorption spectrophotometry (AAS) because they each use a flame as the atomizer. We discuss here the factors affecting absorption and because of the close relationship of atomic absorption and flame photometry, we shall make comparisons between the two techniques where appropriate. 

The sample solution is aspirated into a flame as in flame emission spectrometry, and the sample element is converted to atomic vapor. The flame then contains atoms of that element. Some are thermally excited by the flame, but most remain in the ground state, as shown dramatically in Table 17.1. These ground-state atoms can absorb radiation of a particular wavelength that is produced by a special source made from that element (see Sources). The wavelengths of radiation given off by the source are the same as those absorbed by the atoms in the flame. 

Atomic absorption spectrophotometry is identical in principle to absorption spectrophotometry described in the previous chapter. The absorption follows Beer s law. That is, the absorbance is directly proportional to the pathlength in the flame and to the concentration of atomic vapor in the flame. Both of these variables are difficult to determine, but the pathlength can be held constant and the concentration of atomic vapor is directly proportional to the concentration of the analyte in the solution being aspirated. The procedure used is to prepare a calibration curve of concentration in the solution versus absorbance. 

The major disadvantage of making measurements by atomic absorption, as we shall see below, is that a different source is required for each element, 

Schematic diagram of atomic absorption instrument. (From G. D. Christian and F. J. Feldman, Atomic Absorption Spectroscopy. Applications in Agriculture, Biology, and Medicine. New York Interscience, 1970. Reproduced by permission of John Wiley Sons, Inc.) 

IR is an absorption method. Analysis utilizing absorption measurements can also be done with spectrophotometry and atomic absorption spectrophotometry (AAS). In the former method a monochromatic light (visible or ultraviolet) is passed through a solution containing a compound of an element with unknown concentration. The light absorption is measured and converted to concentration. 

In the AAS method a solution is vaporized in an acetylene-oxygen flame so that the atoms of the unknown element are present in the flame. If, for instance, the intention is to determine the molybdenum concentration, radiation emitted from a cathode ray lamp with a molybdenum electrode is allowed to pass through the flame. 

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Quigley, M. N. Determination of Calcium in Analgesic Tablets Using Atomic Absorption Spectrophotometry, ... 

Koscielniak and Parczewski investigated the influence of A1 on the determination of Ca by atomic absorption spectrophotometry using the 2 factorial design shown in the following table. ... 

Provide an SOP for the determination of cadmium in lake sediments by atomic absorption spectrophotometry using a normal calibration curve. 

Determination of gold concentrations to ca 1 ppm in solution via atomic absorption spectrophotometry (62) has become an increasingly popular technique because it is available in most modem analytical laboratories and because it obviates extensive sample preparation. A more sensitive method for gold analysis is neutron activation, which permits accurate determination to levels < 1 ppb (63). The sensitivity arises from the high neutron-capture cross section (9.9 x 10 = 99 barns) of the only natural isotope, Au. The resulting isotope, Au, decays by P and y emission with a half-life of 2.7 d. 

Concurrent with requirements for low levels of mercurials in discharge water is the problem of their deterrnination. The older methods of wet chemistry are inadequate, and total rehance is placed on instmmental methods. The most popular is atomic absorption spectrophotometry, which rehes on the absorption of light by mercury vapor (4). Solutions of mercury compounds not stabilized with an excess of acid tend to hydrolyze to form yeUow-to-orange basic hydrates. These frequendy absorb onto the walls of containers and may interfere with analytical results when low levels (ppm) of mercury are determined. 

Analytical deterrnination of nickel in solution is usually made by atomic absorption spectrophotometry and, often, by x-ray fluorescence spectroscopy. 

Potassium is analyzed in chemicals that are used in the fertilizer industry and in finished fertilizers by flame photometric methods (44) or volumetric sodium tertraphenylboron methods (45) as approved by the AO AC. Gravimetric deterrnination of potassium as K2PtClg, known as the Lindo-Gladding method (46), and the wet-digestion deterrnination of potassium (47) have been declared surplus methods by the AO AC. Other methods used for control purposes and special analyses include atomic absorption spectrophotometry, inductively coupled plasma (icp) emission spectrophotometry, and a radiometric method based on measuring the radioactivity of the minute amount of the isotope present in all potassium compounds (48). 

Spectrophotometric deterrnination at 550 nm is relatively insensitive and is useful for the deterrnination of vitamin B 2 in high potency products such as premixes. Thin-layer chromatography and open-column chromatography have been appHed to both the direct assay of cobalamins and to the fractionation and removal of interfering substances from sample extracts prior to microbiological or radioassay. Atomic absorption spectrophotometry of cobalt has been proposed for the deterrnination of vitamin B 2 in dry feeds. Chemical methods based on the estimation of cyanide or the presence of 5,6-dimethylben2irnida2ole in the vitamin B 2 molecule have not been widely used. 

Alkaline-earth metals are often deterruined volumetricaHy by complexometric titration at pH 10, using Eriochrome Black T as indicator. The most suitable complexing titrant for barium ion is a solution of diethylenetriaminepentaacetic acid (DTPA). Other alkaline earths, if present, are simultaneously titrated, and in the favored analytical procedure calcium and strontium are deterruined separately by atomic absorption spectrophotometry, and their values subtracted from the total to obtain the barium value. 

BeryUium aUoys ate usuaUy analyzed by optical emission or atomic absorption spectrophotometry. Low voltage spark emission spectrometry is used for the analysis of most copper-beryUium aUoys. Spectral interferences, other inter-element effects, metaUurgical effects, and sample inhomogeneity can degrade accuracy and precision and must be considered when constmcting a method (17). 

