Isotope methods, determination

Wheat samples are extracted with dilute ammonia on the ASE200. The extracts are amended with isotopically labeled internal standards. The extracts are purified by sequential octadecyl reversed-phase solid-phase extraction (Cig SPE) and ethylenediamine-iV-propyl anion exchange (PSA) SPE. The samples are analyzed by LC/MS/MS. This method determines crop residues of flucarbazone-sodium and A-desmethyl flucarbazone with a limit of quantitation (LOQ) of 0.01 mgkg for each analyte. 

Sharama et al. [479] compared results obtained in the determination of cobal-amins in ocean waters by radioisotope dilution and bioassay techniques. These workers showed that the isotopic methods measured both biologically active and inactive cobalamins indiscriminately when porcine factor was used as the B12-specific binder. 

Epstein and Yapp [4] state "it is obviously necessary to calibrate more specifically the relationship between 6(D) records in cellulose nitrate from tree-ring records and known climatic records. This can probably be done best by the analysis of tree rings from widely different, well-documented environments. Such data will allow the comparison of a large variety of trees and determine the versatility of using the isotopic method for climatic temperature determination". We concur with this statement as long as "from the same population" is inserted. 

Plasma levels of dobutamine hydrochloride are determined by reaction of the drug with 3H-methyl-S-adenosylmethionine in the presence of catechol O-methy1-transferase. The radioactivity of the labeled methyl derivative is determined by a liquid scintillation counter using an external standard. The final recovery of added dobutamine as 3H-CH3-dobutamine is 24.9 1.3% in the range of 2 to 170 ng/ml (4). When 14C-dobutamine is administered the samples are counted by a double isotope method. 

Boeda et al. (1996) identified bitumen on a flint scraper and a Levallois flake, discovered in Mousterian levels (about 40 000 BP) at the site of Umm el Tlel in Syria. The occurrence of polyaromatic hydrocarbons such as fluoranthene, pyrene, phenanthrenes and chrysenes suggested that the raw bitumen had been subjected to high temperature. The distribution of the sterane and terpane biomarkers in the bitumen did not correspond to the well-known bitumen occurrences in these areas. In other studies of bitumen associated with a wide variety of artefacts of later date, especially from the 6th Millennium BC onwards, molecular and isotopic methods have proved successful in recognizing different sources of bitumen enabling trade routes to be determined through time (Connan et al., 1992 Connan and Deschesne, 1996 Connan, 1999 Harrell and Lewan, 2002). 

Isotopic methods for estimating calcium absorption have been evaluated by several researchers (49,55-58). From the human data of Harrison et al. (55), the relationship between percent calcium absorption determined by isotope dilution (Y) and excreta counting... 

The isotope dilution gas chromatography-mass spectrometry method described by Lopez-Avila et al. [16] and discussed in section 5.3.1.3 has been applied to the determination of Atrazine in soil. In this method known amounts of labelled Atrazine were specked into soil samples before extraction with acetone-hexane. The ratio of the naturally abundant compound and the stable-labelled isotope was determined by high-resolution gas chromatography-mass spectrometry with the mass spectrometer in the selected ion monitoring mode. Detection limits of 0.1-l.Oppb were achieved. Accuracy was >86% and precision better than 8%. 

Corrections for instrumentally-produced mass fractionation that preserve natural mass dependent fractionation can be approached in one of two ways a double-spike method, which allows for rigorous calculation of instrumental mass fractionation (e.g., Dodson 1963 Compston and Oversby 1969 Eugster et al. 1969 Gale 1970 Hamelin et al. 1985 Galer 1999 see section Double-spike analysis ), or an empirical adjustment, based on comparison with isotopic analysis of standards (Dixon et al. 1993 Taylor et al. 1992 1993). The empirical approach assumes that standards and samples fractionate to the same degree during isotopic analysis, requiring carefully controlled analysis conditions. Such approaches are commonly used for Pb isotope work. However, it is important to stress that the precision and accuracy of isotope ratios determined on unknown samples may be very difficult to evaluate because each filament load in a TIMS analysis is different. 

Some of the methods commonly used for the determination of thorium in biological materials are given in Table 6-1. The colorimetric methods are not capable of isotope-specific determination of thorium isotopes. Alpha spectrometric and neutron activation analysis are useful in the quantification of isotope-specific thorium and thorium-232, respectively, and have better sensitivities than colorimetric methods. Alpha spectrometry is the commonly used isotope-specific analysis for the determination of thorium-232 and the thorium-230 derived from the decay of uranium-238 (Wrenn et al. 1981). Standard reference materials (SRMs) containing thorium in human liver (SRM-4352) and human lung (SRM-4351) necessary for the determination of absolute recovery in a given sample are available from the National Institute of Standards and Technology (Inn 1987). 

Nitz, S., Kollmannsberger, H., Weinreich, B., Drawer , F. (1991) Enantiomeric distribution and 13C/12C isotope ratio determination of y-lactones—appropriate methods for the differentiation between natural and non-natural Flavours ]. Chromatogr. 557 187-197. 

JF Gregory, PR Trumbo, LB Bailey, JP Toth, TG Baumgartner, JJ Cerda. Bioavailability of pyridox-ine-5 -/S-D-glucoside determined in humans by stable-isotopic methods. J Nutr 121 177-186, 1991. 

The investigations which have been described in this article point up the usefulness of isotopic methods for the determination of the fine details of reactions involving photosensitization by Hg 6 (3Pi) atoms. Such techniques would appear particularly valuable for gaining insight into the nature of the transition states in these systems. 

