Speciation possibilities

The choice of techniques for a specific problem depends not only on the nature of the material in question but also on the types of answer required. Methods for the determination of metal speciation range from very simple separations based on bulk physical properties to detailed structural analyses, which can measure interatomic distances and orientations. The complete chemical identification of a total unknown is a different problem from one in which a distinction must be made between, perhaps as few as two, possibilities. The foregoing chapter concentrates on techniques for chemical identification, which, together with the separation techniques which are discussed in depth elsewhere, make the determination of metal speciation possible. 

The main features of LMMS are summarised in Table 3.27. Laser microprobe mass spectrometry is a valuable tool for inorganic and oi anic analysis. Element location and quantification on the )u.m scale can be achieved (spot analysis) and speciation possibilities are available, which are unsurpassed by other... 

The distribution of metals between dissolved and particulate phases in aquatic systems is governed by a competition between precipitation and adsorption (and transport as particles) versus dissolution and formation of soluble complexes (and transport in the solution phase). A great deal is known about the thermodynamics of these reactions, and in many cases it is possible to explain or predict semi-quantita-tively the equilibrium speciation of a metal in an environmental system. Predictions of complete speciation of the metal are often limited by inadequate information on chemical composition, equilibrium constants, and reaction rates. 

The full appreciation of the overriding importance of metal speciation in evaluating the transport and effects of metals in an environment is a relatively recent event. As more information is gathered on the forms in which metals exist and are transported through various environmental compartments, it will become possible to predict more accurately the response of the biological communities exposed to the metals and hopefully avert or mitigate the adverse effects. 

The artificial separation between organic and inorganic mass-spectrometric methods is now narrowing, as shown by speciation studies (Section 8.8). Plasma-source MS (PS-MS), mainly as ICP-MS and MIP-MS, has been particularly effective when applied to speciation analysis. Direct speciation is also possible with electrospray MS (ESI-MS). 

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. 

Mass spectrometry can be specific in certain cases, and would even allow on-line QA in the isotope dilution mode. MS of molecular ions is seldom used in speciation analysis. API-MS allows compound-specific information to be obtained. APCI-MS offers the unique possibility of having an element- and compound-specific detector. A drawback is the limited sensitivity of APCI-MS in the element-specific detection mode. This can be overcome by use of on-line sample enrichment, e.g. SPE-HPLC-MS. The capabilities of ESI-MS for metal speciation have been critically assessed [546], Use of ESI-MS in metal speciation is growing. Houk [547] has emphasised that neither ICP-MS (elemental information) nor ESI-MS (molecular information) alone are adequate for identification of unknown elemental species at trace levels in complex mixtures. Consequently, a plea was made for simultaneous use of these two types of ion source on the same liquid chromatographic effluent. 

The interpretation of previous attempts at measuring the impact of metals on microbially mediated processes has been hindered by the use of a wide range of experimental conditions and measurements. Already, a shift from studies based on total metal concentration to those based on bioavailable metal concentrations has occurred. The next step will entail accurately predicting and measuring metal speciation patterns in order to identify microbial responses to metal speciation. Only then will it be possible to develop more effective methods to quantify and mitigate deleterious effects of metals on the myriad processes that microbes mediate in the environment. 

Luther et al. [92] have described a procedure for the direct determination of iodide in seawater. By use of a cathodic stripping square-wave voltammetry, it is possible to determine low and sub-nanomolar levels of iodide in seawater, freshwater, and brackish water. Precision is typically 5% (la). The minimum detection limit is 0.1 - 0.2 nM (12 parts per trillion) at 180 sec deposition time. Data obtained on Atlantic Ocean samples show similar trends to previously reported iodine speciation data. This method is more sensitive than previous methods by 1-2 orders of magnitude. Triton X-100 added to the sample enhances the mercury electrode s sensitivity to iodine. 

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]. 

Krznaric [799] studied the influence of surfactants (EDTA, NTA) on measurements of copper and cadmium in seawater by differential pulse ASV. Adsorption of surfactants onto the electrode surface were shown to change the kinetics of the overall electrode charge and mass transfer, resulting in altered detection limits. Possible implications for studies on metal speciation in polluted seawater with high surfactant contents are outlined. 

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