Metal Extraction. As with other carboxyhc acids, neodecanoic acid can be used in the solvent extraction of metal ions from aqueous solutions. Recent appHcations include the extraction of zinc from river water for deterrnination by atomic absorption spectrophotometry (105), the coextraction of metals such as nickel, cobalt, and copper with iron (106), and the recovery of copper from ammoniacal leaching solutions (107). 

Mean values from duplicate analyses of each of three samples by atomic absorption spectrophotometry. 

Metal Content. Two common analytical methods for determining metal content are by titration and by atomic absorption spectrophotometry (aas). The titration method is a complexiometric procedure utilizing the disodium salts of ethylenediaminetetraacetic acid (EDTA). The solvent, indicator. 

Testing and Control. Analysis and testing are required whenever a new plating solution is made up, and thereafter at periodic intervals. The analyses are relatively simple and require Httie equipment (78—80). Trace metal contaminants can be analy2ed using spot tests, colorimetricaHy, and with atomic absorption spectrophotometry (see Trace and residue analysis). Additives, chemical balance, impurity effects, and many other variables are tested with small plating cells, such as the Hull cell developed in 1937 (81,82). 

Further techniques which may be applied directly to the solvent extract are flame spectrophotometry and atomic absorption spectrophotometry (AAS).13 The direct use of the solvent extract in AAS may be advantageous since the presence of the organic solvent generally enhances the sensitivity of the method. However, the two main reasons for including a chemical separation in the preparation of a sample for AAS are ... 

Theory. Conventional anion and cation exchange resins appear to be of limited use for concentrating trace metals from saline solutions such as sea water. The introduction of chelating resins, particularly those based on iminodiacetic acid, makes it possible to concentrate trace metals from brine solutions and separate them from the major components of the solution. Thus the elements cadmium, copper, cobalt, nickel and zinc are selectively retained by the resin Chelex-100 and can be recovered subsequently for determination by atomic absorption spectrophotometry.45 To enhance the sensitivity of the AAS procedure the eluate is evaporated to dryness and the residue dissolved in 90 per cent aqueous acetone. The use of the chelating resin offers the advantage over concentration by solvent extraction that, in principle, there is no limit to the volume of sample which can be used. 

In the illustrative experiment described here, copper(II) ions in a brine solution are concentrated from 0.1 to about 3.3 ppm prior to determination by atomic absorption spectrophotometry. 

Sediment pollution. The concentrations of pollutants in the dated sediment cores have been determined in our laboratory by atomic absorption spectrophotometry (AAS). Donazzolo et al. (15) and Pavoni et al. (16) reported mainly heavy metal concentrations. Marcomini et al. (17) and Pavoni et al. (18) discussed the concentration profiles of organic pollutants such as chlorinated hydrocarbons and polycyclic aromatic hydrocarbons. 

R. W. "Determination of Lead In Whole Blood by Graphite Furnace Atomic Absorption Spectrophotometry". Amer. Ind. Hyg. Assoc. J. (1974), 566-570. 

Evenson, M. A. and Anderson, C. T., Jr. Ultramlcro Analysis for Copper, Cadmium and Zinc In Human Liver Tissue by Use of Atomic Absorption Spectrophotometry and the Heated Graphite Tube Atomizer". Clin. Chem. (1975), 2, 537-543. 

Evenson, M. A. and Pendergast, D. D. "Rapid Ultramicro Direct Determination of Erythrocyte Lead Concentration by Atomic Absorption Spectrophotometry with Use of a Graphite Tube Furnace". Clin. Chem. (1974), 20, 163-171. 

J. "Determination of Copper In Serum With a Graphite Rod Atomizer for Atomic Absorption Spectrophotometry". Anal. Chlm. Acta (1971), 263-269. 

Kubaslk, N. P. and Volosln, M. T. "A Simplified Determination of Urinary Cadmium, Lead, and Thallium, with Use of Carbon Rod Atomization and Atomic Absorption Spectrophotometry . Clin. Chem. (1973), 19, 954-958. 

Kurz, D., Roach, J., and Eyrlng, E. J. "Determination of Zinc by Flameless Atomic Absorption Spectrophotometry . 

Variation In Lead Concentration Along Single Hairs as Measured by Non-Flame Atomic Absorption Spectrophotometry". Nature (1972), 238, 162-163. 

Ross, R. T. and Gonzalez, J. G. "Direct Determination of Trace Quantities of Manganese In Blood and Serum Samples Using Selective Volatilization and Graphite Tube Reservoir Atomic Absorption Spectrophotometry". Bull. Environ. 

Zacharlasen, H., Andersen, I., Kostol, C., and Barton, R. "Technique for Determining Nickel In Blood by Flameless Atomic Absorption Spectrophotometry". Clin. Chem. (1975),... 

Atomic absorption spectrophotometry Flame Flameless Simple, versatile measures total metal content. Knowledge of interfering effects important. 10 = to 10 M 10 to 10 M... 

The technique that has been widely apphed for analyzing total merctrry in aqttatic biota since the late 1960s (cold vapor atomic absorption spectrophotometry) lemains a vahd analytical method. Accordingly, we irtfer that most of the historical data for total merctrry in fish tissues are vahd. Moreover, the historical data on total merctrry concentrations in fish tissues provide defettsible estimates of prior MeHg concentrations. 

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