Sensitivity of Activation Analysis

Sensitivity of the activation analysis method for a particular element refers to the minimum mass of that element that can be reliably detected. The minimum detectable mass is determined from Eq. 15.4 by assuming the most favorable conditions for the measurement and by setting an upper limit for the acceptable error of the result. The process is similar to the determination of the minimum detectable activity discussed in Sec. 2.20. 

Assuming that the observer is willing to accept a maximum error such 

Example 15.1 What is the minimum mass of gold that can be detected by neutron activation analysis under the conditions listed below  

Answer Using Eq. 15.12, the minimum acceptable counting rate is 

Example 15.2 What is the absolute minimum mass of an element that can be detected under the most favorable conditions  

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Although the high sensitivity of activation analysis is perhaps its most striking advantage, there are a number of other favorable aspects as well. Activation analysis is basically a multielemental technique. Many elements in the sample will become radioactive during the irradiation and if each of these elements can be isolated chemically or instrumentally, their abundances may be determined simultaneously. Activation analysis can be a nondestructive method of analysis. Numerous tests have shown that with careful experimental manipulation, activation analysis is an accurate ( 1% accuracy) and precise ( 5% precision) method of measuring elemental concentrations. 

Activation analysis can be used for determination of trace elements, in particular heavy metals and essential elements, in various parts or organs, respectively, of plants or animals and man. Making use of the high sensitivity of activation analysis, small samples of the order of several milligrams taken from selected places give information about the concentration of the elements of interest. The results of activation analysis of trace elements also allow conclusions with respect to diseases or malfunctions and are valuable aids to diagnosis. Examples are the determination of Se in man or of trace element concentrations in bones or other parts, with respect to the sufficient supply of essential elements and metabolism. In vivo irradiation has also been proposed. 

The sensitivity of activation analysis is directly proportional to the particle flux employed. In consequence it is the aim of the analyst inter-... 

The sensitivity of analytical methods for trace elements needs to be high, and ideally the reaction or property utilized should be entirely specific. The sensitivity of activation analysis is dependent on the element estimated and in many cases it is extremely high. Specificity is covered by three parameters, half-life of the nuclide produced, its maximum beta energy, and its gamma spectrum. 

The high sensitivity of activation analysis has made it very useful in environmental pollution studies. Table 9.2 lists the limits of detection for some elements in sea water under the conditions specified in the table. 

One of the most appropriate uses of stable activable tracers is in measuring removal of pollutants from the atmosphere. The concentrations of species are extremely low even in tracer experiments (/ ng/L), and the good detection sensitivity of activation analysis is a necessity along with the controlled emission of the tracer. Much of this work has been carried out by the Battelle Pacific Northwest Laboratories and is summarized in annual reports of that laboratory from 1972-1976. 

Although not primarily concerned with activation analysis, the article by Nicholson (646) states that the characteristic of any analytical method is dynamic and that the determination of the sensitivity of the method of choice is a statistical problem. Heydorn (394,395,396) and Rakovic and Prochazkova (748) report specific instances of obtaining accuracy and correctness for activation analysis results. Jurs and Isenhour (438) and Smith (878) give a more general treatment of statistical techniques and their relationship to the sensitivity of activation analysis. 

A number of reports already issued show that activation analysis can be a significant analytical methodology for research investigations concerned with the biochemistry of natural sources. The sensitivity of activation analysis has made it possible for Castro and Schmitt (164) to analyze samples of metalloenzymes weighing less than 1 mg for trace Co, Cu, Mn, Mo and Zn. This particular investigation showed that a separation had to be made of the metalloenzyme fraction before activation and that special techniques had to be followed in handling the separated materials to prevent contamination. Evans and Fritze (255) used similar techniques to identify metal-protein complexes. 

A method has been developed for the determination of technetium-99 in mixed fission products by neutron activation analysis Tc is separated from most fission products by a cyclohexanone extraction from carbonate solution, the stripping into water by addition of CCI4 to the cylohexanone phase, and the adsorption on an anion exchange column. Induced Tc radioactivity is determined using X-ray spectrometry to measure the 540 and 591 keV lines. The sensitivity of the analysis under these conditions is approximately 5 ng. The method has been successfully applied to reactor fuel solutions. 

