Absorption, determination radioactivity

In order to avoid many of the technical difficulties of sedimentation in homogeneous media (convection, associations, etc.), a continuous density gradient is often used. After a sedimentation run in a buffered gradient, established before the run in the centrifuge tube, a hole is punched in the bottom of the tube, and the solution collected drop by drop. Monitoring is by absorption, determination of radioactivity and enzyme activity, etc., and, frequently, a marker of known molecular weight is added to the solution. 

The analysis of steady-state and transient reactor behavior requires the calculation of reaction rates of neutrons with various materials. If the number density of neutrons at a point is n and their characteristic speed is v, a flux effective area of a nucleus as a cross section O, and a target atom number density N, a macroscopic cross section E = Na can be defined, and the reaction rate per unit volume is R = 0S. This relation may be appHed to the processes of neutron scattering, absorption, and fission in balance equations lea ding to predictions of or to the determination of flux distribution. The consumption of nuclear fuels is governed by time-dependent differential equations analogous to those of Bateman for radioactive decay chains. The rate of change in number of atoms N owing to absorption is as follows ... 

G. J. Mullaney, Temperature Determination in Flames by X-ray Absorption Using a Radioactive Source, Rev. Sci. Instr., 29, 87 (1958). The x-ray source used was radioactive Fe-55. 

The uptake of TRA into cervical tissue was determined by measuring tissue radioactivity following insertion of the collagen sponge cervical cap containing tritium-labeled TRA. The TRA concentrations peaked at 4 hr and then diminished rapidly by 24 hr. Since measurements of blood samples revealed that no systemic absorption had occurred, high local concentrations over an extended period of time may be possible without systemic side effects. 

In an actual Mdssbauer transmission experiment, the radioactive source is periodically moved with controlled velocities, +u toward and —d away from the absorber (cf. Fig. 2.6). The motion modulates the energy of the y-photons arriving at the absorber because of the Doppler effect Ey = Eq + d/c). Alternatively, the sample may be moved with the source remaining fixed. The transmitted y-rays are detected with a y-counter and recorded as a function of the Doppler velocity, which yields the Mdssbauer spectrum, r(u). The amount of resonant nuclear y-absorption is determined by the overlap of the shifted emission line and the absorption line, such that greater overlap yields less transmission maximum resonance occurs at complete overlap of emission and absorption lines. 

The enteral absorption of bromocriptine from an aqueous solution amounts to 30 - 40 % as determined from the sum of the cumulative biliary and urinary excretion of radioactivity (parent drug + metabolites) in bile duct cannulated animals. 

For the quantitation of in vivo absorption of 5Ca2+, 200 jd 1 of radioactive solutions were administered by gastric gavage to male mice kept on a deionized water diet during the previous 18 hours. After 4 hours blood was obtained by axillary incision and Ca2+ was extracted with TCA and determined by liquid scintillation counting (26). 

To determine absorption rate, another group of animals was immersed in a 5 mg/1 solution of PNA in Instant OceanR (closed chamber) and the water sampled at intervals to measure the loss of radioactivity compared to controls (Fig. 3). 

Photoionization, where electrons are released by molecules following the absorption of energy from photons, has long been viewed as a non-radioactive means to ionize explosives in the vapor phase [39]. In recent years, two teams have sought to employ laser ionization with IMS for explosive determinations. A team at Implant Sciences Corporation has utilized a laser (or flash lamp) for sampling surfaces and for ionization of sample vapors in an IMS analyzer [40, 41]. In their approach, the sample is removed from a surface with an increased temperature from laser exposure. Gases (and presumably particulate matter) from over the surface are drawn into an IMS drift tube using a wall-free inlet vida supra). In the IMS drift tube, resonance multi-photon ionization by a laser is used to produce ions from the explosives. Their... 

Wolfe has presented an excellent description of the systematic application of stable and radioactive isotope tracers in determining the kinetics of intestinal fat absorption, hepatic triglyceride synthesis, lipid mobilization, triglyceride-fatty acid recycling, and cholesterol turnover. 

The radioactivity can be measured by a beta counter. The metal at trace concentrations can be determined by an atomic absorption or emission spectrophotometer. 

PET offers the possibility to quantitatively measure the myocardial blood flow (MBF). MBF tracers can be divided into two groups. The first group is freely diffusible and represented by [ 0]H20. These tracers do not show any specific absorption and their distribution is completely determined by diffusion. Consequently, the measurement of the MBF is based on the first-pass extraction and clearance data. Because of the low heart-to-blood radioactivity ratio, the freely diffusible tracers provide myocardial images with low signal-to-background ratios. The second class is composed of highly extractable heart tracers. The tracer p NjNHs belongs to this family. These radiolabeled compounds are characterized by a selective extraction and retention in the myocardium. The... 

