Speciation problems

Separation processes in liquid chromatography (LC) are discussed in Chapter 2 and referred to in many other chapters. The great majority of hybrid LC applications to speciation problems have used HPLC coupled to different detectors as discussed below (Sections 4.4.2-4.4.4). 

Below are given some simple examples to illustrate the use of the above concepts in solving equilibrium speciation problems. To keep things relatively simple no... 

Leppard, G.G. (1992) Size, morphology and composition of particulates in aquatic ecosystems solving speciation problems by correlative electron microscopy. Analyst, 117, 595-603. 

The usual method of elemental speciation - also in human milk samples - is the use of already well-established separation and detection methods, which have to be combined in novel ways and modified according to the relevant speciation problems. Off-line combination and on-line hyphenation of separation techniques... 

As reported above, one fundamental characteristic of DPASV with respect to the speciation problem is the ability to differentiate the total dissolved metals in two fractions on the basis of electrochemical properties of chemical forms in which the metals are present. When the methodology is applied to analyze an untreated (undigested) sample, the measurement is sensitive to the so-called electroactive fraction of total dissolved metal concentration, which, under carefully controlled conditions (especially as regards the deposition potential and the rotation speed of the electrode), is practically composed of the ionic and the inorganically complexed fractions of metal. The metal aliquot strongly bound to organic ligands remains undetected (59, 67, 68). 

The separation and quantification of cyano complexes of various metals have been successfully carried out by Hilton and Haddad [45]. Cyano complexes of Cu(I), Ag(I), Fe(II), Fe(III), Co(II), Au(I), Au(IlI), Pd(II), and Pt(II) were analyzed by ion-pairing chromatography. The methods can be extended to many speciation problems. 

The lack of adequate CRMs for many speciation problems necessitates resort to reconsideration of the multiple steps in speciation analysis and use of multiple (different) approaches. The following section includes a summary of the most important strategies. 

Ludwig, Chr. GRIFIT, A Program for Solving Speciation Problems Evaluation of Equilibrium Constants, Concentrations and other Physical Parameters, Internal Report, University of Berne, 1992. 

Otherwise, and assuming that equilibrium conditions apply at the beginning of the experiment, the initial concentrations of the species at the beginning of the simulation are calculated from the corresponding chemical speciation problem. 

A large number of analytical techniques and strategies have been developed to tackle modern speciation problems. There are four main approaches possible computational, direct species-specific detection, hybrid techniques, and physicochemical characterization techniques. 

Notwithstanding the above constrains, computational methods are useful for a first approximation for a speciation problem or to predict speciation changes caused by changes in a parameter of the environmental conditions. Published practical examples of speciation by using a computational approach are the aluminum speciation in serum and distribution of trace elements in fresh waters. 

An additional four unknowns are the activity coefficients of each of the four species for which we could write four more equations, making a total of eight equations and eight unknowns. This is a routine speciation problem. Program SPECIES gives No. of iterations ... 

Direct introduction of a sample into an ICP produces information only on the total element content. It is now recognised that information on the form of the element present, or trace element speciation, is important in a variety of applications. One way of obtaining quantitative measurement of trace element speciation is to use the separation power of chromatography with the ICP as a detector. Since the majority of interesting trace metal speciation problems concern either nonvolatile or thermally unstable species, high-performance liquid chromatography (HPLC) becomes the separation method of choice. The use of HPLC as the separation technique requires the introduction of a liquid sample into the ICP with the attendant sample introduction problem. 

Collette and co-workers have recently developed a spectroscopy-based method for organic chemical speciation that meets all of these requirements [94,95]. Their initial test study was based on the calculation of the microenthalpies for the individual microequilibria of the metahydroxypyridine (MHP) zwitterionic system (see Fig. 11a) via UV spectroscopic measurements at different temperatures. By varying the temperature, the need for isolating the zwitterion and neutral species is removed, which is important because physical separation generally cannot be achieved due to rapid species interconversion. On the other hand, the more common isothermal, pH-variant spectroscopic methods of analysis are not informative for this type of speciation problem due to the fact the zwitterion and... 

One of the most important advantages of lEC in speciation analysis is its high separation efficiency and wide applicability. Solutions for many speciation problems are provided. Frocesses involved in lEC are based on the utilization of exchange equilibria between charged solute ions and the oppositely charged surface of the stationary phase. The equilibrium is determined by three variables pH, ionic strength of the mobile phase, and nature of the ion exchanger. 

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