Atomic absorption analysis, trace metal extraction

Pretreatment of the collected particulate matter may be required for chemical analysis. Pretreatment generally involves extraction of the particulate matter into a liquid. The solution may be further treated to transform the material into a form suitable for analysis. Trace metals may be determined by atomic absorption spectroscopy (AA), emission spectroscopy, polarogra-phy, and anodic stripping voltammetry. Analysis of anions is possible by colorimetric techniques and ion chromatography. Sulfate (S04 ), sulfite (SO-, ), nitrate (NO3 ), chloride Cl ), and fluoride (F ) may be determined by ion chromatography (15). 

The overall precision of the APCD-MIBK extraction technique coupled with atomic absorption analysis was obtained by analysis of data obtained from rephcate analysis of line water over a period of a few months. For each experiment performed, two, three, or more extractions of line water were always performed in the normal fashion (4 ml 5% APCD added). Thus, although the trace metal composition of the line water changed over this period of time, an estimate of the precision of... 

Table VII. Overall Precision of APC1>-MIBK Trace Metal Extraction Scheme with Atomic Absorption Analysis...

Although the co-precipitation method was initially conceived with large volume extraction coupled with flame atomic absorption analysis in mind, its utihty may prove to lie in other areas. In particular, the introduction of improved fiameless atomizers will allow the determination of trace metals using much smaller sample volumes. We have combined pre-concentration by this method on 100 ml samples with use of a Perkin-Elmer HGA-2100 graphite furnace for the analysis of copper. 

A wide variety of inorganic materials have been used to precipitate or collect trace metals from solution. The most direct approach is a cementation process, which is one that removes the trace pollutants from solution by reduction with a metal and plating onto that metal surface. Although this process may be slow, the filtration is usually quick, since decantation is often sufficient. Finely divided cadmium extracts copper, selenium, and mercury from nitric and sulfuric acid solutions (66). When copper was used to preconcentrate mercury from water or biological fluids prior to atomic absorption analysis, the detection limit was 1-2 X 10 g (67, 68). Iron (69), zinc (70), and tungsten (71), as metals, have also been used to obtain a deposit of several trace metals from aqueous systems as dilute as 10 ppb for subsequent analysis. Elemental tellurium can be produced in solution by reduction using tin(II) chloride or sulfur dioxide, and coprecipitates silver (72) and selenium (73). Granulated silicon-metal alloys were used to remove metal ions from water and brine by reduction as well (74, 75). 

In the analysis of seawater, isotope dilution mass spectrometry offers a more accurate and precise determination than is potentially available with other conventional techniques such as flameless AAS or ASV. Instead of using external standards measured in separate experiments, an internal standard, which is an isotopically enriched form of the same element, is added to the sample. Hence, only a ratio of the spike to the common element need be measured. The quantitative recovery necessary for the flameless atomic absorption and ASV techniques is not critical to the isotope dilution approach. This factor can become quite variable in the extraction of trace metals from the salt-laden matrix of seawater. Yield may be isotopically determined by the same experiment or by the addition of a second isotopic spike after the extraction has been completed. 

Gardner and Yates [26] developed a method for the determination of total dissolved cadmium and lead in estuarine waters. Factors leading to the choice of a method employing extraction by chelating resin, and analysis by carbon furnace atomic absorption spectrometry, are described. To ensure complete extraction of trace metals, inert complexes with humic-like material are decomposed by ozone [27]. The effect of pH on extraction by and elution from chelating resin is discussed, and details of the method were presented. These workers found that at pH 7 only 1-2 minutes treatment with ozone was needed to completely destroy complexing agents such as EDTA and humic acid in the samples. 

Atomic absorption spectrometry is one of the most widely used techniques for the determination of metals at trace levels in solution. Its popularity as compared with that of flame emission is due to its relative freedom from interferences by inter-element effects and its relative insensitivity to variations in flame temperature. Only for the routine determination of alkali and alkaline earth metals, is flame photometry usually preferred. Over sixty elements can be determined in almost any matrix by atomic absorption. Examples include heavy metals in body fluids, polluted waters, foodstuffs, soft drinks and beer, the analysis of metallurgical and geochemical samples and the determination of many metals in soils, crude oils, petroleum products and plastics. Detection limits generally lie in the range 100-0.1 ppb (Table 8.4) but these can be improved by chemical pre-concentration procedures involving solvent extraction or ion exchange. 

