Isotopic, preparation

Stereochemically defined, isotopically prepared methyl groups used to determine the stereochemical course of methyltransferases . One typically utilizes [(S)-or [(i )- H,2H,iH]CH3X where X is the remaining part of the substrate. This process cannot be observed when all three hydrogen atoms are H, because they are equivalent and stereochemically indistinguishable. 

The isotope first isolated resulted from beta emission by Np238. In the spring of 1941 Seaborg s group isolated a new isotope, prepared by neutron bombardment of U238. The series of reactions was ... 

Peak intensity calibration is another essential procedure required for correct determination of the isotope concentrations from the observed spectra. Here we should note that the peak intensity per one OH bond is not necessary equal to that per one OD bond. Calibration spectra are taken for an ice specimen containing homogeneously distributed H and D isotopes prepared by freezing an... 

Q1, Americium Am at. wt (most stable isotope) 243 at no. 95 valence 3 4, 5, 6, Completely man-made element. Isotopes (mass numbers) 237-246 all are radioactive, First isotope prepared (Tw 458 years a-emit ... 

This technique takes advantage of the observation (Ecker and Smith, 1968) that increases in the endogenous amino acid pool do not appear to alter the rate of flow of amino acid through the pool. Only the incorporation kinetics are changed, and these changes can be predicted. Thus, two different incorporation curves are obtained when two different con-contrations of leucine are injected into oocytes (Fig. 6). The differences between the two curves result from differences in the amount by which the endogenous leucine pool was expanded by the two isotope preparations and can be described mathematically as follows ... 

The presence of radioactive decay products In lead sources or tracers obtained from natural sources is undesirable for some applications. A number of lead Isotopes can be produced in carrier-free form and In high yield by helium-Ion bombardment of mercury or by protnn or deuteron bombardment of thallium targets. The isotopes prepared in this way and the relevant reactions are indicated In Table I. 

Several sources of background counts are present in this assay and need to be considered. One is the free Pi in the commercially obtained isotope preparation. This background steadily increases as the nucleotide preparation ages due to spontaneous hydrolysis of the GTP and radiation damage. Another is caused by the presence of nucleotidases and nucleoside... 

M. Oberst, J. Klein, and T. Halfmann. Enhanced four-wave mixing in mercury isotopes, prepared by stark-chirped rapid adiabatic. Optics Communications 2006 Aug 15 264(2) 463-470. 

If a particular Isotope preparation is said to be 2 Cf. how many Becquerels do you think it has ... 

The new elements neptunium and plutonium have been produced in quantity by neutron bombardment of uranium. Subsequently many isotopes have been obtained by transmutation and synthetic isotopes of elements such as Ac and Pa are more easily obtained than the naturally occurring species. Synthetic species of lighter elements, e.g. Tc and Pm are also prepared. 

Figure Bl.25.9(a) shows the positive SIMS spectrum of a silica-supported zirconium oxide catalyst precursor, freshly prepared by a condensation reaction between zirconium ethoxide and the hydroxyl groups of the support [17]. Note the simultaneous occurrence of single ions (Ff, Si, Zr and molecular ions (SiO, SiOFf, ZrO, ZrOFf, ZrtK. Also, the isotope pattern of zirconium is clearly visible. Isotopes are important in the identification of peaks, because all peak intensity ratios must agree with the natural abundance. In addition to the peaks expected from zirconia on silica mounted on an indium foil, the spectrum in figure Bl. 25.9(a)...

Boron exists naturally as 19.78% lOB isotope and 80.22% IIB isotope. High-purity crystalline boron may be prepared by the vapor phase reduction of boron trichloride or tribromide with hydrogen on electrically heated filaments. The impure or amorphous, boron, a brownish-black powder, can be obtained by heating the trioxide with magnesium powder. 

Naturally occurring krypton contains six stable isotopes. Seventeen other unstable isotopes are now recognized. The spectral lines of krypton are easily produced and some are very sharp. While krypton is generally thought of as a rare gas that normally does not combine with other elements to form compounds, it now appears that the existence of some krypton compounds is established. Krypton difluoride has been prepared in gram quantities and can be made by several methods. A higher fluoride of krypton and a salt of an oxyacid of krypton also have been... 

