Identification of Metallic Species

Identification of metal particles dispersed in pictorial or decorative layers in works of art is often difficult for microscopy techniques because of (i) their presence as highly diluted components and concentrated in microparticles that, in turn, are included in binding media and attached to priming and protective layers (ii) the coexistence of metals with priming, ground or pigmenting layers in the samples and (iii) the presence of products resulting from the alteration of metals. 

It is therefore frequently difficult to find punctual areas in the sample having a sufficient concentration of the desired analyte to be detected by the x-ray microanalysis system. Thus, identification and eventually quantitation of metals in dec-orative/protective layers of pictorial samples by SEM/EDX frequently require an accurate and often time-consuming scanning process. 

6 Identification of Species Using Reductive/Oxidative Dissolution Process 

A number of electrochemical processes involving solid materials can be described in terms of the reductive or oxidative dissolution of such materials. Within this type of processes, one can include the stripping of metal deposits previously mentioned. In the context of archaeometry, conservation, and restoration sciences, the reductive dissolution of iron oxide-type materials is of particular interest. Thus, application of the voltammetry of the microparticles approach for identifying iron pigments has been described [108, 137-139]. 

A typical example of these processes is shown in Fig. 2.13, where the cyclic voltammetry of (a) hematite, and (b) goethite-modified PIGEs immersed into 0.10 M HCl are depicted. As can be seen in this figure, a prominent cathodic peak is 

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Identification of metal species. The following is an attempt to convey how the structural identity of the metal species (surface complex, sorbate precipitate...) can be recovered by EXAFS spectroscopy. For pedagogic purposes, we first consider a laboratory system aimed at showing how EXAFS spectroscopy enables to distinguish all sorption mechanisms described in the introduction (Figs. 1 and 2), then a case study taken from a real-world system is presented. 

There have been a number of reviews in the literature on the identification of metal species by LC/AAS (40-42) but to successfully utilize the combination, both the LC and the spectrometer system have to be optimized and this has also been the subject of a number of publications (43-45). It has been claimed (44) that the poor sensitivity obtained from the LC/AAS system relative, to that obtained from the atomic spectrometer alone, was due to the dispersion that takes place in the column. Although substantially true, this misunderstanding arises from the fact that the spectroseopist views the chromatograph as just another sampling device and not as a separation system. The point of interfacing a liquid chromatograph with an atomic spectrometer is to achieve a separation before detection and consequently, the important dispersion characteristics are not those that occur in the column but those that occur in the interfaces between the detector and the spectrometer and in the spectrometer itself. 

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