Isotopic dilution methods, determination

In most alpha and mass spectrometric methods for which sample preparation is extensive and chemical recoveries can vary considerably from sample to sample, precise elemental concentrations are determined by isotope dilution methods (e.g., Faure 1977). This method is based on the determination of the isotopic composition of an element in a mixture of a known quantity of a tracer with an unknown quantity of the normal element. The tracer is a solution containing a known concentration of a particular element or elements for which isotopic composition has been changed by enrichment of one or more of its isotopes. 

Quantitative analysis using FAB is not straightforward, as with all ionisation techniques that use a direct insertion probe. While the goal of the exercise is to determine the bulk concentration of the analyte in the FAB matrix, FAB is instead measuring the concentration of the analyte in the surface of the matrix. The analyte surface concentration is not only a function of bulk analyte concentration, but is also affected by such factors as temperature, pressure, ionic strength, pH, FAB matrix, and sample matrix. With FAB and FTB/LSIMS the sample signal often dies away when the matrix, rather than the sample, is consumed therefore, one cannot be sure that the ion signal obtained represents the entire sample. External standard FAB quantitation methods are of questionable accuracy, and even simple internal standard methods can be trusted only where the analyte is found in a well-controlled sample matrix or is separated from its sample matrix prior to FAB analysis. Therefore, labelled internal standards and isotope dilution methods have become the norm for FAB quantitation. 

A mass-isotope dilution method for determining the gamma isomer of benzene hexachloride, in which gamma-hexadeuterobenzene hexachloride is used as a tracer molecule and the dilution is determined by use of infrared spectrophotometry, has been developed by Trenner et al. (52). Impurities have no effect on the accuracy of this method. 

Thus, accurate determination of the specific activities SAa and SAa immediately provides the value of the unknown Ax. Note Whenever Axisotope dilution method suffers from any statistical variation in the measurements of quantities that are nearly identical. Should this prove to be true for your measurements, use a lesser amount of isotopic probe, so that Ax A. 

Bechtold WE, Sabourin PJ, Henderson RF. 1988. Reverse isotope-dilution method for determining benzene and metabolites in tissues. J Anal Toxicol 12(4) 176-179. 

The isotope dilution method consists of mixing a natural sample with an artificial spike and measuring the isotopic ratios of the mixture using mass spectrometry, providing very precise quantitative determination of the concentrations of elements in trace quantities. A spike is a solution that contains a known concentration of the element, artificially enriched in one of its minor isotopes. For example, natural Rb samples have 27.84% and 72.16% Rb (Fig. 11.lA). Rb spikes are made by artificially enrich the minor isotope Rb. And a solution with 90% Rb and 10% Rb (Fig. 11.IB) is a Rb spike. Of course, a solution with 99.99% Rb and 0.01% Rb is a better Rb spike. When the known quantities (mass) of the sample and spike are mixed, the resulting isotopic compositions can be used to calculate the concentration of the element in the sample. 

The isotopic dilution method can be extended to non-radioactive tracers by using mass spectrometry or NMR to determine the variation in isotopic ratios. This method can be used for the measurement of molecules or elemental species (about 60 elements have stable isotopes). This approach allows ultra-trace analysis because, contrary to radioactive labelling where the measurement relies on detecting atoms that decompose during the period of measurement, all labelled atoms are measured. Isotopic mass spectrometers are well suited for these measurements. 

In the interim between the initial development and application of Methods 624 and 625, the Effluent Guidelines Division, under contract, developed isotopic dilution methods for purgeable volatiles and semivolatile priority pollutants (11). The primary difference between Methods 624 and 625 and their 1624 and 1625 counterparts is that stable, isotopically labeled analogs of the compounds of interest are added to the sample prior to extraction, and quantitative determination is made by using the isotope ratio values for the compounds determined relative to their labeled analogs. In addition, Method 1625 is designed as a capillary column method and uses a 30-m X 0.25-mm silicone-bonded-phase fused silica column (J W DB-5). Method 1624 uses the same column materials as Method 624. Methods 1624 and 1625 are presented as acceptable alternatives to Methods 624 and 625. 

