Isotope dilution application

Three common quantitative applications of radiochemical methods of analysis are considered in this section the direct analysis of radioactive isotopes by measuring their rate of disintegration, neutron activation, and the use of radioactive isotopes as tracers in isotope dilution. 

Radioactivity. Methods based on the measurement of radioactivity belong to the realm of radiochemistry and may involve measurement of the intensity of the radiation from a naturally radioactive material measurement of induced radioactivity arising from exposure of the sample under investigation to a neutron source (activation analysis) or the application of what is known as the isotope dilution technique. 

Hintelmann H, Evans RD. 1997. Application of stable isotopes in environmental tracer studies — measurement of monomethyhnercury (CKjEg ) by isotope dilution ICP-MS and detection of species transformation. Fresenius J Anal Chem 358 378-385. 

The development of new fiber coatings in the near future should further improve the specificity of SPME and overcome some of the observed matrix effects. Quantification by stable isotope dilution gas chromatography/mass spectrometry (GC/MS) may assist in improving analytical performance. Along with the possible application of micro LC and capillary LC columns to in-tube SPME, the development of novel derivatization methods and the potential for the analysis of fumigant pesticides, SPME appears to be a technique with a future in the analysis of pesticide residues in food. 

Table 8.62 shows the main characteristics of ICP-MS, which is widely used in routine analytical applications. The ICP ion source has several unique advantages the samples are introduced at atmospheric pressure the degree of ionisation is relatively uniform for all elements and singly charged ions are the principal ion product. Theoretically, 54 elements can be ionised in an ICP with an efficiency of 90 % or more. Even some elements that do not show ionic emission lines should be ionised with reasonable efficiency (namely, As, 52 % and P, 33%) [381]. This is one of the advantages of ICP-MS over ICP-AES. Other features of ICP-MS that make it more attractive than ICP-AES are much lower detection limits ability to provide isotopic ratio information and to offer isotope dilution capabilities for quantitative analysis and clean and simple spectra. The... 

Isotope dilution (ID) is a technique for the quantitative determination of element concentrations in a sample, on the basis of isotope ratios [382]. An important prerequisite for isotope dilution is the availability of two stable isotopes, although in some cases the use of long-lived radionuclides allows the application range to be further extended [420]. 

Applications The application of the isotope dilution technique is especially useful in carrying out precise and accurate micro and trace analyses. The most accurate results in mass spectrometry are obtained if the isotope dilution technique is applied (RSDs better than 1 % in trace analysis). Therefore, application of IDMS is especially recommended for calibration of other analytical data, and for certification of standard reference materials. The technique also finds application in the field of isotope geology, and is used in the nuclear industry for quantitative isotope analysis. 

The thermal ion mass spectrometer was specifically developed for the measurement of isotope abundances and is capable of excellent precision. Although the spark source mass spectrometer used in this work lacks some of this precision, it has proved very useful in stable isotope dilution work. It has a number of advantages, including greater versatility, relatively uniform sensitivity, and better applicability to a wide range of elements. 

Kuehl, D.W., B.C. Butterworth, J. Libal, and P. Marquis. 1991. An isotope dilution high resolution gas chromatographic-high resolution mass spectrometric method for the determination of coplanar polychlorinated biphenyls application to fish and marine mammals. Chemosphere 22 849-858. 

A most important technique which has been developed as an extension of the isotope dilution principle is that of radioimmunoassay (RIA). Analyses by this method employ substoichiometric amounts of specific binding immuno-chemical reagents for the determination of a wide range of materials (immunogens) which can be made to produce immunological responses in animals such as sheep or rabbits. It is possible to combine the specificity of an immunochemical reaction with the extreme sensitivity of radiotracer detection. Analytical methods based upon these principles have achieved wide applicability in the determination of organic compounds at trace levels. 

Part—VI has been solely devoted to Miscellaneous Assay Methods wherein radioimmunoassay (RIA) (Chapter 32) has been discussed extensively. Various arms of theoretical aspects viz., hapten determinants and purity importance of antigenic determinants and analysis of competitive antibody binding of isotopically labeled compounds. The applications of RIA in pharmaceutical analysis, such as morphine, hydromorphone and hydrocordone in human plasma clonazepam, flurazepam in human plasma chlordiazepoxide in plasma barbiturates, flunisolide in human plasma have been described elaborately. Lastly, the novel applications of RIA-techniques, combined RIA-technique-isotope dilution and stereospecificity have also been included to highlight the importance of RIA in the analytical armamentarium. 

In Section III.D various methods were mentioned for determination of the 15N to 14N isotope ratio. Some applications to amines that appeared in the recent literature are presented here. Isotope dilution with a known aliquot of labelled compound allows solving some of the problems related to nonquantitative recovery yields of analyte in the analytical processing of a sample. However, the possibility of isotopic fractionation has to be taken into consideration. 

Mitchell PCH (1999) Molybdenum compounds. In Ullmann s Encyclopedia of Industrial Chemistry. Volume A16. Arpe H-J (ed) John Wiley and Sons, New York, p 675-82 Moore LJ, Machlan LA, Shields WR, Gamer EL (1974) Internal normalization techniques for high accurate isotope dilution analyses. Application to molybdenum and nickel in standard reference materials. Anal Chem 46 1082-1089... 

