Copper speciation analysis

Garcia-Monco Carra et al. [296] have described a hybrid mercury film electrode for the voltammetric analysis of copper (and lead) in acidified seawater. Mercury plating conditions for preparing a consistently reproducible mercury film electrode on a glassy carbon substrate in acid media are evaluated. It is found that a hybrid electrode , i.e., one preplated with mercury and then replated with mercury in situ with the sample, gives very reproducible results in the analysis of copper in seawater. Consistently reproducible electrode performance allows for the calculation of a cell constant and prediction of the slopes of standard addition plots, useful parameters in the study of copper speciation in seawater. 

A rather rare example indicating the possibility of speciation analysis is the determination of ionic copper, in the range from 20 to 90 pg H using a copper-selective electrode in a sample of wine when the total copper content is in the range from 0.10 to 1 mgH The sample is modified very little by the addition of a 10% volume of ImolH KNO3, as should be done with standard solutions. 

FIGURE 1,6 Example of manifolds developed for flow injection speciation analysis, (a) Manifold of the reversed FIA system for speciation of chromium with pH measurements and spectrophotometric detection, q—peristaltic pumps, V—injection valves, L— reaction coil, M.E.—pH glass electrode. (Adapted from Ruz, J. et al. 1986. J. Autom. Ghent. 8 70-74.) (b) Manifold of branched FIA system for simultaneous biamperomet-ric determination of nitrate and nitrite. (Cd)Cu—reductive column with copperized cadmium, L—mixing coils. (Adapted from Trojanowicz, M., W. Matuszewski, and B. Szostek. 1992. Anal. Chim. Acta 261 391-398.) (c) Example signal recordings obtained in the FIA system shown in (b) 1-6—standard solutions, A-I—natural water samples. Concentration in standard solutions 1—0.075 2—0.050 3—0.025 mM nitrate and 4—7.5 5—5.0 6—2.5 pM nitrite. 

Sunda and Hanson [247] have used ligand competition techniques for the analysis of free copper (II) in seawater. This work demonstrated that only 0.02 -2% of dissolved copper (II) is accounted for by inorganic species. (i.e., Cu2+, CuC03, Cu(OH)+, CuCl+, etc.) the remainder is associated with organic complexes. Clearly, the speciation of copper (II) in seawater is markedly different from that in fresh water. 

Prior to the introduction of ion-selective electrode techniques, in situ monitoring of free copper (II) in seawater was not possible due to the practical limitations of existing techniques (e.g., ligand competition and bacterial reactions). Ex situ analysis of free copper (II) is prone to experimental error, as the removal of seawater from the ocean can lead to speciation of copper (II). Potentially, a copper (II) ion electrode is capable of rapid in situ monitoring of environmental free copper (II). Unfortunately, copper (II) has not been used widely for the analysis of seawater due to chloride interference that is alleged to render the copper nonfunctional in this matrix [288]. 

CCcu and K determination by DPASV analysis. To illustrate the possibilities of speciation studies, an example of the application to natural waters will be presented in this section, with special reference to the determination of the complexation capacity for copper of marine waters. 

A value of 14.3 was used in the species calculations in the first part of this paper. Log 3 was changed to 11.8 and the speciation calculation was repeated with all other input data held constant. Since Cu(0H)p was the dominant copper species in the first calculation, the effect of changing the stability constant by a factor of 300 has a marked effect on the calculated activities (Table VII and Table IX). The resulting activities were used as input for factor analysis. In Table X are shown the factors or the 26 point case. Decreasing the stability constant of CuCOH) V/300 fold causes a shift in the loading of Cu(0H)p from Factor 1 to Factor 2, of CuCO from Factor 3 to Factor 1 and of Cu(OH) from Factor 1 to Factor 2, with minimal changes for the other species. Examination... 

Similar demands for speciation of trace elements exist for food analysis. Substantial differences in the biological availability are known for several essential elements and depend on the form in which they are present in the diet. The chemical bases for these differences are known for cobalt, iron, and chromium but not for zinc, copper, and selenium. The importance of speciation in food analysis is best demonstrated by the example of iron. That element, when part of heme compounds, is well absorbed, and there is little influence on the absorption by other factors in the diet. Nonheme iron, on the other hand, is not readily absorbed and, in addition, is subject to many influences from dietary ingredients those influences are poorly understood and probably not completely known (14). 

Soares, H.M.V.M., Vasconcelos, M.T.S.D., 1995. Application of potentiometric stripping analysis for speciation of copper complexes with adsorbable ligands on the mercury electrode. Anal. Chim. Acta 314, 241-249. 

It may not be trivial for an analyst to say that the method actually does measure what it purports to. When there are possibilities of speciation, or different isomers, then the analyst must be aware of the client s requirements and ensure that the method is specific for them. For example, analysis for total copper is different from bioavailable copper , and with the latter the definition of bioavailable must be carefully considered. 

Several complexing ligands can be employed to determine copper in seawater. However, the best results are obtained when copper is determined after adsorptive pre-concentration as a complex with salicylaldoxime (SA) Campos and van den Berg, 1994). Analysis is carried out at neutral pH values (pH 6.5-8.5 is suitable) which means that the original sample pH can be maintained which is useful for speciation studies. 

In the environment one may be interested in some single components like NOs , Cl, NH/, Cu, etc. but nowadays there is an increasing need to know the form in which elements like copper are present, i.e. not only free metal ion concentrations but also the concentrations of the complexes and other compounds in which the metal is present and the inertness or the lability of these compounds. The reason for this need for more detailed information is the difference in toxicity of the various forms in which copper and other elements are present. For instance, the organo-mercury compounds are much more toxic than the inorganic mercury compounds whereas for arsenic the opposite is true. The detailed analysis of the forms in which elements are present is often denoted as speciation and asks, in general, for combinations of sophisticated techniques. For the moment it is difficult to see the role pTAS can play in such speciation studies but for... 

More recently there has been an explosion of interest in using collision/reac-tion cell/interface technology for the analysis of biomedical samples because of the benefits it brings to the determination of many of the toxologically and nutritionally significant elements snch as arsenic, selenium, chromium, iron, and copper. Traditionally, these elements have been very difficnlt to analyze by ICP-MS because of the spectral interferences derived from a combination of the matrix, solvent/acid, and plasma gas ions. This approach is allowing significant improvements in detection capability for both the total and speciated forms of these elements in biomedical-related samples, such as blood serum and tissue samples. 

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