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. 

Element-specific detection Molecular-specific detection... 

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. 

Reynolds, J. G., and Biggs, W. R., Application of Size Exclusion Chromatography Coupled With Element-Specific Detection to the Study of Heavy Crude Oil and Residua Processing Accounts of Chemical Research, 1988. 21 pp. 319-326. 

Element-specific detection combined with capillary GC has become a key technique in the chemical communication studies of our laboratory. An effective detector of this type is based on the microwave plasma emission (Wylie and Quimby 1989), with a tunable selectivity for several elements and a prominent sensitivity for sulfur-containing compounds, which is significantly greater than... 

A method with LOQ at ppt levels was developed based on LLE followed by GC-AFID for the determination of trace concentrations of nitrobenzene, l-chloro-2-nitrobenzene and synthetic fragrances such as musk xylene (223) and musk ketone (224). The method was applied to study the distribution of these compounds in environmental samples of North Sea waters460. GC with atomic emission detection (AED) has been successfully applied to the determination of nitro musks in human adipose tissues, at ppb concentration levels. A clean-up procedure for nonpolar substances and element-specific detection with AED enabled for the first time target screening analysis for lipophilic nitro aromatic compounds. The lack of sensitivity of AED was compensated by higher concentrations of the extracts... 

Some combinations of IC with MS detection are available, including ICP-MS (element-specific detection), particle beam MS and MS with atmospheric pressure ionization (API) operated in either electrospray ionization (ESI) or atmospheric pressure chemical ionization (APCI) modes that give information on molecular ions or adducts. 

The choice of the detector becomes crucial when the concentration of analyte species in the sample is very low and low limits of detection are required. For element-specific detection, the major atomic spectrometric techniques, flame AAS, OES, AFS and ICP-MS, are specially suited as chromatographic... 

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. 

Most important features of atomic spectrometry are the element specific detection and the superior sensitivity. Features such as the large dynamic range, the relative freedom from matrix effects even when atomic spectrometry is coupled to chromatography can be used more extensively to save time and to earn more accurate data using coupling techniques. 

The application of further element-specific detection systems like ICP-MS (Inductively Coupled Plasma-Mass Spectrometry) can result in the element distribution in the SEC fractions (Schmitt et al., 2003). 

Figure 10.8. Experimental setup for element-specific detection of field-flow fractionation.

Lintchinger, J., Koch, I., Serves, S., Feldmann, J. and Cullen, W.R. (1997) Determination of antimony species with high-performance liquid chromatography using element specific detection. Fresenius J. Anal. Chem., 359, 484-491. 

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. 

Nygren, O., Nilsson, C.-A. and Freeh, W. (1988) On-line interfacing of a liquid chromatograph to a continuously heated graphite furnace atomic absorption spectrometer for element-specific detection. Anal. Chem., 60, 2204-2208. 

Commonly used techniques for this purpose are chromatography and hyphenated methods, mostly coupled with element-specific detection, for example, GC-AED,221 GC coupled with atomic absorption detection (GC-AAS),222 or ICP-MS. The last mentioned provides superior sensitivity as well as the possibility of using isotopically labeled standards for quantification and quality control.223... 

Tanzer, D. and K.G. Heumann. 1990. GC determination of dimethyl selenide and trimethyl selenonium ions in aquatic systems using element specific detection. Atmos. Environ. 24 3099-3102. 

Separation of individual species and element-specific detection (extraction, sorption, ion exchange, gas permeation, and electrolysis)... 

Estes el al. [722] described a method for the measurement of triethyl- and trimethyllead chloride in potable water, using fused silica capillary column gas chromatography with microwave excited helium plasma lead specific detection. Element specific detection verified the elution of lead species, a definite advantage to the packed column method. The method involved the initial extraction of trialkyllead ions from water into benzene, which was then vacuum reduced to further concentrate the compounds. Direct injection of the vacuum concentrated solutions into the gas chromatography-microwave excited helium plasma system gave delectability of triethyllead chloride at the 30mg L 1 level... 

J. Zheng, M. Ohata, N. Furuta, W. Kosmus, Speciation of selenium compounds with ion-pair reversed-phase liquid chromatography using inductively coupled plasma mass spectrometry as element-specific detection. J. Chromatogr. A, 874 (2000), 55-64. 

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)... 

Direct (Element-Specific) Detection Methods Inductively coupled plasma-mass spectrometry (CE-ICP-MS)... 

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. 

Finally, reference is made to the element-specific detection via atom spectroscopic techniques, which include atomic absorption and atomic emission spectroscopy. 

For element-specific detection in gas chromatography, diode laser AAS is very powerful. When using several diode lasers simultaneously, signals for several elements can be determined at the same time and the composition of molecular species determined, e.g. Cl at the 837.60 nm line and Br at the 827.24 nm line and this with detection limits of down to 3 ng/mL or with respect to the injection volume 0.1 pg/s or 1 pg absolute. 

Fig. 81. Element-specific detection of Cl and Br in a modulated low-pressure helium plasma coupled to a gas chromatograph.

The oldest of the spectroscopic radiation sources, a flame, has a low temperature (see Section 4.3.1) but therefore good spatial and temporal stability. It easily takes up wet aerosols produced by pneumatic nebulization. Flame atomic emission spectrometry [265] is still a most sensitive technique for the determination of the alkali elements, as eg. is applied for serum analysis. With the aid of hot flames such as the nitrous oxide-acetylene flame, a number of elements can be determined, however, not down to low concentrations [349]. Moreover, interferences arising from the formation of stable compounds are high. Further spectral interferences can also occur. They are due to the emission of intense rotation-vibration band spectra, including the OH (310-330 nm), NH (around 340 nm), N2 bands (around 390 nm), C2 bands (Swan bands around 450 nm, etc.) [20], Also analyte bands may occur. The S2 bands and the CS bands around 390 nm [350] can even be used for the determination of these elements while performing element-specific detection in gas chromatography. However, SiO and other bands may hamper analyses considerably. 

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... 

Also low pressure ICPs and MIPs, often used as ion sources (see e.g. Creed et al. [606]) are very similar in their performance to glow discharges, except for them being electrodeless discharges. They have shown particular merits for element-specific detection in chromatography, where detection limits down to the pg/s level... 

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). ... 

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