Note According to the enormous usage of isotope dilution, H- (D) and Relabeled standards [47] are commereially available for a wide range of applications. It is important not to select compounds with acidic hydrogens exchanged for deuterons. Other restrictions for internal standards apply analogously. 

Barker SA, Littlefield-Chabaud MA, David C. 2001. Distribution of the hallucinogens N,N-dimethyltryptamine and 5-methoxy-N,N-dimethyltryptamine in rat brain following intraperitoneal injection application of a new solid-phase extraction LC-APCI-MS-MS-isotope dilution method. J Chromatogr B Biomed Appl 751 37. 

The sensitivity and specificity conferred by making isotopically labeled derivatives of drugs in vitro can be used clinically (K18, K19), as can isotope dilution methods (H2, 07). The great imposition upon time, skills, and resources that such techniques demand, limits their application to selected research studies. They nevertheless provide a basis against which other more practicable methods can be assessed. 

A major attraction is the ability to perform isotope ratio measurements, e.g. in many geological applications to determine the age of rocks, and isotope dilution analysis. The latter in particular is gaining-popularity as a highly accurate, precise and hence traceable, method of analysis, so it is worthwhile describing these techniques in more detail. 

Hoffmann GF, Sweetman L, et al. (1991) Facts and artefacts in mevalonic aciduria development of a stable isotope dilution GCMS assay for mevalonic acid and its application to physiological fluids, tissue samples, prenatal diagnosis and carrier detection. Clin Chim Acta 198 209-227... 

In some applications like newborn screening and filter paper blood spots, the internal standard that is labeled cannot be mixed with blood. It can only be present in the extraction solvents. Therefore, only the extracted metabolites can be quantitatively measured. I have denoted a term called pseudo-isotope dilution to account for the differences between traditional isotope dilution and the technique commonly used in newborn screening by MS/MS. A special analysis is capable using this technique, however, in terms of an extraction efficiency experiment. With isotope-labeled standards you can perform an experiment whereby a traditional isotope-dilution technique (internal standard added to liquid blood and spotted) is compared to pseudo-isotope dilution techniques (internal standard is added to the extraction matrix). The ratio of the results of these two analysis (pseudo/traditional) is the extraction efficiency. 

As discussed before, quadrupole based ICP-MS allows multi-element determination at the trace and ultratrace level and/or isotope ratios in aqueous solutions in a few minutes as a routine method with detection limits of elements in the sub pgml-1 range and a precision for determined trace element concentration in the low % range (RSD - relative standard deviation). The precision for isotope ratio measurements varies between 0.1% and 0.5% RSD. This isotope ratio precision is sufficient for a multitude of applications, e.g., for evidence of contamination of sample with depleted or enriched uranium in urine (this technique is used in the author s laboratory in a routine mode14) or the isotope dilution technique for the quantitative determination of trace element and species concentration after doping the sample with enriched isotope spikes. 

Figure 5.21 Schematic of hyphenated ICP mass spectrometric techniques HPLC-ICP-MS for the application of isotope dilution techniques. (K. G. Heumann, L. Rottmann, j. Vogl, ]. Anal. At. Spectrom., 12, 1381 (1994). Reproduced by permission of The Royal Society of Chemistry.)...

An excellent possibility for quantifying analytical data in inorganic mass spectrometry is isotope dilution analysis (IDA) based on more precise isotope ratio measurements. IDA uses highly enriched isotope spikes of analytes of known concentration for calibration and is the method of choice if a high accuracy for element concentrations is required. The principles and applications of this method will be discussed below. 

The fundamentals and several applications of isotope dilution mass spectrometry requiring accurate isotope ratio measurements are reviewed by Heumann.50,51 Today isotope dilution mass spectrometry (IDMS) is recognized as a primary measurement method, by means of which accurate results with sufficiently small uncertainties can be achieved and therefore it has been used in certifying the composition of reference materials. A requirement of isotope dilution analysis in mass spectrometry is to achieve equilibration of spike and sample so that very careful sample preparation steps, especially in solid mass spectrometry, are necessary when a homogeneous sample spike mixture is to be prepared. 

For application in the isotope dilution technique and for tracer experiments using isotope enriched spikes, Merck (Darmstadt, Germany) have launched the production of new sets of isotopically enriched materials, such as 53Cr or mCd enriched spike calibration solutions, which were characterized by the Institute for Reference Materials and Measurements (IRMM, Geel, Belgium).52... 

A further application of the isotope dilution technique is the determination of selenomethionine in human blood serum by capillary HPLC-ICP(ORC) MS. 

Further details of different strategies in solution calibration are described in the literature.1 29 76 79 Precise and accurate measurements of isotope ratios, which is one of the major advantages of mass spectrometric techniques, are a requirement for the application of isotope dilution techniques in trace analysis, which is also the main goal of the application of isotope dilution in solution based calibration in LA-ICP-MS. 

For measurements of isotope ratios or isotope abundances, any of the mass spectrometers discussed in the previous chapters, such as SSMS, LIMS, GDMS56 and LA-ICP-MS,6 are of benefit for the direct isotope analysis of solid samples. SSMS and LIMS are rarely applied in isotope analysis due to their relatively low precision. Several applications of the isotope dilution technique as a calibration strategy in SSMS, mostly on geological samples, are known.57-59 GDMS has been mostly applied in multi-element trace analysis and depth profiling and plays only a minor role... 

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