In most of the examples, the isotopic methods are used essentially for analytical purposes. In this connection, their great value lies in extreme sensitivity and specificity. Many of the problems are concerned with the accurate determination of low concentrations of particular groups in polymer molecules. Another type of problem involves the determination of small weights of substances of low molecular weight formed simultaneously with polymer, or formed from polymer by chemical reactions for this purpose, the technique of isotope dilution analysis is of great value. The substances are usually present as minor components in rather complex mixtures so that determination by conventional methods would be extremely difficult. The method of isotope dilution analysis involves the addition to the mixture of a substantial quantity of the substance to... 

A complete set of 13C kinetic isotope effects determined (by a natural abundance CMR method) for addition of lithium dibutylcuprate to cyclohexenone, in THF at —78°C, have been shown to be consistent with those calculated theoretically for ratedetermining reductive elimination from an intermediate square-planar copper complex.120 Thus, die KIE (12k/13k) = 1.020-1.026 at C(3) is indicative of substantial bonding change, and partial alkyl transfer can explain the significant low KIE = 1.011-1.016 for Ca of die butyl group. 

During this century there has been considerable interest in the application of thermoluminescence studies to the recent history of meteorites. Natural TL provides a means of exploring radiation history and thermal environment in a manner which is complementary to isotopic methods, and the measurement of natural TL is now routine for the numerous meteorites being returned each year from the Antarctic (3,4). However, induced TL measurements have also proved of considerable interest, because the measurements have implications for the earliest history of meteorites. Essentially, the induced TL properties of meteorites are determined, with a few notable exceptions, by the amount and the nature of the feldspar in them, and feldspar is very sensitive to the major processes experienced by meteorites. In the present paper, we describe our recent work on the induced TL properties of meteorites and briefly discuss how these data relate to early meteorite history. We emphasize the relationship between the TL data and mineral properties. We also present here detailed descriptions of the cathodoluminescence properties of primitive meteorites, as these provide new insights into mineralogical controls on TL properties. 

Due to the inability to reliably detect beet sugar additions, the site-specific natural isotope fractionation nuclear magnetic resonance (SNIF-NMR) method used widely in the wine industry was adapted for the maple industry (Martin et al., 1996). This method determines the site specific isotope concentrations of organic compounds by nuclear magnetic resonance of ethanol fermented from the suspect sample. 

Ketterer, M.E. (1992) Assessment of overall accuracy of lead isotope ratios determined by inductively coupled plasma mass spectrometry using batch quality control and the Youden two-sample method. J. Anal. Atomic Spectrom. 7, 1125-1129. 

Introduction to Derivatization For many years derivatization by alkylation -especially as ethylation, but also as propylation - has been applied to transform Hg species into volatile Hg species before measurement with hyphenated techniques [2, 50, 52]. Sodium tetraphenylborate (NaBPh4) was also used for derivatization prior to measurement with GC-MIP-AED [53], Studies of possible species transformation, for example, during the analytical procedure, have been carried out with isotope-specific determination methods. The results showed that a direct ethylation of Me-Hg in an atmospheric precipitation sample by NaBEt4 produced no significant amount of artifactual Me-Hg [54]. Others investigated the species transformation processes in synthetic solutions to simulate environmental matrices. From the experiments it could be concluded that the species conversion, for example, of Me-Hg into zerovalent Hg, depends on the concentration levels of the halide [2]. Furthermore, the procedural order is of great importance, for example, ethylation should be done after addition of the organic phase to avoid species transformation [55]. 

When GC/MS analysis Is used, the metal of Interest Is chelated to a volatile organic compound and analyzed. This method Is the fastest and least expensive method of analysis, but It lacks the precision we require for reliable Isotopic ratio determinations. 

The problem of excess substrate addition in tracer experiments has been largely overcome by the advent of more sensitive mass spectrometers, however, and estimates obtained under conditions approaching in situ are possible. Owing to the great sensitivity of isotope ratio mass spectrometry, much shorter incubations (compared to the inhibitor and inventory methods) are possible (a few hours to 24 h are commonly used). Although they have not yet been widely applied for this purpose, the recently introduced sensitive isotope methods for determination of content ofN02 and N03 (Mcllvin and Altabet, 2005 Sigman et al., 2001) should make true tracer level incubations possible. Details on the most commonly used isotope tracer methods for... 

There have been very few comparisons of different methods for determination of nitrification rates. Enoksson (1986) compared three isotopic methods, incorporation with N-serve to inhibit nitrification, and N03 isotope dilution. She found ammonium oxidation measured with was 2-7 times greater than determined by the other methods, but pointed out that both different incubation times (from 1 to 12 days) and different substrate additions were used which would greatly alter the measured rates. Jones (1992) found a good relationship between a CO method and for measuring ammonium oxidation. Although most researchers have switched to direct assessment using there is stiU usage of the 002 method (e.g., Brion et al, 2000). 

The reduced partition functions of isotopic molecules determine the isotope separation factors in all equilibrium and many non-equilibrium processes. Power series expansion of the function in terms of even powers of the molecular vibrations has given explicit relationships between the separation factor and molecular structure and molecular forces. A significant extension to the Bernoulli expansion, developed previously, which has the restriction u = hv/kT < 2n, is developed through truncated series, derived from the hyper-geometric function. The finite expansion can be written in the Bernoulli form with determinable modulating coefficients for each term. They are convergent for all values of u and yield better approximations to the reduced partition function than the Bernoulli expansion. The utility of the present method is illustrated through calcidations on numerous molecular systems. 

Strong JM, Dutcher JS, Lee W-K, Atkinson AJ Jr. Absolute bioavailability in man of N-acetylprocainamide determined by a novel stable isotope method. Clin Pharmacol Ther 1975 18 613-22. 

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