The major anions and cations in seawater have a significant influence on most analytical protocols used to determine trace metals at low concentrations, so production of reference materials in seawater is absolutely essential. The major ions interfere strongly with metal analysis using graphite furnace atomic absorption spectroscopy (GFAAS) and inductively coupled plasma mass spectroscopy (ICP-MS) and must be eliminated. Consequently, preconcentration techniques used to lower detection limits must also exclude these elements. Techniques based on solvent extraction of hydrophobic chelates and column preconcentration using Chelex 100 achieve these objectives and have been widely used with GFAAS. 

Cloud point extraction of metal ions. The use of cloud point extraction as a separation technique was first introduced by Watanabe for the extraction of metal ions forming sparingly water soluble complexes [109], Since then, the technique has been applied successfully to the extraction of metal chelates for spectrophotometric, atomic absorption, or flow injection analysis of trace metals in a variety of samples [105-107,110]. Other metal complexes such as AUCI4 or thiocyanato-metal complexes can be extracted directly using nonionic surfactants such as polyoxyethylene... 

Cadmium in acidified aqueous solution may be analyzed at trace levels by various instrumental techniques such as flame and furnace atomic absorption, and ICP emission spectrophotometry. Cadmium in solid matrices is extracted into aqueous phase by digestion with nitric acid prior to analysis. A much lower detection level may be obtained by ICP-mass spectrometry. Other instrumental techniques to analyze this metal include neutron activation analysis and anodic stripping voltammetry. Cadmium also may be measured in aqueous matrices by colorimetry. Cadmium ions react with dithizone to form a pink-red color that can be extracted with chloroform. The absorbance of the solution is measured by a spectrophotometer and the concentration is determined from a standard calibration curve (APHA, AWWA and WEF. 1999. Standard Methods for the Examination of Water and Wastewater, 20th ed. Washington, DC American Public Health Association). The metal in the solid phase may be determined nondestructively by x-ray fluorescence or diffraction techniques. 

Inorganic extractables/leachables would include metals and other trace elements such as silica, sodium, potassium, aluminum, calcium, and zinc associated with glass packaging systems. Analytical techniques for the trace analysis of these elements are well established and include inductively coupled plasma—atomic emission spectroscopy (ICP-AES), ICP-MS, graphite furnace atomic absorption spectroscopy (GFAAS), electron microprobe, and X-ray fluorescence. Applications of these techniques have been reviewed by Jenke. " An example of an extractables study for certain glass containers is presented by Borchert et al. ". ... 

Willis (W12) has recently summarized the principles and applications of this method. A short note appeared recently regarding the use of atomic absorption spectrometry for serum and urine copper analysis (B15). The sensitivity of this method for copper is rather less than for such other biologically important trace metals as magnesium, zinc, and sodium. The sensitivity can be improved by extracting the copper as dithiocarbamate or pyrollidinedithiocarbamate complex (A7) into methyl isobutyl ketone. While this method is less sensitive than some others, it is nevertheless very specific and the apparatus is only moderately expensive. 

Kotz etal. (1972, Decomposition of biological materials for the determination of extremely low contents of trace elements in limited amounts with nitric acid under pressure in a Teflon tube) Hartstein et al. (1973, Novel wet-digestion procedure for trace-metal analysis of coal by atomic absorption) Jackson etal. (1978), Automated digestion and extraction apparatus for use in the determination of trace metals in foodstuffs) Campos etal. (1990, Combustion and volatilization of solid samples for direct atomic absorption spectrometry using silica or nickel tube furnace atomizers) Erber et al. (1994, The Wickbold combustion method for the determination of mercury under statistical aspects) and Woit-tiez and Sloof (1994, Sampling and sample preparation). 

It is apparent that the spectrophotometric procedures are rather time consuming since several multiple extractions must be performed in order to minimize interferences. Atomic absorption spectroscopy has enjoyed wide popularity in recent years as a trace metal analysis tool. This is due to a number of factors including high sensitivity, selectivity, and ease of sample preparation. With biological fluids, often no sample preparation at all is required, depending on the element analyzed, its concentration and the sample matrix. Because of its advantages, this technique will be treated in some detail. 

Several authors have used ion-exchange methods to preconcentrate trace metals for further analysis from geothermal water 410) for x-ray determination 411, 412) for y-spectroscopy (475) and for atomic absorption spectroscopy (474, 475). For x-ray analysis, an attractive method for preconcentration is the use of ion-exchange resin-loaded paper (476-422). Although these methods often need slow filtration of the sample or several passes to obtain quantitative extraction of the metals, the sorbed material is presented in a convenient form for X ray, and low-concentration solutions and large sample volumes can be used. 

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