Sensitivity of radioactivity

Let us briefly discuss again the limitations of radioactive decay measurement. The observation of the radioactive decay of a single atom is possible, consequently, with efficient apparatus for the detection of the decay particles and a radioactive species with a half-life of seconds and minutes, it is possible to detect all or nearly all of a small number of radioactive atoms in the presence of a large number of nonradioactive atoms with radiation detection techniques. However, as the half-life increases, the time taken to carry out an experiment with a small number of radioactive atoms naturally increase, for half-lives, of say, 10 years efficient detection of the radioactive decay products becomes impossible unless the measurement can be continued for 10 years. Therefore, studies of long-lived radioactive isotopes invariably use very large numbers of atoms and the apparatus detects the decay of only a small fraction of the total during the experiment. In this situation the mass spectrometric detection sensitivity surpasses by far the sensitivity of radioactive counting methods. 

Labeled compounds offer many advantages for food science. The extreme sensitivity of radioactivity measurement enables one to determine and characterize biologically important trace elements and compounds (e.g., pesticides and pesticide residues) in food samples. 

For example, when the activity is determined by counting 10,000 radioactive particles, the relative standard deviation is 1%. The analytical sensitivity of a radiochemical method is inversely proportional to the standard deviation of the measured ac-... 

Few of the naturally occurring elements have significant amounts of radioactive isotopes, but there are many artificially produced radioactive species. Mass spectrometry can measure both radioactive and nonradioactive isotope ratios, but there are health and safety issues for the radioactive ones. However, modem isotope instmments are becoming so sensitive that only very small amounts of sample are needed. Where radioactive isotopes are a serious issue, the radioactive hazards can be minimized by using special inlet systems and ion pumps in place of rotary pumps for maintaining a vacuum. For example, mass spectrometry is now used in the analysis of Pu/ Pu ratios. 

The methods for detection and quantitation of radiolabeled tracers are deterrnined by the type of emission, ie, y-, or x-rays, the tracer affords the energy of the emission and the efficiency of the system by which it is measured. Detection of radioactivity can be achieved in all cases using the Geiger counter. However, in the case of the radionucHdes that emit low energy betas such as H, large amounts of isotopes are required for detection and accurate quantitation of a signal. This is in most cases undesirable and impractical. Thus, more sensitive and reproducible methods of detection and quantitation have been developed. 

Concerns over safe handling of radioactive materials and issues around the cost and disposal of low level radioactive waste has stimulated the development of nonradiometric products and technologies with the aim of replacing radioactive tracers in research and medical diagnosis (25). However, for many of the appHcations described, radioactive tracer technology is expected to continue to be widely used because of its sensitivity and specificity when compared with colorimetric, fluorescent, or chemiluminescent detection methods. 

Although performance varies with the isotopes for which they are intended, and with the balance in the design between resolution and efficiency, the overall sensitivity of a y-camera collimator is on the order of 5000 counts/(MBqmin) (several hundred counts/(/iCi-min)). In terms of photons detected per photon emitted, this is equivalent to about 2 x lO ". In other words, about two photons out of 10,000 emitted arrives at the crystal. This necessitates exposure times that range from several minutes to the better part of an hour. Fortunately, the large number of photons available from a modest injected radioactive dose more than offsets the poor detector sensitivity. The camera s abiUty to resolve small objects, however, is ultimately limited by the collimator inefficiency. 

Isotope Dilution Assay. An isotope dilution assay for biotin, based on the high affinity of avidin for the ureido group of biotin, compares the binding of radioactive biotin and nonradio active biotin with avidin. This method is sensitive to a level of 1—10 ng biotin (82—84), and the radiotracers typically used are p C]biotin (83), [3H]biotin (84,85) or an I-labeled biotin derivative (86). A variation of this approach uses I-labeled avidin (87) for the assay. 

For monitoring the day-by-day fluctuation of the sensitivity of luminometer, the radioactive luminescence standard described by Hastings and Weber (1963), and Hastings and Reynolds (1966) is most useful and convenient (see also Section 2.6). 

Limitations encountered in routine use include (a) the use of radioactive materials, (b) limited sensitivity in the presence of high protein concentrations, (c) long assay time of up to 5 days,... 

After each series of experiments with beams of various intensity the section plate would be removed from the cell and disassembled, with radioactive silver washed out by nitric acid. Radioactivity of the solutions obtained was measured by a multichannel spectrometric scintillation y-counter with sensitivity of up to 10 G, i. e. around 10 of atoms which, according to calculations, is 10 times lower than sensitivity of ZnO sensor 10 G or 10 of Ag atoms respectively [28]. This difference in sensitivity lead to great inconveniences when exposing of targets was used in above methods. Only a few seconds were sufficient to expose the sensor compared to several hours of exposure of the scintillation counter in order to let it accumulate the overall radioactivity. It is quite evident that due to insufficient stability during a long period of exposure time an error piled up. 

Mans, R. J., and Novelh, G. D. (1960). A convenient, rapid, and sensitive method for measuring the incorporation of radioactive amino acids into protein. Biochem. Biophys. Res. Commun. 3, 540-543. 

These types of detections are capable of very high sensitivity so that very small numbers of radioactive atoms can be detected. To illustrate this idea the number of atoms of Rn-222 for a concentration of 10 pCi/1 can be calculated... 

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