Krypton clathrates have been prepared with hydroquinone and phenol. 85Kr has found recent application in chemical analysis. By imbedding the isotope in various solids, kryptonates are formed. The activity of these kryptonates is sensitive to chemical reactions at the surface. Estimates of the concentration of reactants are therefore made possible. Krypton is used in certain photographic flash lamps for high-speed photography. Uses thus far have been limited because of its high cost. Krypton gas presently costs about 30/1. 

Eighteen isotopes of niobium are known. The metal can be isolated from tantalum, and prepared in several ways. 

Twenty five isotopes of polonium are known, with atomic masses ranging from 194 to 218. Polonium-210 is the most readily available. Isotopes of mass 209 (half-life 103 years) and mass 208 (half-life 2.9 years) can be prepared by alpha, proton, or deuteron bombardment of lead or bismuth in a cyclotron, but these are expensive to produce. 

Gr. aktis, aktinos, beam or ray). Discovered by Andre Debierne in 1899 and independently by F. Giesel in 1902. Occurs naturally in association with uranium minerals. Actinium-227, a decay product of uranium-235, is a beta emitter with a 21.6-year half-life. Its principal decay products are thorium-227 (18.5-day half-life), radium-223 (11.4-day half-life), and a number of short-lived products including radon, bismuth, polonium, and lead isotopes. In equilibrium with its decay products, it is a powerful source of alpha rays. Actinium metal has been prepared by the reduction of actinium fluoride with lithium vapor at about 1100 to 1300-degrees G. The chemical behavior of actinium is similar to that of the rare earths, particularly lanthanum. Purified actinium comes into equilibrium with its decay products at the end of 185 days, and then decays according to its 21.6-year half-life. It is about 150 times as active as radium, making it of value in the production of neutrons. 

A clear cut answer was provided by Irving Roberts and Harold C Urey of Colum bia University in 1938 They prepared methanol that had been enriched in the mass 18 isotope of oxygen When this sample of methanol was esterified with benzoic acid the... 

A solution to the question of the mechanism of these reactions was provided by John D Roberts m 1953 on the basis of an imaginative experiment Roberts prepared a sample of chlorobenzene m which one of the carbons the one bearing the chlorine was the radioactive mass 14 isotope of carbon Reaction with potassium amide m liquid... 

Isotope Dilution Another important quantitative radiochemical method is isotope dilution. In this method of analysis a sample of analyte, called a tracer, is prepared in a radioactive form with a known activity. Ax, for its radioactive decay. A measured mass of the tracer, Wf, is added to a sample containing an unknown mass, w, of a nonradioactive analyte, and the material is homogenized. The sample is then processed to isolate wa grams of purified analyte, containing both radioactive and nonradioactive materials. The activity of the isolated sample, A, is measured. If all the analyte, both radioactive and nonradioactive, is recovered, then A and Ax will be equal. Normally, some of the analyte is lost during isolation and purification. In this case A is less than Ax, and... 

The previous discussion demonstrates that measurement of precise isotope ratios requires a substantial amount of operator experience, particularly with samples that have not been examined previously. A choice of filament metal must be made, the preparation of the sample on the filament surface is important (particularly when activators are used), and the rate of evaporation (and therefore temperature control) may be crucial. Despite these challenges, this method of surface ionization is a useful technique for measuring precise isotope ratios for multiple isotopes. Other chapters in this book discuss practical details and applications. 

A major advantage of this hydride approach lies in the separation of the remaining elements of the analyte solution from the element to be determined. Because the volatile hydrides are swept out of the analyte solution, the latter can be simply diverted to waste and not sent through the plasma flame Itself. Consequently potential interference from. sample-preparation constituents and by-products is reduced to very low levels. For example, a major interference for arsenic analysis arises from ions ArCE having m/z 75,77, which have the same integral m/z value as that of As+ ions themselves. Thus, any chlorides in the analyte solution (for example, from sea water) could produce serious interference in the accurate analysis of arsenic. The option of diverting the used analyte solution away from the plasma flame facilitates accurate, sensitive analysis of isotope concentrations. Inlet systems for generation of volatile hydrides can operate continuously or batchwise. 

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