Figure 13.2 Table of activation analysis sensitivities as offered by General Atomic Company, San Diego, California.

D.Bezier, "Methodes Modemes d Analyse Quantitative Min rale, Masson Cie, Paris(1955) l6)W.W.Meinke, Science 121, 177-84(1955) and Rus translation by N.G. Polianskii, UspekhiKhimii 15,770-80(1956) (Trace element sensitivity , comparison of activation analysis with other methods) 17)G.Charlot D.Bezier, "Quantitative Inorganic Analysis, translated from the French by R.GMurray, Methuen Co, London (19 57), 295 9 18)L.Meites, H.C.Thomas R.P.Bauman "Advanced Analytical Chemistry, McGraw-Hill,NY( 1958), 344-69 19)... 

The kinetic mode of bioselector operation is employed when the sensitivity of the analysis depends on the activity of the biological material (i.e. on the biochemical reaction), but not on the diffusion stages of the process. Put more simply, this means that the biochemical reaction rate is limited by the substrate transformation process, but not by its transportation to the bioselector. 

Many of the early applications of activation analysis were designed specifically to take advantage of its high sensitivity for rarer elements and its freedom from sources of contamination. In these trace element... 

Isotope enrichment is a major activity of the nuclear industry. The naturally occurring chemical elements are composed of isotopes with varying nuclear properties. By enriching the chemical element in the nuclide with the desirable nuclear property, the performance of the nuclear processes is enhanced. In addition, the sensitivity of isotopic analysis permits the use of enriched chemical elements in place of radioisotopes as tracers in research, medicine, and industry. 

The precision of activation analysis is, when all other variables have been eliminated, limited by statistical variations in the counting rate. Consequently the precision will tend to decrease as the amount of element estimated and the activity present in the final source derived from it becomes smaller. However, statistical variations in the counting rate are rarely of importance except at extreme sensitivity and then the effect can be minimized by increasing the counting period. 

All methods of activation analysis are very accurate and sensitive, and a precision of approximately 2% RSD is easily attainable. Detection limits in parts per per billion or lower, depending on the element and sample matrix, are easily attainable. As many as 60 different elements that can form radionuclide can be analyzed using NAA. NAA finds wide application in a number of other fields, and these are summarized in Table 6. 

One of the greatest advantages of activation analysis is its ability to detect most of the isotopes with an extremely high sensitivity. Other advantages are that the method... 

For optimum analytical sensitivity it is desirable to produce the maximum amount of activity in the sample to determine and measure that activity with the maximum efficiency. The measurement of activities with short half-Hves should be carried out as soon as possible after the irradiation. It follows from the basic equation of activation analysis that 50 % of the maximum activity is obtained after irradiation time equal to one half-life and more then 90% after four half-lives. 

Many of the review articles cited above emphasize the sensitivity limits of activation analysis. Experimenters constantly seek techniques to improve these limits for example, a number of recent reports show that some experimenters consider very short-lived species (with radioactive half-lives of seconds or minutes) to be most easily used to obtain sensitivity and a rapid analysis (18,171,177,565,625,883). Other reports (17, 70,157,326,328,461,566,730,832) give specific information about optimum conditions for sensitivity, and other experimenters have evaluated the maximum sensitivity that can be obtained in the presence of interfering radionuclides, a condition that sometimes becomes a major disadvantage in the application of activation analysis methodology to a given sample material (404,729,730,740). 

In recent years, medical research workers have reported that an apparent correlation exists between trace elements and certain types of diseases. As a result, many hypotheses about the abnormal role of certain trace elements to cancer, hypertension, arthritis, multiple sclerosis, cystic flbrosis and other diseases have evolved. Some of these researchers have considered activation analysis to be a most important tool for this type of research, because it is sensitive and requires only a small amount of material for an assay. Typical applications of activation analysis to this major area of medical research are given in Table IV. 

