Direct speciation methods

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

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). 

McElroy, F. F., V. L. Thompson, D. M. Holland, W. A. Lonneman, and R. L. Sella, Cryogenic Preconcentration-Direct FID Method for Measurement of Ambient NMOC Refinement and Comparison with GC Speciation, JAPCA, 36, 710-714 (1986). 

Direct speciation of dissolved americium and plutonium is possible only for solutions with appreciable concentration ([Am(DI)] > 10-6 M and [Pu(VT)] s 10-5 M) using a spectrophotometer with cumulative data recording (21). Typical spectra measured for the Am3+ ion at pH = 6.5 are shown in Figure 5 for 1, 5, 10, and 40 times cumulation at 503 nm. With this method it is shown that only trivalent americium ions are present in both equilibrium solutions from Am02 and Am(0H)3>nH20 solids. For plutonium solutions, the spectrophotometric study indicates the presence of polymers as shown in Figure 4. 

Total metal concentrations are often very low in natural systems and in performing speciation analysis it is necessary to measure even lower concentrations on attempting to resolve component species. Therefore, very sensitive methods are needed and there is a high risk of contamination, alteration and/or adsorption losses. The ideal speciation method would be sufficiently sensitive and selective to be used directly on natural water samples, would involve minimal perturbation of the sample, and would furnish an analytical signal directly dependent on the (chemical) reactivity of the element of interest (Buffle, 1981a) (Fig. 8.1). 

Variable recovery is a principal cause of non-equivalence of data and there is no straightforward solution to this problem [26], Artificially made reference samples or pure compounds added to test material cannot be used for estimations of recovery of analytes. Direct speciation analysis from the solid sample [27] is not feasible at present, although analytical methods are appearing that could be useful in the future (X-ray absorption spectrometry, laser mass spectrometry, static secondary ion mass spectrometry). 

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. 

Since LPAS application to actinide chemistry is in its infancy, only a limited number of works are available in the published literature. Experiments hitherto performed are confined to either hydrolysis, complexation reactions with carbonate, EDTA and humate ligands and a variety of speciation works for Am(III) and to much lesser extent for U(IV), U(VI) Np(IV), Np(V), Np(VI) Pu(IV), Pu(VI). Of considerable interest is the LPAS application to the direct speciation of actinides in natural aquifer systems, where the solubility of actinides is in general very low and multi-component constituent elements as well as compounds are in much higher concentrations than actinide solubilities. The study of the chemical behaviour of actinides in such natural systems requires a selective spectroscopic method of high sensitivity. LPAS is an invaluable method for this purpose but its application to the problem is only just beginning. 

Fluorescence spectroscopy has, however, several advantages over most other methods for studying metal-HS complexation in aqueous media. The method is relatively rapid since no separation is required between bound and free metal ion thus, errors associated with the separation step in most speciation methods are avoided. Unlike most other methods, it allows direct measurement of the CC of the ligand through a determination of the concentration of free ligands, thus... 

These samples were prepared at another FDA laboratory in Denver, CO, wherein known levels of both Se(IV) and Se(VI) species were intentionally added to authentic animal feed premixes. The Denver FDA laboratory did not perform any Se speciation studies. The WEAC laboratory worked-up these samples, and quantitated using a matrix matching technique with a blank feed premix sample spiked at known levels of Se species. Table 9.5 summarizes the data obtained for levels spiked and individual Se speciation. We have indicated average standard deviation values for all determinations (n = 3). In general, the levels of Se species determined via HPLC-DCP were in good agreement with the actual levels spiked. Table 9.5 also summarizes data obtained for total Se content via direct-DCP methods, together with the summation... 

Instrumental Quantitative Analysis. Methods such as x-ray spectroscopy, oaes, and naa do not necessarily require pretreatment of samples to soluble forms. Only reUable and verified standards are needed. Other instmmental methods that can be used to determine a wide range of chromium concentrations are atomic absorption spectroscopy (aas), flame photometry, icap-aes, and direct current plasma—atomic emission spectroscopy (dcp-aes). These methods caimot distinguish the oxidation states of chromium, and speciation at trace levels usually requires a previous wet-chemical separation. However, the instmmental methods are preferred over (3)-diphenylcarbazide for trace chromium concentrations, because of the difficulty of oxidizing very small quantities of Cr(III). 

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. 

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. 

Ruzic [278 ] considered the theoretical aspects of the direct titration of copper in seawaters and the information this technique provides regarding copper speciation. The method is based on a graph of the ratio between the free and bound metal concentration versus the free metal concentration. The application of this method, which is based on a 1 1 complex formation model, is discussed with respect to trace metal speciation in natural waters. Procedures for interpretation of experimental results are proposed for those cases in which two types of complexes with different conditional stability constants are formed, or om which the metal is adsorbed on colloidal particles. The advantages of the method in comparison with earlier methods are presented theoretically and illustrated with some experiments on copper (II) in seawater. The limitations of the method are also discussed. 

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. 

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