Direct radioactivity counting

The range of applicability of equation 11.122 depends on the limits of detection of in the sample. The current maximum age attained by direct radioactivity counting is about 4 X 10" a. To measure residual radioactivity, the total carbon in the sample is usually converted to CO2 and counted in the gas phase, either as purified CO2 or after further conversion to C2H2 or CH4. To enhance the amount of counted carbon, with the same detection limit (about 0.1 dpm/g), counters attain volumes of several liters and operate at several bars. More recent methods of direct detection (selective laser excitation Van de Graaif or cyclotron acceleration) has practically doubled the range of determinable ages (Muller, 1979). 

In a typical IRMA, Tg antibodies in diluted serum specimens bind to human Tg in coated test tubes. After incubation for 1 hour at 37 C, the tubes are washed and T-labeled protein A is added. After another 1 hour of incubation at 37 C, the supernatants are washed again and the radioactivity counted. The amount of bound radioactivity in each tube is directly proportional to the amount of anti-Tg antibody present. Interassay precision is about 3% at 36 U/mL. 

A 20 pi reaction mix, containing 30 pCi (i.e., 3 pi of 10 mCi/ml) is assumed. After incorporation of precursor, the mix is diluted twice and 2 pi is diluted 100-fold with TE containing 10 pg of carrier nucleic acid. Half of the TE-diluted sample (100 pi) is directly counted in a scintillation counter whereas 2 ml of cold 5% TCA (containing 20 mM pyrophosphate) is added to the other half, then mixed and incubated on ice for 5 min and the precipitate collected on GF/C glass-fiber filters. The tube should be washed once with 5% TCA and twice with 95% ethanol and the washes also passed through the filter (alternatively, the TE-diluted sample can be applied to the filter and then rinsed with the TCA solution Table 8.11A). The radioactivity counted on the filter indicates the amount incorporated. For example, 1X 10 /2X10 or 50% is incorporated. 

The absorptivity of IR bands depends on a large number of factors and must be calibrated with suitable standards. We may go about evaluating the composition of the standards in several ways we can blend the homopolymers in variable proportions, or we can analyze some copolymers either by an absolute method (NMR) or by a relative, but highly reliable method (radioactive counting on labelled copolymers), or we can directly compare absorptivities in copolymers and in the corresponding homopolymers. Each method has advantages and drawbacks. 

The test sections are equipped with a radioactive counting station using sodium iodide detectors about two thirds up from the bottom of the tube. It is considered that after approximately this distance the velocity profile will have been established. The length after the counting station is included to avoid any interference by the exit of the liquid from the tube. After passing through the test section, liquid can be directed to either of the feed tanks or into the glass vessel (for flow measurement). Fig. 17.4 shows data obtained from the test apparatus [Williamson 1990],... 

The adsorption of prothrombin onto the lipid monolayer was followed directly by counting the surface radioactivity of the H labelled protein using a gas-flow counter equipped with an ultrathin window as described elsewhere ( ). By calibrating the counter as previously described ( ), it is possible to determine the surface concentration of the radioactive protein, T. ... 

At its simplest, radioactivity counting involves a source, a suitable detector for the radiation emitted by the source, a means of counting those decay events that are detected and a timer. If we measure the rate of detection of events, we can directly relate this to the number of radioactive atoms present in the source. The basic premise is that the decay rate of the source (/ ) is proportional to the number of atoms of radioactive nuchde present (N), the proportionality constant being the decay constant, X. Thus ... 

The use of stable isotopes can provide richmetabolic information that differs from that of radioisotopes. For example, quantihcation of radioisotopes is in terms of energy emitted from the loss of an electron from the nucleus (i.e. isotopic decay), where the energy emitted is directly proportional to the number of radioactive atoms ( C, H, S) present per sample or molecule weight (i.e. specific activity). Often, specific activity of a whole molecule (e.g. glucose, fatty acids) is determined, and this has three particular disadvantages. First, the concentration of the molecule must be determined and the compormd must be isolated for radioactive counting. Both analytical techniques can introduce error in measurement. Second, with or H tracers, total molecule specific activity is usually measured. 

A modem variant is to count the number of atoms directly in a mass spectrometer.) The practical limit is about 50000 y since by this time the activity has fallen to about 0.2% of its original valuable and becomes submerged in the background counts. is also extremely valuable as a radioactive tracer for mechanistic studies using labelled compounds, and many such compounds, particularly organic ones, are commercially available (p. 310). 

