Speciation and Element-Specific Detection

Various forms of off- and on-line AES/AAS can achieve element specific detection in IC. The majority of atomic emission techniques for detection in IC are based on ICP. In the field of speciation analysis both IC-ICP-AES and IC-ICP-MS play an important role. Besides the availability of the ICP ion source for elemental MS analysis, structural information can be provided by interfaces and ion sources like particle beam or electrospray. 

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. 

Every coupling application favors one part of the coupling system. A dominating chromatography part leads to the speciation analysis [5,6,26,27]. The elemental specific detection facilities of atomic spectrometry are strongly favored over the multielement capabilities. An inversion of this construction leads to multielement trace analysis in complex matrices with the use of chromatographic equipment as powerful preconcentration and matrix elimination tool [13k The ability of chromatography for a further time resolution between the separated traces is not really required because of the excellent elemental specific detection capabilities of atomic spectrometry. 

Michalke, B. and Schramel, P. (1996) Hyphenation of capillary electrophoresis to inductively coupled mass spectrometry as an element-specific detection method for metal speciation. J. Chromatogr. A, 750, 51-62. 

Element-specific detection by ICP-MS has been widely used in the characterization of metallothioneins (MTs). The biological importance of these proteins is due to their role in homeostatic regulation of essential heavy metals like Cu and Zn. On the other hand, MT protects the cells from harmful chemicals, like nonessential and excessive essential heavy metals, reactive oxygen species, radicals, and alkylating agents. Fararello et reviewed different chromatographic approaches with ICP-MS detection for the multielemental speciation in MTs and MT-like proteins. 

Garcia, S.D., Montes-Bayon, M., Blanco, G.E., Sanz-Medel, A. Speciation studies of cis-platin adducts with DNA nucleotides via elemental specific detection (P and Pt) using liquid chromatography-inductively coupled plasma-mass spectrometry and structural characterization by electrospray mass spectrometry. J. Anal. At. Spectrom. 21, 861-868 (2006)... 

ICP-MS and on-hne LC-ICP-MS are primarily used for element speciation in environmental, food-related, and clinical applications. Its use in metabolism stndies is more recent [66-67], Bromine-specific detection by LC-ICP-MS in conjnnction with LC-MS has been apphed to selectively find the bromine-containing metabolites of 4-bromoaniline [66] in rat mine, while chlorine and sulfur-specific detection was applied in a metabohte stndy on diclofenac [67], Pharmaceutical applications of LC-ICP-MS were reviewed [68],... 

The most definitive assessment of the metal composition of metalloproteins comes from the application of element-specific detection methods. CE-ICP-MS provides information not only about the type and quantity of individual metals bound to the proteins but also about the isotopes of each element as well [11,12]. Elemental speciation has become increasingly important to the areas of toxicology and environmental chemistry. Such analytical capability also opens up important possibilities for trace element metabolism studies. Figure 1 depicts the separation of rabbit liver metallothionein containing zinc, copper, and cadmium (the predominant metal) using CE-ICP-MS with a high-sensitivity, direct injection nebulizer (DIN) interface. UV detection (200 nm) was used to monitor the efficiency of the CE separation of the protein isoforms (MT-1 and MT-2), whereas ICP-MS detection made it possible to detect and quantify specific zinc, copper (not shown), and cadmium isotopes associated with the individual isoform peaks. 

Gas and liquid chromatography Coupled with ICP-AES these are useful approaches for speciation work, as shown by the early work of Cox et al. [418] for Crm/Crvl speciation, where AI2O3 filled columns and elution with acids and NaOH were used. Anion exchangers could also be used. New impetus has arisen from the interfacing of thermospray and high-pressure nebulization to ICP-AES. ICP-AES remains of interest for the element-specific detection in the speciation of silicon compounds and is a good alternative to ICP-MS, where considerable spectral interferences hamper the power of detection that can be obtained. 

Helium MIPs are excellent for element-specific detection in gas chromatography, as has been commercially realized [451]. In this way, not only are the halogens and other elements relevant in pesticide residue analysis but also organolead and or-ganotin compounds determined down to low concentrations. This makes MIP-AES very useful for speciation work [321, 452]. It has been shown that the delocalized helium MIP gave low detection limits for elements with high excitation potentials... 

