Hybrid methods of speciation

In general, to identify and quantify different elemental species, it is necessary to use at least two analytical techniques. The first is used to separate the species, which may then be identified and quantified by the second analytical technique. Among the few exceptions to this rule are the electrochemical techniques (see Chapter 2) which may be able to identify and quantify different oxidation species of an element simultaneously. 

Many different separation and detection systems have been used for speciation. For example, size fractionation and ultra-filtration have been used for separation with the separated species then being determined by neutron activation (Tanizaki et al., 1992). These physico-chemical separation processes are, however, time consuming and the species have to be collected and then determined separately. Although the techniques are invaluable for certain types of speciation where the interaction of the species with colloids and sediments is important, hybrid or coupled techniques are usually preferred. 

In a hybrid technique the separation process and elemental detection occurs on-line. The two separate techniques must therefore be coupled by an interface. In designing the most effective coupled technique the detection system must be compatible with the separation process. The separation process in these systems is usually some form of chromatography, but the detectors normally used for chromatography lack the selectivity and sensitivity required for speciation studies. The favoured detectors for hybrid systems are the very sensitive and selective element-specific detectors. Selectivity is very useful in speciation studies because it means that only species of the element of interest will be detected, thus simplifying the sample preparation step. 

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Van Landeghem GF, D Haese PC, Lamberts LV, et al. 1994. Quantitative HPLC/ETAAS hybrid method with an on-line metal scavenger for studying the protein binding and speciation of aluminum and iron. Anal Chem 66 216-222. 

It would be difficult to find more comprehensive or more detailed studies on the physical chemistry of seawater than those done at the University of Miami (Millero, 2001). Several programs were developed for calculation of activity coefficients and speciation of both major ions and trace elements in seawater. The activity coefficient models have been influenced strongly by the Pitzer method but are best described as hybrid because of the need to use ion-pair formation constants (Millero and Schreiber, 1982). The current model is based on Quick Basic computes activity coefficients for 12 major cations and anions, 7 neutral solutes, and more than 36 minor or trace ions. At 25 °C the ionic strength range is 0-6 m. For major components, the temperature range has been extended to 0-50 °C, and in many cases the temperature dependence is reasonably estimated to 75 °C. Details of the model and the parameters and their sources can be found in Millero and Roy (1997) and Millero and Pierrot (1998). Comparison of some individual-ion activity coefficients and some speciation for seawater computed with the Miami model is shown in Section 5.02.8.6 on model reliability. 

Although NATs are thought to be unsuitable for speciation analysis, in recent years, more and more nuclear analysts have endeavored to use NATs for speciation studies by combinating NATs with specific pre-separation procedures to provide chemical species information. A variety of physical, chemical, or biological separation procedures have been developed in combination with NATs. The general experimental methods of the hybrid NATs being used for chemical speciation studies are listed in Table 8.3. ... 

Accordingly many procedures have been proposed for speciation. Amongst the most popular are the so-called hybrid techniques which couple separatory methods such as chromatography with detection methods such as optical or mass spectrometry. This has led to a proliferation of abbreviations, such as GC-MS, GC-AAS, GC-ICP-AES, GC-ICP-MS, HPLC-AAS, HPLC-ICP-MS, HPLC-ICP-AES, SFC-ICP-MS and CZE-ICP-MS, which read more like alphabet soup than science. 

Elemental speciation is becoming more and more important, since the environmental toxicity and biological importance of many elements depend on their oxidation states and different chemical forms. It is accepted today that the most reliable approaches for speciation are tandem techniques, such as hybrid analytical methods involving interfeced chromatography/atomic emission spectrometry (AES) or chromatography/inductively coupled plasma-mass spectrometry (ICP-MS). The low level det ion capability of ICP-MS makes it especially attractive as an element-specific chromatographic detector in chromatography. 

CE-MS has been used in analyses of anions and cations as a method simultaneously providing positive identification and quantitation. CE-ESI MS is particularly well suited for analyses of quaternary ammonium salts. Speciation analysis of As can be carried out using a CE-ICP-MS. Problems in interfacing the ICP-MS detection to CE are associated with low flow rates and small samples analyzed. Detection limits in ppb region can be attained, using postcapillary hybridization prior to ICP-MS. 

Many analytical strategies and methods have been described for elemental speciation. However, the so-called hyphenated (coupled or hybrid) techniques, consisting in the on-line coupling of an efficient separation technique [such as gas chromatography (GC), HPLC, or more recently capillary electrophoresis... 

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