Metal trace

One of the characteristics of the cycle of metal mobilization and deposition is that the form of the 

One of the characteristics of the cycle of metal mobilization and deposition is that the form of the metal is changed. This change in speciation of a metal has a profound effect on its fate. The link between metal speciation and fate is the central theme of this chapter. 

In existing reference materials the concentrations of several key analyfes, including iron, are too high to be useful for scientists making open ocean analysis. For instance, the concentration of iron in seawater standards provided by NRC-Canada is about 100 times greater than expected in surface ocean wafers (Table 3.1). 

TABLE 3.1 Comparison of Mefal Concenfrafions in an Existing Reference Seawater (NASS-5, from NRC-Canada) and in Oceanic Seawater 

Metal NASS-5 nM Pacific Surface Water nM Pacific Deep Water nM Reference 

There are other metals for which compelling cases can be made to produce contamination-free oceanic reference seawater. These include other bioactive metals (e.g., zinc, cobalt, cadmium, and copper), tracers of anthropogenic contamination (e.g., lead. Box 3.1), and non-bioactive metals used as tracers of geochemical and physical processes (e.g., aluminum). 

The primary and immediate need is for a trace metal reference material, but a certified reference material would provide even greater benefits. A technique based on isotope dilution with detection by inductively-coupled plasma mass spectrometry (ICP-MS) (Wu and Boyle, 1998) most clearly meets the traceability criteria required for a certified reference material. Although useful for iron and several other metals, isotope dilution is not possible for monoisotopic elements like cobalt, so other techniques must also be used. Indeed, it is advisable that several techniques be used to certify a trace metal reference material. 

The interaction of natural organics with cations result in various species, but due to the unknown equilibrium constants and the different interactions by various organics involving several mechanisms, the available data is limited. Matlack (1992) modelled the interactions of natural organics with cations in order to determine speciation. He found a general underestimation of the interactions by speciation software due to the assumption of competition between cations. 

Little literature has been published on the binding energy of calcium and natural organics. Clark et al published binding energies between calcium and natural organics measured by XPS as 349.2 to 349.7 eV (Clark and Jucker (1993)). However, it was not specified in which chemical state calcium and HSs were and much mote work is needed to identify the species involved. 

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Anodic-stripping voltaimnetry (ASV) is used for the analysis of cations in solution, particularly to detemiine trace heavy metals. It involves pre-concentrating the metals at the electrode surface by reducmg the dissolved metal species in the sample to the zero oxidation state, where they tend to fomi amalgams with Hg. Subsequently, the potential is swept anodically resulting in the dissolution of tire metal species back into solution at their respective fomial potential values. The detemiination step often utilizes a square-wave scan (SWASV), since it increases the rapidity of tlie analysis, avoiding interference from oxygen in solution, and improves the sensitivity. This teclmique has been shown to enable the simultaneous detemiination of four to six trace metals at concentrations down to fractional parts per billion and has found widespread use in seawater analysis. 

Adsorptive stripping analysis involves pre-concentration of the analyte, or a derivative of it, by adsorption onto the working electrode, followed by voltanmietric iiieasurement of the surface species. Many species with surface-active properties are measurable at Hg electrodes down to nanoniolar levels and below, with detection limits comparable to those for trace metal detemiination with ASV. 

Batley, G. E. Gardner, D. Sampling and Storage of Natural Waters for Trace Metal Analysis , Wat Res. 1977,11, 745-756. 

Benoit, G. Hunter, K. S. Rozan, T. F. Sources of Trace Metal Contamination Artifacts During Collection, Handling, and Analysis of Freshwaters, Anal. Chem. 1997, 69, 1006-1011. Keith, L. H., ed. Principles of Environmental Sampling, American Chemical Society Washington, DC, 1988. 

Atomic absorption using either flame or electrothermal atomization is widely used for the analysis of trace metals in a variety of sample matrices. Using the atomic absorption analysis for zinc as an example, procedures have been developed for its determination in samples as diverse as water and wastewater, air, blood, urine, muscle... 

Quigley, M. N. Vernon, E. Determination of Trace Metal Ion Concentrations in Seawater, /. Chem. Educ. 1996, 73, 671-675. 

Trace metals in sea water are preconcentrated either by coprecipitating with Ee(OH)3 and recovering by dissolving the precipitate or by ion exchange. The concentrations of several trace metals are determined by standard additions using graphite furnace atomic absorption spectrometry. 

