Half-life measurement

We are applying the principles of enzyme mechanism to organometallic catalysis of the reactions of nonpolar and polar molecules for our early work using heterocyclic phosphines, please see ref. 1.(1) Here we report that whereas uncatalyzed alkyne hydration by water has a half-life measured in thousands of years, we have created improved catalysts which reduce the half-life to minutes, even at neutral pH. These data correspond to enzyme-like rate accelerations of >3.4 x 109, which is 12.8 times faster than our previously reported catalyst and 1170 times faster than the best catalyst known in the literature without a heterocyclic phosphine. In some cases, practical hydration can now be conducted at room temperature. Moreover, our improved catalysts favor anti-Markovnikov hydration over traditional Markovnikov hydration in ratios of over 1000 to 1, with aldehyde yields above 99% in many cases. In addition, we find that very active hydration catalysts can be created in situ by adding heterocyclic phosphines to otherwise inactive catalysts. The scope, limitations, and development of these reactions will be described in detail. 

In essence, NAA involves converting some atoms of the elements within a sample into artificial radioactive isotopes by irradiation with neutrons. The radioactive isotopes so formed then decay to form stable isotopes at a rate which depends on their half-life. Measurement of the decay allows the identification of the nature and concentration of the original elements in the sample. If analysis is to be quantitative, a series of standard specimens which resemble the composition of the archaeological artifact as closely as possible are required. NAA differs from other spectroscopic methods considered in earlier chapters because it involves reorganization of the nucleus, and subsequent changes between energy levels within the nucleus, rather than between the electronic energy levels. 

Superoxide has a chemical half-life measured in microseconds, but in even this short time serious damage can be caused to all types of biological macromolecules. Peroxidation of membrane lipids could cause haemolysis but the oxidation of ferrous (Fe2+) to ferric (Fe3+) iron in haemoglobin due to free radical action is a more immediate cause for concern within the red cell (Figure 5.17). 

Most protein pharmaceuticals exhibit a short half-life—measured in minutes—and high clearance because they are smaller than 30kDa and are readily cleared by glomerular filtration in the kidneys. Some pharmacokinetic features of protein phar-... 

The liver appears to be the principal organ for clearance. Excess vitamin D is stored in adipose tissue. The metabolic clearance of calcitriol in humans indicates a rapid turnover, with a terminal half-life measured in hours. Several of the l,25(OH)2D analogs are bound poorly by the vitamin D-binding protein. As a result, their clearance is very rapid, with a... 

Like most natural events, radioactive decay is not a uniform function. Consequently, the term half-life is meant to describe the value that would result if an infinite number of half-life measurements were made and the average calculated. Individual decays, however, follow a Poisson distribution, i.e.. the standard deviation is equal lu the square root of the number of observed decay events. This fact enables the experimenter to calculate the probable accuracy of his result, assuming no instrumentation inaccuracy. 

Age determinations using radioactive nuclides may be looked upon as processes that are the inverse of half-life measurements. If a radionuclide of known half life exists within an object, the age of that object can be determined either by measuring the number of radionuclides that remain or the number of product nuclides of the radioactive decay. In these determinations it is assumed that, if we know the half life of the radionuclide, an elapsed time t. or age, for the object can be found by using the formula t = (lnA /AL)/A, where k is the decay constant of the... 

Figure 3. Determination of oxygen from eiq half-life measurements.

Human chorionic gonadotropin is well absorbed after intramuscular administration and has a biologic half-life of 8 hours, compared with 30 minutes for LH. The difference may lie in the high sialic acid content of hCG compared with that of LH. It is apparently modified in the body prior to urinary excretion, because the half-life measured by immunoassay far exceeds that measured by bioassay. 

In conclusion thre first half-life measurements of light neutron rich nuclei using the MSU Reaction Product Mass Separator has resulted in the measurement of eight half-lives,two of which represent first time measurements and three of which are second measurements.The RPMS coupled with fast beam switching has proven to provide a very clean environment in which to study the decays of neutron rich nuclei. 

