Isotopes commonly used

Table 13.1 provides a list of several isotopes commonly used as tracers. The half-lives for these isotopes also are listed. What is the rate constant for the radioactive decay of each isotope ... 

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,... 

Isotopes that are not radioactive are called stable isotopes. If a particular isotope contains a greater number of neutrons than the most common form of the element, the isotope is frequently referred to as a heavy isotope. Stable isotopes commonly used In biochemical research include H (deuterium), C , N , O", S Ca, Fe Fe , Zn . and Zn . 

Besides deuterium and tritium, isotopes commonly used in organic chemistry include i C, available a > CH30H and Ba CO 0, as Ha 0 as 5NO3, N02 Cl, as chlorine or chloride as iodide. 

Table 10.4 Isotopes Commonly Used in Nuclear Medicine ...

The kinetic energy spectrum of emitted radiation is characteristic of the isotope. The energy is commonly measured in electron volts (eV). One electron volt is the energy acquired by an electron falling through a potential of 1 V. The isotopes commonly used in the clinical laboratory have energy spectra which range from 18 keV to 3.6 MeV. 

Positron-emitting isotopes are more numerous and more varied. They also tend to have a wide range of half-lives, although all have the same photon energy owing to the way in which the photons are produced. Among the isotopes commonly used for PET are "C (half-life 0.3 h),... 

Table 1 Physical properties of some isotopes commonly used in metabolism studies...

H, known as tritium, is an isotope commonly used in radioactive work. It has a half-life of 12.3 years, which makes it stable enough to use in various laboratory procedures. Another radioactive material sometimes used in laboratory experiments is that has a long half-life of 5730 years, which makes it convenient for dating materials of historical interest up to many tens of thousands of years old. 

A moderator, a substance that slows down neutrons, is required if uranium-235 is the fuel and this isotope is present as a small fraction of the total fuel. The neutrons that are released by the splitting of uranium-235 nuclei are absorbed more readily by uranium-238 than by other uranium-235 nuclei. However, when the neutrons are slowed down by a moderator, they are more readily absorbed by uranium-235, so it is possible to sustain a chain reaction with low fractional abundance of this isotope. Commonly used moderators are heavy water (1H2O), light water, and graphite. 

Equations 13.31 and 13.32 are only valid if the radioactive element in the tracer has a half-life that is considerably longer than the time needed to conduct the analysis. If this is not the case, then the decrease in activity is due both to the effect of dilution and the natural decrease in the isotope s activity. Some common radioactive isotopes for use in isotope dilution are listed in Table 13.1. 

Special sample inlet devices such as nebulizers, furnaces, and gas inlets are commonly used to avoid cross-contamination and accidental fractionation of isotopes. 

A.ccekrator-Producedlsotopes. Particle accelerators cause nuclear reactions by bombarding target materials, which are often enriched in a particular stable isotope, with rapidly moving protons, deuterons, tritons, or electrons. Proton reactions are most commonly used for production purposes. 

The most commonly used catalysts are palladized charcoal or calcium carbonate and platinum oxide. For better isotopic purity, the use of platinum oxide may be preferred for certain olefins since the substrate undergoes fewer side reactions while being chemisorbed on the platinum surface as compared to palladium.Suitable solvents are cyclohexane, ethyl acetate, tetrahydrofuran, dioxane or acetic acid-OD with platinum oxide. 

Polonium is extremely toxic at all concentrations and is never beneficial. Severe radiation damage of vital organs follows ingestion of even the minutest concentrations and, for the most commonly used isotope, °Po, the maximum permissible body burden is 0.03/zCi, i.e. 1100 Bq (=1100s ), equivalent to 7 x 10 g of the element. Concentrations of airborne Po compounds must be kept below 4 x 10" " mgm . ... 

Uranium-235 is the isotope of uranium commonly used in nuclear power plants. How many... 

Suggestions that phosphatic minerals in mammals could be used, however, revived the interest in climate reconstruction in continental interiors. Aquatic, cold-blooded animals like fish have body temperatures and body water oxygen isotopic compositions that are directly dependent on the water in which they live. For these animals, a commonly used equation describes the relationships among temperature, water oxygen isotopic composition and phosphate oxygen isotopic composition as (Longinelli and Nuti 1973 verified by Kolodny et al. 1983, among others) ... 

The first step in determination of a structure by NMR spectroscopy involves assignment of individual proton resonances. Development of high-field spectrometers and the use of a second dimension (2D-NMR) along with isotopic substitution (11) and sophisticated pulse sequences (12) make it possible to almost completely assign the proton spectrum of proteins of about 15 kD molecular weight (13—17). Some 2D-pulse sequences commonly used in the study of macromolecules are shown in Figure 1. 

Tracers have been used to label fluids in order to track fluid movement and monitor chemical changes of the injected fluid. Radioactive materials are one class of commonly used tracers. These tracers have several drawbacks. One drawback is that they require special handling because of the danger posed to personnel and the environment. Another drawback is the alteration by the radioactive materials of the natural isotope ratio indigenous to the reservoir— thereby interfering with scientific analysis of the reservoir fluid characteristics. In addition, the half life of radioactive tracers tends to be either too long or too short for practical use. 

Decay of the nuclide itself. The conceptually simplest approach is to take a known quantity of the nuclide of interest, P, and repeatedly measure it over a sufficiently long period. The observed decrease in activity with time provides the half-life to an acceptable precision and it was this technique that was originally used to establish the concept of half-lives (Rutherford 1900). Most early attempts to assess half lives, such as that for " Th depicted on the front cover of this volume, followed this method (Rutherford and Soddy 1903). This approach may use measurement of either the activity of P, or the number of atoms of P, although the former is more commonly used. Care must be taken that the nuclide is sufficiently pure so that, for instance, no parent of P is admixed allowing continued production of P during the experiment. The technique is obviously limited to those nuclides with sufficiently short half-lives that decay can readily be measured in a realistic timeframe. In practice, the longest-lived isotopes which can be assessed in this way have half-lives of a few decades (e.g., °Pb Merritt et al. 1957). 

In this chapter we discuss improvements documented in the literature over the past decade in these areas and others. Chemical procedures, decay-counting spectroscopy, and mass spectrometric techniques published prior to 1992 were previously discussed by Lally (1992), Ivanovich and Murray (1992), and Chen et al. (1992). Because ICPMS methods were not discussed in preceding reviews and have become more commonly used in the past decade, we also include some theoretical discussion of ICPMS techniques and their variants. We also primarily focus our discussion of analytical developments on the longer-lived isotopes of uranium, thorium, protactinium, and radium in the uranium and thorium decay series, as these have been more widely applied in geochemistry and geochronology. 

In these situations, addition of a tracer of unique isotopic composition is required, and the nature of the tracers added depends on the measurement technique. For example, short-lived and Th (with respective half-lives of 70 and 1.9 years) are commonly used as a tracer for alpha spectrometric analysis of U and Th, whereas longer-lived... 

L234 are about 3 per mil lower than those calculated with commonly used X234 values, hence the revised modem sea water of 145.8 1.7 per mil (Cheng et al. 2000b), compared to earlier values about 3 per mil higher. In general half-lives are now known precisely enough so that their contribution to error in age is comparable to or smaller than typical errors in isotope ratios (determined with mass spectrometric techniques). 

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