Van Loon, J. G. Selected Methods of Trace Metal Analysis Biological and Environmental Samples. Wiley-lnterscience New York, 1985. 

Control led-Potential Coulometry The majority of controlled-potential coulometric analyses involve the determination of inorganic cations and anions, including trace metals and halides. Table 11.8 provides a summary of several of these methods. 

Differential pulse polarography and stripping voltammetry have been applied to the analysis of trace metals in airborne particulates, incinerator fly ash, rocks. 

Speciation of Soluble Trace Metals According to the Scheme of Batley and Florence ... 

Miscellaneous Samples Besides environmental and clinical samples, differential pulse polarography and stripping voltammetry have been used for the analysis of trace metals in other samples, including food, steels and other alloys, gasoline, gunpowder residues, and pharmaceuticals. Voltammetry is also an important tool for... 

Trace metal Trace metal contents Trace metals Tracer applications Tracer bullets Tracers... 

Biocides should not be present in water used for aquaculture. Sources of herbicides and pesticides are mnoff from agricultural land, contamination of the water table, and spray drift from crop-dusting activity. Excessive levels of phosphoms and nitrogen may occur where mnoff from fertilized land enters an aquaculture faciUty either from surface mnoff or groundwater contamination. Trace metal levels should be low as indicated in Tables 4 and 5. 

Quality Specifications. Because of the extreme sensitivity of polyamide synthesis to impurities ia the iagredients (eg, for molecular-weight control, dye receptivity), adipic acid is one of the purest materials produced on a large scale. In addition to food-additive and polyamide specifications, other special requirements arise from the variety of other appHcations. Table 8 summarizes the more important specifications. Typical impurities iaclude monobasic acids arising from the air oxidation step ia synthesis, and lower dibasic acids and nitrogenous materials from the nitric acid oxidation step. Trace metals, water, color, and oils round out the usual specification Hsts. 

Analytical Procedures. Standard methods for analysis of food-grade adipic acid are described ia the Food Chemicals Codex (see Refs, ia Table 8). Classical methods are used for assay (titration), trace metals (As, heavy metals as Pb), and total ash. Water is determined by Kad-Fisher titration of a methanol solution of the acid. Determination of color ia methanol solution (APHA, Hazen equivalent, max. 10), as well as iron and other metals, are also described elsewhere (175). Other analyses frequendy are required for resia-grade acid. For example, hydrolyzable nitrogen (NH, amides, nitriles, etc) is determined by distillation of ammonia from an alkaline solution. Reducible nitrogen (nitrates and nitroorganics) may then be determined by adding DeVarda s alloy and continuing the distillation. Hydrocarbon oil contaminants may be determined by ir analysis of halocarbon extracts of alkaline solutions of the acid. 

To measure trace metals to the levels required in the guidelines involves the use of state-of-the-art instmmentation such as inductively coupled plasma/mass spectrometry (icp/ms). 

Citric acid is used in carbonated beverages to provide tartness, modify and enhance flavors, and chelate trace metals. It is often added to jams and jellies to control pH and provide tartness. It is used in cured and freeze-dried meat products to protect the amino acids (qv) and improve water retention. Bakers use it to improve the flavor of fmit fillings in baked goods. Because citric acid is a good chelator for trace metals, it is used as an antioxidant synergist in fats and oils, and as a preservative in frozen fish and shellfish (7) (see Antioxidaisits). 

As opposed to gaseous, pure formaldehyde, solutions of formaldehyde are unstable. Both formic acid (acidity) and paraformaldehyde (soHds) concentrations increase with time and depend on temperature. Formic acid concentration builds at a rate of 1.5—3 ppm/d at 35°C and 10—20 ppm/d at 65°C (17,18). Trace metallic impurities such as iron can boost the rate of formation of formic acid (121). Although low storage temperature minimizes acidity, it also increases the tendency to precipitate paraformaldehyde. 

Chemical Composition. Chemical compositional data iaclude proximate and ultimate analyses, measures of aromaticity and reactivity, elemental composition of ash, and trace metal compositions of fuel and ash. All of these characteristics impact the combustion processes associated with wastes as fuels. Table 4 presents an analysis of a variety of wood-waste fuels these energy sources have modest energy contents. 

Table 8. Trace Metal Concentrations in Ash from Agricultural Biofuels and Wood-Fired Boilers, mg/kg...

Table 9. Trace Metals in Municipal Solid Waste and Solid Waste Ash, ppm by wt...

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