Dermal absorption was also studied in rats. [ C]Diethanolamine was applied to 19.5 cm of the dorsal skin (20 mg/cm, 1500 mg/kg bw) and covered for 48 h (no washing) or for 6 h before it was removed by washing. Absorbed [ Qdiethanolamine was determined in 48-h urine and faeces and from sampled tissues. FInwashed rats absorbed 1.4% and washed animals 0.64% of the dose, while the majority of [ C]diethanolamine was recovered in the occlusive wrappings (80%) and in skin of the dose site (3.6%). The radioactivity was found in carcass, liver or kidneys but very little in urine (0.11%), faeces or blood (Waechter etal., 1995, cited by Knaak etal, 1997). 

Once a diagnosis of megaloblastic anemia is made, it must be determined whether vitamin B12 or folic acid deficiency is the cause. (Other causes of megaloblastic anemia are very rare.) This can usually be accomplished by measuring serum levels of the vitamins. The Schilling test, which measures absorption and urinary excretion of radioactively labeled vitamin B12, can be used to further define the mechanism of vitamin Bi2 malabsorption when this is found to be the cause of the megaloblastic anemia. 

Leaching onto Ion-Exchange Resins. A more sophisticated procedure for determining leachabilities of radioactive waste forms has been developed(7). Flowing deionized water is continuously circulated across the sample surface and then through ion-exchange resins where the leached ions are adsorbed. The adsorbed ions are subsequently eluted from the resin columns for atomic absorption analyses. Leachabilities measured by this procedure are claimed to be lower and more consistent than those made in stagnant water in the absence of continuous ionic control (7 ). 

Potassium is analyzed in chemicals that are used in the fertilizer industry and in finished fertilizers by flame photometric methods (44) or volumetric sodium tertraphenylboron methods (45) as approved by the AO AC. Gravimetric determination of potassium as IC PtClknown as the Lindo-Gladding method (46), and the wet-digestion determination of potassium (47) have been declared surplus methods by the AO AC. Other methods used for control purposes and special analyses include atomic absorption spectrophotometry, inductively coupled plasma (icp) emission spectrophotometry, and a radiometric method based on measuring the radioactivity of the minute amount of the isotope present in all potassium compounds (48). 

The two preceding methods have been combined to determine the tritium isotope effect. In a tritium-labeled substrate in D20, the change in infrared radiation absorption arises almost entirely from release of protons because the concentration of the tritium species in the reactant is small. Thus, the rate constant kH (determined by the change in the DOH absorption) represents release of protons. The constant kT for release of tritium to the solvent is determined from radioactivity measurements of water from the same reaction mixture. In the enolization of... 

Intermediary metabolism of irradiated tissues was examined to attempt to explain the increased respiration on the basis of biochemical mechanics. In these experiments carrot disks 0.5 mm. thick were prepared as before and subjected to substrate utilization studies (9). This involved incubating the slices with the appropriately labeled substrate in 0.03M phospate buffer at 25°C. The incubation involves shaking with gentle aeration and trapping the respired CO2 in alkali. Following incubation the slices were removed, washed, ground with 80% ethanol, and filtered, and both the filtrate and residue were assayed for radioactivity. Thus, a measure of substrate absorption and utilization was determined. 

Mechanism 4 also was bom in this laboratory (1-6,24). A rise in AV could be determined by the penetration and orientation of the ions of electrolyte, H20, and their products of interaction. Such ions could be part of either the hydration shell of the lipid s molecular organization or the thick interfacial viscosity layers that are located above and/or below the mathematical line (25) of surface tension. The measurements of IR absorption and surface radioactivity (see Figures 6,7,8... 

Unlike some classes of polyphenols such as flavonols and flavones, flavanols are almost always present in the nonglycosylated form. Removal of glycoside from flavonoids, usually necessary before the transport across the intestinal barrier, is not required in the case of flavanols [Scalbert and Williamson, 2000]. The absorption of procyanidins by the small intestine was investigated by studying 14C-( + )-catechin, dimer, trimer, and procyanidin polymers permeation through Caco-2 cell cultures [Deprez et al., 2001]. There was little difference in permeability between monomer, dimmer, and trimer, based on the measurement of radioactivity present on the basal side of the cultures, whereas the permeability of the polymers was 10 times lower. The authors reported the absence of catechin metabolism but did not determine whether the radioactivity measured on the basal side of the cultures was from the parent dimers to polymers or from their products of degradation or metabolites, which could have resulted from instability of the parent compounds in the culture... 

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