Long half-lives, determination

In order to rapidly achieve a steady-state level of the compound in the plasma so that the organism is exposed to a fairly constant level, the dosage interval and half-life should be similar (Fig. 3.30). For steady-state conditions, the half-life determines the plasma level. That is, substrates with a long half-life attain a higher steady-state plasma level than compounds with shorter half-lives. It is obviously important to measure this plasma concentration for an assessment of chronic toxicity (Fig. 3.30). 

Because of the affinity of their unpaired electrons, free radicals have short lives, lend to dimerize and thus lose their reactivity. Because of their generally short half-lives (1-100 milliseconds), detection and identification of these entities is essentially through spectrophoiometric methods. However, in solid systems, free radicals can he trapped for appreciable lengths of time and at least one of these, 2,2-diphenyl-l-picrylhydrazyl. has such a long half-life that it is sold as such for the photometric determination of tocophcml. 

Obviously this wide distribution of the 14C formed in the atmosphere lakes time it is believed to require a period of 500-1000 years. This time is not. however, a deterrent to radiocarbon dating because of two factors die long half-life of I4C and the relatively constant rate of cosmic-ray formation of l4C in the earth s atmosphere over the most recent several thousands of years. These considerations lead to the conclusion that the proportion of 14C in the carbon reservoir of the earth is constant, and that the addition by cosmic ray production is in balance with the loss by radioactive decay. If this conclusion is warranted, then the carbon dioxide on earth many centuries ago had the same content of radioactive carbon as the carbon dioxide on earth today, Thus, radioactive carbon in the wood of a tree growing centuries ago had the same content as that in carbon oil earth today. Therefore, if we wish to determine how long ago a tree was cut down to build an ancient fire, all we need to do is to determine the relative 14C content of the carbon in the charcoal remaining, using the value we have determined for llie half life of 14C. If the carbon from Ihe charcoal in an ancient cave has only as much 14C radioactivity as does carbon on earth today, then we can conclude that the tree which furnished llie firewood grew 5730 30 years ago. 

Diphenylthiocarbazone ( dithizone ) forms intensely colored complexes with, for example, Mn, Fe, Co, Ni, Cu, Zn, Pd, Ag, Cd, Pt, Au, and Hg, which are soluble in CCLi and CHCI3, a property which facilitates then-separation and individual determination.Solutions ofmost metal dithizonates in organic solvents have the interesting property of being photochromic (see Photochromism), and the relatively long half-life of the photoexcited mercury(II) dithizonate complexes (ti/2 ca. 1 min) has facilitated then-investigation by conventional specttoscopic techniques. ... 

In the case of Th, the attainment of radioactive equilibrium with the daughter nuclides is very slow, because of the long half-life of Ra (q/2 = 5.75 y). Th can be determined directly by measuring its a radiation, but the measurement of Po is more sensitive (about 10 g Th can be determined in this way in 1 g of rock material). Other methods are based on the separation and measurement of Ra or Rn. In all determinations of Th, the possibility of the presence of radioactive impurities, mainly of members of the uranium and actinium families, has to be taken into account. 

Activation analysis has the advantage over other methods of trace element determination, in that after irradiation it is insensitive to contamination by inactive material. However, where nuclides of long, or moderately long, half-life are estimated in a radiochemical laboratory precautions must be taken to prevent contamination with residual activity from previous analyses ... 

Due to the long half-life of it is not feasible to determine carbon by neutron irradiation. Siie has used the reaction C d,7i-)N for its determination in organic matter 99). Samples of approximately 100 /xg were sealed in silica microtubes and irradiated for periods of 5 min in the deuteron beam from a cyclotron when the following reactions occurred ... 

Clinically, the half-life determines the interdosing interval, how long a pharmacological or toxic effect will persist and drug withdrawal times in food-producing animals and performance horses (Martinez 1998b). The plasma concentration of... 

Half-life also determines the time necessary to attain steady state. Long half-life hypnotics accumulate slowly but extensively, whereas short half-life benzodiazepines reach steady state rapidly and accumulate to a small extent. 

Half-life also determines the time required for drug washout after multidose treatment is discontinued. Short half-life benzodiazepines disappear quickly, whereas long half-life compounds are eliminated slowly following the last dose. 

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