Trace Metal Ion Analysis

Diamond s electrochemical properties are ideally suited for the detection of trace metal ions via anodic stripping voltammetry (ASV) [30,57,115] (1) a large overpotential for hydrogen evolution, (2) a large overpotential for oxygen reduction, (3) no metal complexation with the diamond surface, (4) excellent stability at extreme anodic and cathodic potentials, and (5) chemical inertness and environmental friendliness. These properties make it superior, in some respects, to Hg and other alternative electrode materials. 

Second, when multiple metals are deposited simultaneously, as is the case in a real stripping voltammetric measurement, not only is their interaction with the diamond surface important, but equally critical is their interaction with each other. There is a possibility of intermetallic compounds or alloys forming, both of which will affect the oxidation or stripping potential for each. When these heterogeneous deposits form, the oxidation of a particular metal can occur from different sites on the diamond surface or from another metal surface. Oxidation from these multiple sites leads to peak broadening due to a spread in reaction kinetics. Ideally, for this application, highly dispersed metal deposits of low volume, without any intermetallic interactions, are desired. Even with these complexities, it is supposed that diamond will become a useful electrode for the determination of trace metal ions via anodic-stripping voltammetry. 

Marcel E)EKKERf Inc. 270 Madison Avenue, New York, New York 10016 

Differential pulse anodic-stripping voltammetric curves for Ag(I), PB(II), and Cd(II) for a boron-doped nanocrystalline diamond thin-film electrode in 0.1 M acetate buffer, pH 4.5. The metal ion concentrations are (a) 10, (b) 1, and (c) 0.5 pM. Preconcentration at —1000 mV for 3 min (no stirring). (Reprinted with permission from Chem. Mater. 2003, 15, 879. Copyright (2003) American Chemical Society.) (From Ref 115.) 

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Y.-S. Fung and H.-S. Tung, Capillary electrophoresis for trace metal ion analysis on environmental... 

All analysts are familiar with the principles of potentiometry and potarography and indeed, most analytical laboratories will contain a pH meter and a polarograph. However, electrochemical methods arc, in general, not very important in modern analysis. In contrast, there arc spccifiG applications such as trace metal ion analysis in water and effluents and also some other aspects of environmental analysis for which electrochemical methods are particularly attractive. This is because (1) some methods, especially anodic stripping voltammetry, have a very high sensitivity for heavy-metal ions and the lowest detection limit of from 10 to mol dm is well below that of other available methods (2) electrochemical methods are well suited for modification to on-line and/or portable devices for analysis in the held. Whether the analysis is based on current, conductivity or the response of an ion-selective electrode, both the cell and the control electronics are readily miniaturized and operate on low power Hence, this chapter considers the principles of the electroanalytical methods important in environmental and on-line analysis, together with biochemical applications of electrochemical sensors. 

The detection and quantification of one or more of the above lipid peroxidation produas (primary and/or secondary) in appropriate biofluids and tissue samples serves to provide indices of lipid peroxidation both in ntro and in vivo. However, it must be stressed that it is absolutely essential to ensure that the products monitored do not arise artifactually, a very difiScult task since parameters such as the availability of catalytic trace metal ions and O2, temperature and exposure to light are all capable of promoting the oxidative deterioration of PUFAs. Indeed, one sensible precaution involves the treatment of samples for analysis with sufficient levels of a chainbreaking antioxidant [for example, butylated hydroxy-toluene (BHT)] immediately after collection to retard or prevent peroxidation occurring during periods of storage or preparation. 

As the later chapters indicate, a given question concerning a chemical system usually can be answered by any one of several electrochemical techniques. However, experience has demonstrated that there is a most convenient or reliable method for a specific kind of data. For example, polarography with a static or dropping-mercury electrode remains the most reliable electrochemical method for the quantitative determination of trace-metal ion concentrations. This is true for two reasons (1) the reproducibility of the dropping-mercury electrode is unsurpassed and (2) the reference literature for analysis by polarography surpasses that for any other electrochemical method by at least an order of magnitude. 

In the past, the practice has been to take a sample from any depth in a large metal or (better) plastic container and then transfer the sample to another, usually plastic, container for subsequent analysis by appropriate analytical methods. Obviously, a metal container will contribute to the trace metal content of the sample, and even plastic containers will cause problems. Trace analysis studies have shown that plastic or glass sample containers can both absorb trace metal ions from the sample and/or contribute other metal ions to solution by surface dissolution 12, 13), Thus, the sample cannot be analyzed accurately because of the time-dependent effects on concentration which are related simply to the nature of the container and the conditions used to store the sample. 

M. Bengtsson, F. Malamas, A. Torstensson, O. Regnell, and G. Johansson, Trace Metal Ion Preconcentration for Flame Atomic Absorption by an Immobilized N,N,N -Tri-(2-Pyridylmethyl)ethylene Diamine (TriPEN) Chelate Ion Exchanger in a Flow Injection Analysis. Mikrochim. Acta, III (1985) 209. 

In many cases, nonvolatile substances are converted into more volatile derivatives prior to mass-spectral analysis. Examples include trimethylsilyl derivatives of alcohols or molecules containing sugar groups, ester derivatives of acids, and volatile chelates of trace-metal ions. Usually, a suitable volatile derivative can be synthesized by well established and relatively simple procedures. 

