Direct speciation approaches

After sampling, storage, and sample preparation, species are to be identified and quantified. Direct speciation approaches can provide full information about the species in a sample without any additional (separation) method, and quantify the species directly. Such methods, for example, chemical sensors, biosensors, and nuclear magnetic resonance (NMR), however, have many limitations in sensitivity and/or selectivity when applied to real-world samples as human milk. 

Speciation Approaches Direct and Combined (Hyphenated) Techniques... 

Fig. 3.4 Overview of speciation approaches direct speciation methods or combined (hyphenated) techniques.

SPECIATION APPROACHES DIRECT METHODS OR HYPHENATED TECHNIQUES... 

Application to solid polymer/additive formulations is restricted, for obvious reasons. SS-ETV-ICP-MS (cup-in-tube) has been used for the simultaneous determination of four elements (Co, Mn, P and Ti) with very different furnace characteristics in mg-size PET samples [413]. The results were compared to ICP-AES (after sample dissolution) and XRF. Table 8.66 shows the very good agreement between the various analytical approaches. The advantage of directly introducing the solid sample in an ETV device is also clearly shown by the fact that the detection limit is even better than that reported for ICP-HRMS. The technique also enables speciation of Sb in PET, and the determination of various sulfur species in aramide fibres. ETV offers some advantages over the well-established specific sulfur analysers very low sample consumption the possibility of using an aqueous standard for calibration and the flexibility to carry out the determination of other analytes. The method cannot be considered as very economic. 

A combination of direct observation and extraction may be carried out. The whole soil may be analyzed by various methods and then specific components sequentially extracted and measured. This approach has been used in the investigation of the speciation of metals in soil under various conditions. Using X-ray spectroscopy, metals and their various ionic forms in soil can be directly identified [1],... 

Simple ligands can adsorb on iron oxides to form a variety of surface species including mononuclear monodentate, mononuclear bidentate and binuclear mono or bi-dentate complexes (Fig. 11.2) these complexes may also be protonated. How adsorbed ligands (and cations) are coordinated to the oxide surface can be deduced from adsorption data, particularly from the area/adsorbed species and from coadsorption of protons. Spectroscopic techniques such as FTIR and EXAFS can provide further (often direct) information about the nature of the surfaces species and their mode of coordination. In another approach, the surface species which permit satisfactory modelling of the adsorption data are often assumed to predominate. As, however, the species chosen can depend upon the model being used, this method cannot provide an unequivocal indication of surface speciation confirmation by an experimental (preferably spectroscopic) technique is necessary. 

The application of atomic spectroscopic instruments as element-specific detectors in chromatography has been reviewed by van Loon More recently, Krull has extensively reviewed their use in high pressure liquid chromatography (HPLC). Atomic spectrometry has found wide acceptance in the field of liquid chromatography because, in most cases, the fractions can be directly analysed after elution from the column. However, it is possible to use the technique for the analysis of solid samples without first dissolving the matrix. This is particularly useful after electrophoresis, where the fractions are fixed either in a gel or on paper. Kamel et al. have shown that it is possible to cut the appropriate sections and insert them into the carbon furnace for analysis. The disadvantage of this approach is that the precision is usually poorer (about 10%) and it is difficult to calibrate the instrument. Nevertheless, this approach is very useful if it is used for qualitative speciation. 

Another important speciation refers to Mg bound to RNA, which is essential to the folding and function of this macromolecule. A computational approach to this analysis was presented for site-bound and diffusively bound Mg(II) ions in RNA. This method confirmed the locations of experimentally determined sites and pointed to potentially important sites not currently annotated as Mg binding sites but deserving experimental follow-up in that direction ". 

Regarding the liquid/liquid extraction from the metal standpoint is rather different. This is the classical approach of coordination chemistry (most of the publications in this area). Today, it is still difficult to establish a direct link between the two descriptions of the organic extractant phases. To better understand liquid/ liquid extraction, the aggregation number and coordination number must be measured separately for each system and set of initial conditions. This is the only way to determine the role of the aggregates in the extraction efficiency. This important point was emphasized by Yaita et al. (61). In this way, Gannaz et al. has used an approach combining studies on both supramolecular and molecular speciation of extractant systems of the DIAMEX-SANEX process (36). 

The ideal speciation procedure is one which allows positive identification and quantitative evaluation of one particular species. Some of the better known approaches are summarised in Table 2.1. Techniques (usually spectroscopic) which have been used to identify and determine directly a particular chemical species in biological samples, are summarised in Table 2.2 and the topic of Direct Methods of Metal Speciation is dealt with in Chapter 3. 

There are few methods which can measure well-defined metal fractions with sufficient sensitivity for direct use with environmental samples (approach B in Fig. 8.2). Nevertheless, this approach is necessary in the experimental determination of the distribution of compounds that are labile with respect to the time scales of the analytical method. Recent literature indicates that high-performance liquid (HPLC) and gas chromatographic (GC) based techniques may have such capabilities (Batley and Low, 1989 Chau and Wong, 1989 van Loon and Barefoot, 1992 Kitazume et al, 1993 Rottmann and Heumann, 1994 Baxter and Freeh, 1995 Szpunar-Lobinska et al, 1995 Ellis and Roberts, 1997 Vogl and Heumann, 1998). The ability to vary both the stationary and mobile phases, in conjunction with suitable detector selection (e.g. ICP-MS), provides considerable discriminatory power. HPLC is the superior method GC has the disadvantage that species normally need to be derivatised to volatile forms prior to analysis. Capillary electrophoresis also shows promise as a metal speciation tool its main advantage is the absence of potential equilibria perturbation, interactions... 

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