In applying the isotope dilution method to the analysis of HMX/RDX mixts, an accurately weighed amt of carbon-14 labeled HMX of known specific activity (counts per min per g) is added to a known wt of the mixt. The mixt is made homogeneous by dissolving it in acetone. A small amt of pure HMX is then isolated from the mixt by fractional recrystallization from acetone. The specific activity of the pure HMX sample, both before and after dilution, is determined by either dissolving or suspending a known wt in a scintillator soln or gel and counting with a commercial liq scintillation counter. 

One of the sampling sites was located on Isle Royale in the northwest part of Lake Superior near the Canadian border and far from the US Corn Belt. Atrazine, presumably from the Com Belt, was detected and verified by GC/MS analysis in samples from several rains at this site during June 1990. These data prompted the collection of water samples from Lake Superior and from four small lakes on Isle Royale in late September 1990. The atrazine concentration in these samples, determined by isotope dilution methods, was 6.5 nanograms per liter (ng/L) for Lake Superior,... 

Li, L.J., Jiang, K., 1995. Determination of toxic equivalents of PCDD/Fs in two Chinese commercial PCBs by C isotope dilution method. Acta Sci. Circumst. (Chinese) 15, 433 139. 

Ou, S.M., 2006. Determination of flame retardant—polybrominated diphenyl ethers (PBDEs) in the sediments from Xiamen coastal area and Yuan Dan Lake by GC/MS isotope dilution method. Fujian Anal. Test. (Chinese) 15, 1-3. 

This compound was synthesized by Dr. Nowacki by reacting 11+C-IAA-imidazole with myo-inositol (40). Application of these labeled compounds to corn kernels, followed immediately by homogenization of the tissue in acetone permitted us to determine the amounts of each constiuent in the kernel by the isotope dilution method of Rittenberg and Foster (41). An extension of this method, whereby the kernels are incubated for varying periods of time after application of the isotopically labeled compound permits determination of the "turnover" of the pool. Such data are shown in... 

In studies where radioisotopes can be used, using stable isotopes is usually less convenient and more time consuming. For trace element determinations, other methods like atomic absorption spectrometry are usually faster, more convenient and require simpler instrumenatlon. Other than for tracer studies in humans, stable Isotope dilution methods perhaps serve best in establishing the accuracy of other methods. 

In clinical chemistry the determination of stable elements by radiochemical methods offers no outstanding advantages over alternative methods, but the use of radioisotopes for determining organic compounds is developing rapidly. In isotope dilution methods (G6), a pure but radioactive form of the compound to be measured is mixed with the sample, a fraction is isolated, and its activity is determined. In radio-metric or derivative analysis (W14), a radioactive reagent is allowed to react with the analyte the labeled compound is separated and its activity is measured. The isotopes commonly used are C,... 

The epoxide metabolites of inhaled 1,3-butadiene, used in industry, are reported to be carcinogenic and mutagenic in rodents, and their in vivo concentration following inhalation exposure to butadiene has to be determined by gas chromatography/mass spectroscopy, the isotope dilution method utilizing 8 as an internal standard. Commercially available [De]-propylene oxide has been used previously as an internal standard to monitor in vivo blood propylene oxide levels following inhalation exposure to propylene. ... 

Stanley, E. M., Dawson, K. B., Field, E. O., and Glicksman, A. 8., An isotope dilution method for determining the absolute amounts and specific activities of proline and hydroxyproline in tissues. Intern. J. Appl. Radiation Isotopes 17, 467-477 (1966). 

Optical purity of barbituric acid enantiomers was determined by the isotope dilution method using 2-14C-labeled compounds 215 or by an NMR... 

The field of application for the isotope dilution method with radioactive tags, extends to measurements using stable isotope. Mass spectrometry or nuclear magnetic resonance are used to determine the variations in the isotopic concentrations. Chemical labelling using externally introduced tags consists of the addition to a sample of the same analyte but containing a stable isotope (e.g. H, C, N) as an internal standard. This method is as much used for molecular species as for atoms (around 60 have stable isotopes). 

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