In speciation, glow discharges are excellent detectors for GC work as shown earlier. In addition to the low power and pressure ICPs they can be used successfully for element-specific detection for gas chromatography. An rf-GD-MS system has been used as a detector for GC by Olson et al. [661], The set-up should consist of a temperature-controlled transfer line of stainless steel from the exit of the GC to the inlet of the GD source. The system has been tested with tetraethyl-Pb, tetraethyl-Sn and tetrabutyl-Sn and provided useful structural information for the identification of these compounds through the observation of fragment peaks the detection limits were down to 1 pg. 

A common speciation scheme after sample preparation involves a fractionation step followed by the element quantification in the fractions obtained. A clear trend exists toward using the techniques that combine separation and detection steps into one operating on-line system. In these coupled techniques, the selectivity is achieved by application of powerful separation modes (different chromatographic or electrophoretic methods), while the use of atomic spectrometric techniques assures high sensitivity of detection. It should be stressed, however, that coupled techniques with element-specific detection do not provide structural information for the species. If the appropriate standards are available, the assignment of chromatographic peaks can be accomplished by spiking experiments. On the other hand, the identification of unknown forms and/or ultimate confirmation of unexpected compounds observed in the sample require the use of complementary techniques (molecular mass spectrometry or NMR). ... 

There are a number of other GC detectors commercially available. Photoionization detectors (PIDs) are primarily used for the selective, low-level detection of the compounds which have double or triple bonds or an aromatic moiety in their structures. Electrolytic conductivity detectors (ELCDs) are used for the selective detection of chlorine-, nitrogen-, or sulfur-containing compounds at low levels. Chemiluminescence detectors are usually employed for the detection of sulfur compounds. The atomic emission detectors (AEDs) can be set up to respond only to selected atoms, or group of atoms, and they are very useful for element-specific detection and element-speciation work. 

The most definitive assessment of the metal composition of metalloproteins comes from the application of element-specific detection methods. CE/ICP-MS provides information not only about the type and quantity of individual metals bound to the proteins but also about the isotopes of each element as well. Elemental speciation has become increasingly important to the areas of toxicology and environmental chemistry. Such analytical capability also opens up important possibilities for trace element metabolism studies. Fig. 1 depicts... 

The chemical speciation study of trace elements in life sciences has been paid more and more attention in recent years, mainly because it can provide more significant information on the pathway, distribution, accumulation, excretion, and functions of trace elements in biological systems of interest than the traditional bulk composition study. Almost all speciation techniques consist of two steps. The first step involves the separation of species from the sample followed by the second step of element specific detection. The so-called molecular neutron activation analysis (MoNAA) or speciation neutron activation analysis (SNAA) is, in fact, a combination of conventional NAA with physical, chemical, or biological separation procedures in order to meet the ever-increasing need for chemical species study. 

Zheng, J., Ohata, M., Furuta, N., and Kosmus,W. (2000). Speciation of selenium compounds with ion-pair reversed-phase liquid chromatography using inductively coupled plasma mass spectrometry as element-specific detection./ Chromatogr. 874(1), 55. 

K.J. Irgolic and F.E. Brinckman, Liquid chromatography element-specific detection systems for analysis of molecular species , in The importance of chemical speciation in environmental processes , M. Bernhard, EE. Brinckman and P Sadler (Eds.), Dahlem Konferenzen 1986, Springer-Verlag, Berlin, 1986, pp. 667-684. 

This chapter reviews and expands on the material presented by the authors at the 1987 Eastern Analytical Symposium. A short review of the work done in this area by other researchers will be presented for completeness. The primary focus will be on chromatographic sample introduction for plasma source MS. The use of ICP-MS for the detection of high performance liquid chromatography (HPLC) eluates will be discussed. Speciation of several As and Sn compounds found in the environment demonstrates the ability of ICP-MS to provide element specific detection at ultra-trace levels. In addition, gas chromatographic detection using He microwave-induced plasma mass spectrometry (MIP-MS) is shown to be a promising technique, particularly for the determination of halogenated species. 

Applications Electrochemical techniques, while lacking the wide elemental range and long linear response of some atomic and mass spectrometry technologies, offer a valuable alternative in a number of specific applications, and have particular advantages for direct speciation and anion determination. In ion chromatography, amperometric, potentiometric and conductometric detection is widely used [472], see also Section 4.4.2.5. 

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