Isotope tracer method

The isotope tracer method for estimating the total muscle mass of an organism by determining the amount of creatinine clearance. The method relies on the fact that muscle creatine phosphate is converted to creatine, and the latter is metabolized into creatinine. 

An isotope tracer method for estimating the total amount of water within an organism. 

Overall, serum and urinary increment methods have shown that a Ca source (e.g., CaCOs) accompanied by citrate is better absorbed than one that is not (Heaney et ah, 1999). Citrate that is absorbed into circulation is inclined to bind Ca ions and thereby artificially elevate incremental data compared to other salts (Heaney, 2001b). A number of studies utilizing the most sensitive isotopic tracer methods have demonstrated that CCM is highly absorbable compared to other Ca sources (Abrams et ah, 2003 Heaney et ah, 1989b Miller et ah, 1988 Smith et ah, 1987). 

The reaction mechanism of and active sites for the reaction have been studied using various techniques combined with an isotopic tracer method. Infrared spectra were measured for detection of surface adsorbates on the Cu-ZSM5 zeolites ESR, XPS, phosphorescence, diffuse reflectance UV, and Cu-MASNMR have been used to reveal the states of the copper ions in the catalysts. CO adsorption and TPD experiments have been employed to measure quantitativdy the amounts of Cu ions, NO adsorbed, and O2 remaining on the surface. Based on these investigations, we can propose a reasonable reaction mechanism which includes Cu ions as active sites and NO" species as reaction intermediates. The reaction cycle is suggested to be as foDows elevated temp. + 2NO... 

The rate of self-exchange reactions can generally be measured by isotopic tracer methods, but in several cases other techniques (optical rotation, nmr, epr) are more useful. Reactions like (18),... 

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... 

Better rate measurements Although the currently available methods for direct assessment of nitrification rates and the geochemical constraints provide internally consistent estimates of the overall rates and distributions of nitrification, improvements in direct rate methods are sorely needed to investigate the effects of environmental variables on rates under realistic conditions. These improvements will likely come with the application of even greater sensitivity isotope tracer methods, which require less substrate perturbation for detection of signal in the product pool. In addition, stable isotope methods that do not require tracer addition may possess the sensitivity to detect small fractionation... 

Relative to bioassays, advantages of the isotope tracer method include higher sensitivity, shorter incubation times, ability to measure gross uptake (e.g., total uptake corrected for any release of N during the incubation), and the ability to measure uptake in the presence of high rates of regeneration. One of the disadvantages of the approach is that the addition of a tracer may perturb the system if the addition is substantial, relative to the ambient concentration of the added substrate. This is particularly problematic in open ocean studies where ambient NH4+ and N03 are at or near the limit of detection. Under these conditions, even a small trace addition (e.g., 0.03 pM) can be a serious perturbation. 

Isotope Tracer Methods for Studies OF THE Marine Nitrogen Cycle... 

Androsterone and 5)3-androsterone, which are the major metabolites, were thought to be uniquely derived from the plasma androstenedione pool. Korenman et al. [170,388] demonstrated by the double isotope tracer method using carbon-14-labeled testosterone and tritium-labeled androstenedione that neither androsterone nor 5 -androsterone is a unique metabolite of a plasma androstenedione pool. A unique steroid metabolite has been defined by Dorfman [326] as a steroid persisting or formed during metabolism which can be related to one and only one tissue steroid. Korenman et al. obtained different tritium/carbon-14 ratios for androsterone and 5/3-androsterone, suggesting that other pathways of testosterone and androstenedione metabolism also exist. These other pathways, possibly metabolism of testosterone and androstenedione by peripheral tissue, may be responsible for the relative enrichment of either the 5a- or the 5/8-isomer of the urinary metabolites [305]. 

By 1950, such isotopic tracer methods began to revolutionize the study of redox reactions as color alone could not always be used to distinguish product formation see equation (1.9). The importance of H+ and other ions on electron transfer rates was soon discovered. A symposium on electron transfer took place in 1951 at the University of Notre Dame, during which a distinction between outer- and inner-sphere electron transfer was made. 

Lowe NM, Woodhouse LR, Matel JS, King JC. Comparison of estimates of zinc absorption in humans by using 4 stable isotopic tracer methods and compartmental analysis. Am J CUn Nutr 2000 71 523-9. 

Nuclear analytical techniques (NATs) and related isotopic tracer methods are well established as important tools in a wide variety of different kinds of environmental studies. They provide a wealth of information on sources, pathways and effects of many elements of environmental and health-related interest. Apart from being regarded as of particular strength in analytical quality assurance (IAEA, 1997), nuclear and related techniques cover studies of air particulates, solid waste products, sediments, food, water, human tissues, biomonitors and other kinds of environmental samples. 

Diffusion of a compound or element in a matrix containing an excess of the same species (active and inactive) isotopic tracer methods are the only methods available. At < 10 s. half-lifes t < 1 h. half-Iifes t < 3 d. Half-lives t >20 d. 

Many important tracer applications can be substituted by other methods. Even IDA and tracer applications in self-diffusion studies can be replaced by inactive isotope tracer methods using mass spectrometry and other methods for isotope ratio determination. However, because of the extremely high sensitivity of IDA, radioactive tracers are of unique usefulness in radioimmunoassays, radiorelease reagents, radiochromatography, AA, and for systematic studies in trace and ultra-trace analysis, physiological chemistry. IDA, diffusion, isotope exchange, and physical chemistry of solids. 

Comparison of stable-isotope-tracer methods for the determination of magnesiiun absorption in humans. Am. J. CUn. Nutr., 77, 1206-1212. 

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