Combinations of online procedures are often employed in environmental applications. Examples include the analysis of trace metal ions in factory wastewater by a combination of cation-selective exhaustive injection (CSEI) (a form of stacking, CZE format) and sweeping. In this latter case, sweeping was promoted by dynamic complexation with EDTA, used as carrier. The same CSEI principle has also been applied to the analysis of quaternary ammonium herbicides and environmentally relevant aromatic amines in water, both followed by sweeping with SDS. 

Broad classes of organic compounds, such as acids and bases, can be separated by pH control, and trace metal ions complexed with organic reagents can be separated or concentrated prior to spectrometric analysis (Topic ElO). 

The combination of oxidi2ing effect, acidic strength, and high solubiHty of salts makes perchloric acid a valuable analytical reagent. It is often employed in studies where the absence of complex ions must be ensured. The value of wet ashing techniques, in which perchloric acid is used to destroy organics prior to elemental analysis for the determination of trace metals in organics, has been well estabHshed (see Trace and residue analysis). 

An on-line concentration, isolation, and Hquid chromatographic separation method for the analysis of trace organics in natural waters has been described (63). Concentration and isolation are accompHshed with two precolumns connected in series the first acts as a filter for removal of interferences the second actually concentrates target solutes. The technique is appHcable even if no selective sorbent is available for the specific analyte of interest. Detection limits of less than 0.1 ppb were achieved for polar herbicides (qv) in the chlorotriazine and phenylurea classes. A novel method for deterrnination of tetracyclines in animal tissues and fluids was developed with sample extraction and cleanup based on tendency of tetracyclines to chelate with divalent metal ions (64). The metal chelate affinity precolumn was connected on-line to reversed-phase hplc column, and detection limits for several different tetracyclines in a variety of matrices were in the 10—50 ppb range. 

The detection and determination of traces of cobalt is of concern in such diverse areas as soflds, plants, fertilizers (qv), stainless and other steels for nuclear energy equipment (see Steel), high purity fissile materials (U, Th), refractory metals (Ta, Nb, Mo, and W), and semiconductors (qv). Useful techniques are spectrophotometry, polarography, emission spectrography, flame photometry, x-ray fluorescence, activation analysis, tracers, and mass spectrography, chromatography, and ion exchange (19) (see Analytical TffiTHODS Spectroscopy, optical Trace and residue analysis). 

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). 

Wetai Ion Analysis. We have reported a sensitive trace-metal analysis based upon HPLC separation of p-aminophenyl EDTA chelates and fluorescence detection by postcolumn reaction with fluorescamine (23). An application of the pyridone chemistry already discussed leads to a fluorescent-labeled EDTA (VIII). 

The ability of metal ions to form complexes with formazans is utilized to determine these ions either directly (for low valent reducing ions) or indirectly in the presence of a reducing agent. Among others, molybdenum(VI) and vanadium(V) have been determined using this method.442,443 Indirect methods have been reported for the analyses of substances that do not reduce tetrazolium salts. Examples include arsenic in nickel ores436 and traces of selenium.437 A method for the extraction and analysis of a number of metal ternary ion association complexes has been described.444 - 448... 

Table 8.80 shows the present status of speciation methodology. For trace-metal speciation, atomic absorption detectors feature a relatively high absolute detection limit (10 pg level), as compared to the 0.1 to 1 pg sensitivity level for molecular ion MS techniques as well as for MIP-AES. The detection limit of LEI-ToFMS is in the attogram range. Speciation has been reviewed [550]. Various monographs deal with speciation analysis [542,551,552]. 

Nearly every area of measurement science can boast of progress in measuring ever-smaller quantities of chemicals, but several stand out in their stunning trace-analysis capabilities. Trace-metal analysis has come to be dominated by methods that volatilize the sample and then either measure its spectroscopic emission or absorption, or measure the masses of the gaseous metal ions using mass spectrometry. Volatilization is accomplished by various thermal means that include flames, furnaces, and inductively coupled or microwave plasmas. The com-... 

Determination of trace metals in seawater represents one of the most challenging tasks in chemical analysis because the parts per billion (ppb) or sub-ppb levels of analyte are very susceptible to matrix interference from alkali or alkaline-earth metals and their associated counterions. For instance, the alkali metals tend to affect the atomisation and the ionisation equilibrium process in atomic spectroscopy, and the associated counterions such as the chloride ions might be preferentially adsorbed onto the electrode surface to give some undesirable electrochemical side reactions in voltammetric analysis. Thus, most current methods for seawater analysis employ some kind of analyte preconcentration along with matrix rejection techniques. These preconcentration techniques include coprecipitation, solvent extraction, column adsorption, electrodeposition, and Donnan dialysis. 

A logical approach which serves to minimise such uncertainties is the use of a number of distinctly different analytical methods for the determination of each analyte wherein none of the methods would be expected to suffer identical interferences. In this manner, any correspondence observed between the results of different methods implies that a reliable estimate of the true value for the analyte concentration in the sample has been obtained. To this end Sturgeon et al. [21] carried out the analysis of coastal seawater for the above elements using isotope dilution spark source mass spectrometry. GFA-AS, and ICP-ES following trace metal separation-preconcentration (using ion exchange and chelation-solvent extraction), and direct analysis by GFA-AS. These workers discuss analytical advantages inherent in such an approach. 

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