Activation analysis is the other field of radiochemical analysis that has become of major importance, particularly neutron activation analysis. In this method nuclear transformations are carried out by irradiation with neutrons. The nature and the intensity of the radiation emitted by the radionuclides formed are characteristic, respectively, of the nature and concentrations of the atoms irradiated. Activation analysis is one of the most sensitive methods, an important tool for the analysis of high-purity materials, and lends itself to automation. The technique was devised by Hevesy, who with Levi in 1936 determined dysprosium in yttrium by measuring the radiation of dysprosium after irradiation with neutrons from a Po-Be neutron source. At the time the nature of the radiation was characterized by half-life, and the only available neutron sources were Po-Be and Ra-Be, which were of low efficiency. Hevesy s paper was not followed up for many years. The importance of activation analysis increased dramatically after the emergence of accelerators and reactors in which almost all elements could be activated. Hevesy received the 1943 Nobel prize in chemistry for work on the use of isotopes as tracers in the study of chemical processes . 

Nuclear activation methods can provide high sensitivities for many but not all elements. These methods are capable of simultaneous multielement analysis and in some cases they may be essentially nondestructive analytical methods. One of the main disadvantages is the lack of information on chemical form (speciation) of the elements in the analyte. Neutron activation analysis is the most common form of activation analysis, but charged particle and photon activation analysis methods are also applied. 

Abstract This chapter presents the basic principles of activation analysis and details its different types. Emphasis is given to instrumental neutron activation analysis and radiochemical separations for the determination of trace and ultra-trace elements. Location sensitive analysis is also included. 

Another outstanding feature of activation analysis is the selectivity of the methods. Not only can the decay characteristics of the activation product be used for unique identification and quantitation, but selection and exploitation of a variety of physical parameters in activation, decay, and measurement can optimize the sensitivity and selectivity for the desired determination. Moreover, optimization of the physical parameters is well predicted by virtue of the known activation and measurement relationships. 

The primary objective of RAA is to increase the sensitivity and/or the accuracy of the analysis, while preserving the main advantage of the instrumental techniques, their multielement character. The instrumental modes of activation analysis are widely used analytical techniques for the nondestructive and simultaneous determination of trace element mass fractions or concentrations. As a result of chemical separation, interferences caused by other elements/radionuclides are removed thus additional elements of low concentration become detectable and/or the accuracy of the determination is improved. In this sense, RAA complements instrumental AA methods. 

Several methods are available for the determination of total aluminum in biological and other materials. Chemical and physicochemical methods are in most practical situations insensitive and inaccurate X-ray fluorescence is specific but lacks sensitivity neutron activation analysis is complex and subject to interferences, although it is a very sensitive technique. Nuclear magnetic resonance spectroscopy is not very sensitive but useful to get information on speciation [33]. Graphite furnace atomic absorption spectrometry (GFAAS) is the most widely used technique and can produce reliable results, provided that the matrix effects are recognized and corrected. Savory and Wills [19] reviewed chemical and physicochemical methods for the determination of aluminum in biological materials, e.g. X-ray fluorescence, neutron activation analysis, atomic emission spectrometry, flame emission, inductively coupled plasma emission spectroscopy, and AAS. 

MIO - W. W. Melnke, "Trace Element Sensitivity. Comparison of Activation Analysis With Other Methods", Science 121, 177 (1955).. 

Apphcations of activation analysis in TLC are not so far known. The multitude of elements in adsorbent, binder and glass plate would demand an extensive modification of the chromatographic procedure in order to prevent formation of long lived, active isotopes in the carrier material. The advantages of TLC, like high layer capacity and speed would be virtually forfeited in relation to the high sensitivity of the measuring process and the appreciable expense associated with the activation. 

Methods for iodine deterrnination in foods using colorimetry (95,96), ion-selective electrodes (94,97), micro acid digestion methods (98), and gas chromatography (99) suffer some limitations such as potential interferences, possibHity of contamination, and loss during analysis. More recendy neutron activation analysis, which is probably the most sensitive analytical technique for determining iodine, has also been used (100—102). 

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