Techniques isotopic identification

Due to the complexity of organic matter sources in estuaries and the aforementioned problems associated with making only bulk measurements to constrain them, the application of chemical biomarkers has become widespread in estuarine research (see review, Bianchi and Canuel, 2001). The term biomarker molecule has recently been defined by Meyers (2003, p. 262) as compounds that characterize certain biotic sources and that retain their source information after burial in sediments, even after some alteration. This molecular information is more specific and sensitive than bulk elemental and isotopic techniques in characterizing sources of organic matter, and further allows for identification of multiple sources (Meyers, 1997, 2003). 

Plasma source mass spectrometry has also been used in the identification and differentiation of trace metals between anthropogenic and natural sources using an isotopic technique. Although, in the past, isotopic studies were mainly concentrated on Pb (206Pb, 207Pb and 208Pb) (Ault et al., 1970 Callender and Rice, 2000 Hansmann and Koppel, 2000 Semlali et al, 2001, 2004), modem techniques now also enable the characterisation of Cu and Zn isotopic concentrations in soils (Archer and Vance, 2004). 

Bjorge SM. Identification and characterization of drug metabolites using stable isotope techniques. Pharmacochem Libr 1997 26 233-242. 

VOCs can also be determined by isotope dilution GC/MS, where stable isotopically labeled analogs of the compounds of interest are available otherwise, an internal standard method is used. After purging and trapping, the sample is introduced into a gas chromatograph, and the compounds are separated by the GC and detected by a mass spectrometer. The labeled compounds serve to correct the variability of the analytical technique. Identification of a compound (qualitative analysis) is performed by comparing the GC retention time and the background corrected characteristic spectral masses with those of authentic standards. 

Motzer, W.E., Mohr, T.K.G., McCraven, S., Stanin, P. (2006) Stable and other isotope techniques for perchlorate source identification. Environ. Forensics, 7(1), 89-100. 

Purpose. This experiment illustrates a selective cross-coupUng reaction between an alkyl Grignard reagent and an aryl halide in the presence of a nickel phosphine catalyst to form a C—C bond. The reaction is also used to demonstrate the technique of "labeling" specific hydrogen atoms with an isotope for identification purposes. 

Benzoates. The selective debenzoylation of sucrose octabenzoate [2425-84-5] using isopropylamine in the absence of solvents caused deacylation in the furanose ring to give 2,3,4,6,1/3/6 -hepta- and 2,3,4,6,1/6 -hexa-O-benzoyl-sucroses in 24.1 and 25.4% after 21 and 80 hours, respectively (54). The unambiguous assignment of partially benzoylated sucrose derivatives was accompHshed by specific isotopic labeling techniques (54). Identification of any benzoylated sucrose derivative can thus be achieved by comparison of its C-nmr carbonyl carbon resonances with those of the assigned octabenzoate derivative after benzoylation with 10 atom % benzoyl—carbonyl chloride in pyridine. 

Applications With the current use of soft ionisation techniques in LC-MS, i.e. ESI and APCI, the application of MS/MS is almost obligatory for confirmatory purposes. However, an alternative mass-spectrometric strategy may be based on the use of oaToF-MS, which enables accurate mass determination at 5 ppm. This allows calculation of the elemental composition of an unknown analyte. In combination with retention time data, UV spectra and the isotope pattern in the mass spectrum, this should permit straightforward identification of unknown analytes. Hogenboom et al. [132] used such an approach for identification and confirmation of analytes by means of on-line SPE-LC-ESI-oaToFMS. Off-line SPE-LC-APCI-MS has been used to determine fluorescence whitening agents (FWAs) in surface waters of a Catalan industrialised area [138]. Similarly, Alonso et al. [139] used off-line SPE-LC-DAD-ISP-MS for the analysis of industrial textile waters. SPE functions here mainly as a preconcentration device. 

The ability to provide data on the isotopic composition of analyte elements, is a further special feature of the technique. This has already been exploited in a number of ways, including geological dating and the identification of sources of environmental lead contamination and poisoning. The radiogeneric source of lead means that different deposits may have different isotopic composition. In the UK this feature has been used to... 

The first mode of the high resolution C-NMR of adsorbed molecules was recently reviewed Q-3) and the NMR parameters were thoroughly discussed. In this work we emphasize the study of the state of adsorbed molecules, their mobility on the surface, the identification of the surface active sites in presence of adsorbed molecules and finally the study of catalytic transformations. As an illustration we report the study of 1- and 2-butene molecules adsorbed on zeolites and on mixed tin-antimony oxides (4>3). Another application of this technique consists in the in-situ identification of products when a complex reaction such as the conversion of methanol, of ethanol (6 7) or of ethylene (8) is run on a highly acidic and shape-selective zeolite. When the conversion of methanol-ethylene mixtures (9) is considered, isotopic labeling proves to be a powerful technique to discriminate between the possible reaction pathways of ethylene. 

CAI s that were once molten (type B and compact type A) apparently crystallized under conditions where both partial pressures and total pressures were low because they exhibit marked fractionation of Mg isotopes relative to chondritic isotope ratios. But much remains to be learned from the distribution of this fractionation. Models and laboratory experiments indicate that Mg, O, and Si should fractionate to different degrees in a CAI (Davis et al. 1990 Richter et al. 2002) commensurate with the different equilibrium vapor pressures of Mg, SiO and other O-bearing species. Only now, with the advent of more precise mass spectrometry and sampling techniques, is it possible to search for these differences. Also, models prediet that there should be variations in isotope ratios with growth direction and Mg/Al content in minerals like melilite. Identification of such trends would verify the validity of the theory. Conversely, if no correlations between position, mineral composition, and Mg, Si, and O isotopic composition are found in once molten CAIs, it implies that the objects acquired their isotopic signals prior to final crystallization. Evidence of this nature could be used to determine which objects were melted more than once. 

So far, biological Mossbauer spectroscopy is only sensitive to iron (or to be more precise to the Fe isotope) which rules out the use of this technique to directly probe the Ni site. Unfortunately, the iron site seems to remain always in a low-spin state making its identification by Mossbauer very difficult since it will be probably unresolved from the [3Fe-4S] and [4Fe-4S] cluster components. Also, each iron atom in the protein contributes equally to the Mossbauer spectrum the Fe site of the Ni-Fe centre is only 8.3 per cent of the total absorption. 

As with the bulk POM and DOM, the operationally defined fractions of UDOM and humic substances are quantified by elemental analysis and via broad molecular-class detection. Other strategies involve measurement of the natural isotopic composition, both stable and radioactive, of the various fractions. Efforts are underway to develop more sophisticated techniques, such as solid-state NMR and high-resolution mass spectrometry, far identification of specific bonds and functional groups. 

When applied to electron-transfer reactions, this kinetic isotope effect technique can provide information on the real reaction pathway leading to the formation of the product. Frequently, spectroscopic detection of species or identification of products is indicative of radical intermediates. The formation of the intermediates could simply be a blind step. 

Since its introduction some 20 years ago, MALDI-MS has been established as a standard technique for a large variety of applications within the field of bioanalyt-ical mass spectrometry, ranging from protein identification to enzyme activity screening. Quantitative analysis has long been a challenge, but, with the use of isotopically labelled standards, it is steadily obtaining more attention. 

Electrothermal vaporization has many appHcations relating to ICP analysis the abihty to vaporize solvents and to handle small soHd or viscous samples are particularly important. In addition, the measurement of the isotopic ratios of lead in various ores provides a unique identification of the ore and its source. Now that commercial systems are available, the technique will undoubtedly become popular. These techniques seem to have found favour, particularly in Japan. 

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