The difference in rates of release of BCNU from wafers produced by the trituration or solution methods is also seen in vivo (11,14), as is shown in Fig. 6. Wafers of PCPP-SA 20 80 were prepared by either the solution or trituration methods, as described above, and were implanted into the brains of rabbits. The animals were sacrificed at various times after implantation and the brains were removed, fixed, and processed for quantitative autoradiography. To quantitate the percentage of the brain exposed to BCNU released from these wafers, the following calculation was performed. The percentage of the brain in which the radioactivity from the tritiated BCNU released from the wafers exceeded the background counts by at least two standard deviation units was plotted as a function of time following implantation in Fig. 6. A control set of rabbits had a solution of BCNU injected directly into the same location in the... 

Instead of scraping and manual collection of the adsorbent, the band can be sucked off the plate with a Vacuum-cleaner -type apparatus. Dekker [50] described an apparatus for the isolation of compounds from layers by elution and direct Millipore filtration, and Platt [51] designed a zone collector that used vacuum to transfer separated zones from layers direcdy to vials for hquid scintillation counting of radioactivity. 

Figure 2. Alpha spectrum for a radium adsorbing manganese-oxide thin film exposed to a groundwater sample, after Surbeck (2000) and Eikenberg et al. (2001b). A 2x2 cm sheet is exposed to O.l-l.O L of sample for 2 days, capturing nearly all of the radium in the sample. These sample discs can be used directly for low-level alpha spectrometry without the need for further separation and preparation methods to produce planar sample sources. Energy resolution is nearly as good as for electroplated sources, and detection limits are typically 0.2 mBqA (6 fg Ra/L) for Ra and " Ra for a one-week counting period. These sensitivities are comparable to traditional methods of alpha spectrometry. [Used by permission of Elsevier Science, from Eikenberg et al. (2001), J Environ Radioact, Vol. 54, Fig. 4, p. 117]...

In contrast, the direct detection technique for 14C can count approximately one percent of the 14C atoms that are present in a 200 pg sample with virtually zero background in times that are of the order of a few hours. Thus, for milligram quantities of carbon, the improvement in sensitivity of direct counting over radioactivity is of the order 7.2 x 10s. 

Comparison of the kinetic features of different LMWPs revealed that all LMWPs tested so far (such as lysozyme, cytochrome-c and aprotinin) are quickly cleared from the circulation and accumulate rapidly in the kidney [38]. The fractions of the injected LMWP that are reported to be taken up by the kidney vary between 40-80 % of the injected dose. In our studies, using external counting of radioactivity, at least 80 % of the intravenously injected LMWPs was finally taken up by the kidneys, which is in agreement with renal extraction studies [69,70]. However, studies in which the actual amount of LMWP in the kidney was measured directly in the tissue, indicated a lower, but still substantial accumulation of 40% of the injected dose [71,72]. Apart from the kidney, LMWPs do not seem to accumulate elsewhere in the body (Figure 5.8). 

Common Features of NAA Procedures. In all of the procedures discussed in this article, irradiations are made in a high thermal neutron flux (1011 to 1013 neutrons cm"2 sec 1) simultaneously with the samples and standard(s) sealed in polyethylene containers for a short irradiation or in silica containers for a long irradiation. The standard is a known amount, or solution of known concentration, of a pure compound of the element to be determined. The concentration of the element in the sample is determined by comparing its radioactivity with that of the standard, which is either subjected to the same radiochemical separation as the sample with an inactive matrix or diluted. The radioactivity is counted directly if the sample is measured in solution. The radiochemical yield of precipitated samples is determined directly by weighing and that of solutions of samples by aliquot re-irradiation. 

Natural Radioactivity Measurement. The naturally occurring radionuclides U(226Ra), Th(232Th), and K(40K) can also be determined on 100-g samples by direct counting of unirradiated samples using anticoincidence shielded multidimensional y-ray spectrometers (29, 30). These spectrometers use large (12 in. diameter X 8 in thick) principal Nal(Tl)... 

N may be expressed in any convenient units because whatever is used cancels out in the ratio NJN2. A common unit is "counts per minute" (cpm) because, through Equation 26-1, the observed cpm are a direct measure of the number of radioactive atoms present. 

The amount of unreacted target element that eluted was determined by measuring its radioactivity directly in the case of actinides, and by activation analysis in the case of lanthanides. The distribution of the radioactive neutron capture product was determined by counting both the eluate and the eluted zeolite. All irradiations were done in the Oak Ridge Research reactor in a pneumatic tube facility with a thermal neutron flux of about 4 X 1013 neutrons cm-2 sec-1 or, for a few long irradiations, in a tube adjacent to the reactor core at the fluxes stated in Table VI. 

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