Activity radioisotopes

Ao is the activity in disintegrations per second, (in becquerel - Bq) exaaiy at the end of the irradiation (at zero decay time, ta = 0) and ti is the time of irradiation. The expression in parentheses, 1 - e is called the saturation factor or S. It varies from 0 to 1 zero for ti = 0 or an extremely short irradiation, one for tj ti/2 or an infinitely long irradiation reiative to the ti/2 of the radioisotope. In practice, S rapidly and asymptotically approaches the limiting value of one with increasing ti relative to the ti/2 of the activated radioisotope species. At saturation, Ao simply equals NOa (steady state of formation of the radioisotope, rate of formation equaling rate of decay). 

More confident attribution usually is possible when radionuclides in a sample can have only limited origin from, say, atmospheric fallout and one or two nuclear facilities. For tritium, I, or uranium in the environment, for example, the specific activity (radioisotope/stable isotope ratio) can indicate the origin. Certain activation products can be attributed to specific nuclear medicine or reactor facilities. 

Many applications ia tracer technology require products of high specific activity, ie, compounds having a high degree of substitution of specific atoms with radioisotopes. For many labeled compounds nearly 100% labeling can be achieved at one or more locations ia a molecule usiag... 

An important consideration for all radiopharmaceuticals and especially radiolabeled biologically active molecules is specific activity. There are two types of specific activity radionucHdic and biological. RadionucHdic specific activity refers to the ratio of the number of atoms of a particular radioisotope to the total number of atoms of the element. For Tc, the radionuchdic specific activity is the number of Tc atoms to the total number of Tc and Tc atoms. Because all isotopes of an element ate chemically identical, a low specific activity may lead to a low yield in the synthesis of a radiopharmaceutical if a significant proportion of the reagents is consumed by the undesited isotopes. 

Radioisotope dilution assays are based on the principle of competition between radioactive labeled ( Co) vitamin B 2 and cobalamins extracted from matrices for binding sites on the intrinsic factor (a glycoprotein). Binding is in proportion to the concentration of the radioactive and nonradio active B 2 with the concentration of intrinsic factor as the limiting factor. Free cobalamins are separated from those bound on the intrinsic factor by absorption... 

Radioactivity in environmental waters can originate from both natural and artificial sources. The natural or background radioactivity usuaUy amounts to <100 mBq/L. The development of the nuclear power industry as weU as other industrial and medical uses of radioisotopes (qv) necessitates the deterrnination of gross alpha and beta activity of some water samples. These measurements are relatively inexpensive and are useful for screening samples. The gross alpha or beta activity of an acidified sample is deterrnined after an appropriate volume is evaporated to near dryness, transferred to a flat sample-mounting dish, and evaporated to dryness in an oven at 103—105°C. The amount of original sample taken depends on the amount of residue needed to provide measurable alpha or beta activity. 

Another application involves the measurement of copper via the radioisotope Cu (12.6-hour half-life). Since Cu decays by electron capture to Ni ( Cu Ni), a necessary consequence is the emission of X rays from Ni at 7.5 keV. By using X-ray spectrometry following irradiation, sensitive Cu analysis can be accomplished. Because of the short range of the low-energy X rays, near-surface analytical data are obtained without chemical etching. A combination of neutron activation with X-ray spectrometry also can be applied to other elements, such as Zn and Ge. 

Definitive identification of lysine as the modified active-site residue has come from radioisotope-labeling studies. NaBH4 reduction of the aldolase Schiff base intermediate formed from C-labeled dihydroxyacetone-P yields an enzyme covalently labeled with C. Acid hydrolysis of the inactivated enzyme liberates a novel C-labeled amino acid, N -dihydroxypropyl-L-lysine. This is the product anticipated from reduction of the Schiff base formed between a lysine residue and the C-labeled dihydroxy-acetone-P. (The phosphate group is lost during acid hydrolysis of the inactivated enzyme.) The use of C labeling in a case such as this facilitates the separation and identification of the telltale amino acid. 

Thin layer activation (TLA) has a long experience in monitoring or measuring wear and erosion. A small quantity of radioisotope tracer is introduced into the metal surface which can be either a coupon or component. Metal loss due to corrosion (provided the corrosion product is non-adherent) can be detected remotely with high sensitivity . 

The activity of a sample of a radioisotope was found to be 2150 disintegrations per minute. After 6.0 hours the activity was found to be 1324 disintegrations per minute. What is the half-life of the radioisotope ... 

Other radioisotopes known to be produced by cosmic rays include Be, H, Na, Be, and Of these Be, P, and P have activities that are high enough to be measured in rainwater. In several instances, notably 0 and Be, these radioactive elements are useful as tracers. 

Eighteen isotopes of sulfur, 17 of selenium, 21 of tellurium, and 27 of polonium have been registered of these, 4 sulfur, 6 selenium, and 8 tellurium isotopes are stable, while there is no stable isotope of polonium. None of the naturally occurring isotopes of Se is radioactive its radioisotopes are by-products of the nuclear reactor and neutron activation technology. The naturally occurring, stable isotopes of S, Se, and Te are included in Table 1.2. 

Radioisotope-labeled nitrosamines have proven valuable in development of analytical methods and for demonstrating efficiency of recovery of nitrosamines from tobacco products and smoke (37-39). The very high specific activity required for low part-per-billion determinations has discouraged most analysts from using this approach. Unless a radiochromatographic detector with adequate sensitivity is available, samples must be counted independently of the final chromatographic determination, and one of the advantages of internal standardization, correction for variation in volume injected, is lost. 

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