Aging half-lives

The apparent half-life of aging depends upon the enzyme, inhibitor, pH, and temperature. However, NTE inhibited with most neuropathic agents exhibits aging half-lives on the order of several minutes, so that the week delay between exposure and appearance of signs of OPIDN is not due to the time required for aging of phosphylated NTE (Jianmongkol et al, 1999 Johnson, 1982 Kropp and Richardson, 2007 Richardson, 1992). 

Although excretion of aminoglycosides is similar in adults and children >6 months of age, half-lives of the drugs may be prolonged significantly in the newborn. Thus, it is critically important to monitor concentrations of aminoglycosides during treatment of neonates. 

In Eq. (63.13), ARi = activity of reactivated enzyme at faging/ = activity of reactivated enzyme at f = 0, AI = activity of inhibited enzyme without reactivator at faging/ and AIq = activity of inhibited enzyme without reactivator at f = 0. The slope ( 4) of the least-squares best-fit line from a plot of ln(100/% inhibition) versus f is determined using linear regression from the initial linear portion of the curve. From Eq. (63.12), it can be seen that the imits of k are (e.g., min ), and that the aging half-life, fi/2, the time required for 50% aging, can be determined from Eq. (63.14). 

The C exchanges with C in living organisms, but exchange ceases on death. The radioactive content decays with a half-life of 5730 years. Hence the age of a once living material may be established by determining the amount of C. 

Therefore, in these cases it is only necessary that the gaseous members be retained for periods of several of their half-lives for the measured ages to be unaffected by loss of the gases. In the chain, the Rn has a half-life of 3.8 d, so that diffusion out of the region of the may occur, but in the Th chain the Rn has a half-life of only 55 s, so that diffusion would be less likely. 

Streptokinase has an initial plasma half-life (/ 2 of 18 min, and a P half-life of 83 min (73) it is well recognized that the thrombolytic efficacy of the enzyme decreases as the age of the thrombus increases thus, thrombolysis is significantly decreased when therapy is initiated more than three hours after an occlusion (74). 

The effect of hemofiltration on drug elimination can be estimated from serum creatinine (SCr), age, and the MDRD-2 formula to predict the combined effect of filtration rate (eGFR = GFRresidual + HFR) on drug clearance and drug half-life during hemofiltration. 

The constant half-life of a nuclide is used to determine the ages of archaeological artifacts. In isotopic dating, we measure the activity of the radioactive isotopes that they contain. Isotopes used for dating objects include uranium-238, potassium-40, and tritium. However, the most important example is radiocarbon dating, which uses the decay of carbon-14, for which the half-life is 5730 a. 

A carbon sample of mass 1.00 g from wood found in an archaeological site in Arizona underwent 7.90 X 103 carbon-14 disintegrations in a period of 20.0 h. In the same period, 1.00 g of carbon from a modern source underwent 1.84 X 104 disintegrations. Calculate the age of the sample given that the half-life of 14C is 5.73 ka. 

The law of radioactive decay implies that the number of radioactive nuclei decreases exponentially with time with a characteristic half-life. Radioactive isotopes are used to determine the ages of objects. 

Most CO and CO2 in the atmosphere contain the mass 12 isotope of carbon. However, due to the reaction of cosmic ray neutrons with nitrogen in the upper atmosphere, C is produced. Nuclear bomb explosions also produce C. The C is oxidized, first to CO and then to C02 by OH- radicals. As a result, all CO2 in the atmosphere contains some 0, currently a fraction of ca. 10 of all CO2. Since C is radioactive (j -emitter, 0.156 MeV, half-life of 5770 years), all atmospheric CO2 is slightly radioactive. Again, since atmospheric CO2 is the carbon source for photos5mthesis, aU biomass contains C and its level of radioactivity can be used to date the age of the biological material. 

Equation does not contain the concentration of A, so the half-life of a first-order reaction is a constant that is independent of how much A is present. The decomposition reactions of radioactive isotopes provide excellent examples of first-order processes, as Example illustrates. We describe the use of radioactive isotopes and their half-lives to determine the age of an object in more detail in Chapter 22. 

As shown in Example, Equation is used to find a nuclear half-life from measurements of nuclear decays. Equation is used to find how much of a radioactive substance will remain after a certain time, or how long it will take for the amount of substance to fall by a given amount. Example provides an illustration of this t q)e of calculation. In Section 22-1. we show that Equation also provides a way to determine the age of a material that contains radioactive nuclides. 

One other highly useful chronometer is measurement of °Po- b disequilibria. °Po has a half-life of 138.4 days making the chronometer active for 2 yrs. °Po- °Pb fractionation is based on Po but not Pb partitioning into volatiles during degassing (Gill et al. 1985). Ey repeat analysis of °Po, Rubin et al. (1994) constrained the time of eruption of several samples from 9°N EPR to windows of-100 days. These dates are consistent with eruption windows based on submersible observation. Thus, this technique can provide critical age constraints for other U-series parent-daughter pairs but requires that samples be collected and analyzed as soon as possible after eruption. 

Of note are the values for °Th and as the revised values postdate the development of mass spectrometric techniques for measurement of U and Th in natural materials. Data published prior to Cheng et al. (2000b) does use not use the revised values whereas data published subsequently may or may not use the new values. The revised half-lives do have a small, but significant effect on calculated °Th ages, particularly ages older than about 100 ka. Furthermore, the new value for the half-life changes values as these are calculated from measured atomic ratios using... 

In addition to the assumptions of initial conditions, the validity of U-Pb methodology relies on closed system behavior of U, Pb and intermediate nuclides in the decay chain. Concordance between the two U-series decay chains is most likely to be compromised by Rn loss because Rn is the only gas in the decay chains and has a high diffusivity. Radon-222 in the decay chain has a half life of 3.8 days. This is much longer than the half-life of Rn (3.96 s) in the decay chain. Therefore, partial loss of Rn will give rise to an apparent age younger than the true age, whereas the 207p /235p ... 

The element revealed itself through spectacular violet-colored flames and several red spectral lines. The metal melts at 38 °C, is very soft, and extremely reactive (burns in air and reacts violently with water). Rubidium is stored under mineral oil. It is suitable as a scavenger (oxygen capture) in vacuum tubes, where it is deposited on the glass as a mirror. It can also be found in photocells and phosphors for screens (for example, for air-traffic controllers. Not physiologically important. The radioactive rubidium-87 is useful for age determination in geochronology (half-life ca. 50 billion years). 

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