It is clear that biological systems can manage the chemical reactivity of unstable species. For example, oxalo-acetate—a metabolic intermediate in terran metabolism that is a precursor of citric acid, malic acid, and the amino acid aspartic acid—decarboxylates readily, with a half-life measured in minutes at room temperature at neutral pH. The half-life for the decarboxylation of oxaloacetate drops to seconds at high temperatures in pure water. It is not clear how microorganisms that live at high temperatures manage the instability of oxaloacetate, which is a key intermediate in standard biochemistry for the formation of amino acids, such as aspartate, and asparagine. 

The half-life measures the time it takes for the radiation coming off any mass of an isotope to drop to half its value. Short half-lives of hours or minutes mean that these isotopes give off radiation at such a rate that they deliver their radiation in a short time. Often, the shorter the half-life, the more energetic and more harmful the radiation. 

Here, No is the original number of atoms of the isotope, N is the number at time t, and hii is the half-life. Archaeological and geological samples may be dated by half-life measurement (Section 21.2). 

J. I. Vargas, The Chemical Applications of Angular Correlation and Half-life Measurements, in Radiochemistry, International Review of Sciences, Inorganic Chemistry, Series One, Vol. 8 (Ed. A. G. Maddock), Butterworths, London, 1971... 

Radionuclide Half-life Measured radiation Application... 

Note The mean elimination half-life measured in six functionally anephric patients was 41.9 hours (Stec GP, Atkinson AJ Jr, Nevin MJ, Thenot J-P, Ruo TI, Gibson TP, Ivanovich P, del Greco P. N-Acetylprocainamide pharmacokinetics in functionally anephric patients before and after perturbation by hemodialysis. Clin Pharmacol Ther 1979 26 618-28). 

Boron in the form of various oxides is removed from the atmosphere by precipitation and deposition fairly rapidly (a half-life measured in days). Whether via atmospheric deposition, precipitation, or weathering of boron-containing rocks, boron can be expected to migrate to the water column where it will hydrolysis to its weak acid form. Once in the water column the only significant factor that will affect its fate is possible adsorption to soils and sediment. The adsorption process is not well predicted, and will need to be determined for each sediment type being considered. 

Constitutive poly(ADP-ribose) is degraded in a biphasic manner in intact cells with reported half-lives of about 5 minutes and one to several hours. When polymer levels are enhanced by experimentally activating the polymerase, the polymer is turned over rapidly with a half-life measured in minutes or even less. 

We do not have to wait 4.51 X 10 yr to make a half-life measurement of uranium-238. Its value can be calculated from the rate constant using Equation (13.5). 

Figure 10. Foam half-life measurements of foam stability and foam height measurements of foaminess versus dilatational elasticity for aqueous foams stabilized by a-olefin sulfonates. (Reproduced from reference 22 Copyright 1986 American Chemical Society.)...

In mammals, the principal route of excretion of cesium is via the urine, and almost 80% of incorporated cesium is excreted by the kidneys. The biological half-life (measured using jjCs) is short, and depends heavily on the intensity of metabolism in the organism. In humans, the biological half-life is about 20 days for children, but up to 60 days for older adults (Burkhardt and Wirth, 1986). 

The time dependence of radioactive decay is expressed in terms of the half-life which is the time required for one-half of the radioactive atoms in a sanq>le to undergo decay. In practice this is the time for the measured radioactive intensity (or simply, radioactivity of a sanq>le) to decrease to one-half of its previous value (see Fig. 1.1). Half-lives vary from millions of years to fractions of seconds. While half-lives between a minute and a year are easily determined with fairly simple laboratory techniques, the determination of much shorter half-lives requires elaborate techniques with advanced instrumentation. The shortest half-life measurable today is about 10 s. Consequently, radioactive decay which occurs with a time period less than 10 s is considered to be instantaneous. At the other extreme, if the half-life of the radioactive decay exceeds 10 y, the decay usually cannot be observed above the normal signal background present in the detectors. Therefore, nuclides which may have half-lives greater than 10 y are normally considered to be stable to radioactive decay. However, a few unstable nuclides with extremely long half-lives, > 10 y, have been identified. It should be realized that 10 y is about 10 times larger than the age of the universe. 

New and revised half-life measurements results, Nud. Instrum. Meth. Phys. Res. 

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