Radioactive analytes

Direct Analysis of Radioactive Analytes The concentration of a long-lived radioactive isotope is essentially constant during the period of analysis. As shown in Example 13.6, the sample s activity can be used to calculate the number of radioactive particles that are present. 

The method selected to prepare a radionuclide for counting depends on the skills and preferences of the analyst, available detectors, and conditions associated with the radioactive analyte, accompanying radionuclides, and the sample matrix. All aspects have to be considered to obtain a measurement that meets reliability and sensitivity specifications for radionuclide identification and detection. In some cases, the analyst has many options in others, choice is restricted by circumstances such as small samples, low radionuclide concentration, half-life considerations, or unavailability of certain detector types. 

The Marine Environmental Laboratory of IAEA located in Monaco is involved in the measurement and understanding of marine radioactivity. Analytical quality assurance, the use of isotopes as tracers and site-specific radiological assessments are central... 

A radiometric detector, also called a radio-flow detector, is used to measure radioactivity of radioactive analytes in the HPLC eluent passing through a flow cell. Most are based on liquid scintillation technology to detect phosphors caused by the radioactive nuclides. A liquid scintillator can be added post-column with a pump or a permanent solid-state scintillator can be used around the flow cell. This detector is specific only to radioactive compounds and can be extremely sensitive. This detector is used for experiments using tritium or C-14 radiolabeled compounds in toxicological, metabolism, or degradation studies. 

There are many potential advantages to kinetic methods of analysis, perhaps the most important of which is the ability to use chemical reactions that are slow to reach equilibrium. In this chapter we examine three techniques that rely on measurements made while the analytical system is under kinetic rather than thermodynamic control chemical kinetic techniques, in which the rate of a chemical reaction is measured radiochemical techniques, in which a radioactive element s rate of nuclear decay is measured and flow injection analysis, in which the analyte is injected into a continuously flowing carrier stream, where its mixing and reaction with reagents in the stream are controlled by the kinetic processes of convection and diffusion. 

Neutron Activation Analysis Few samples of interest are naturally radioactive. For many elements, however, radioactivity may be induced by irradiating the sample with neutrons in a process called neutron activation analysis (NAA). The radioactive element formed by neutron activation decays to a stable isotope by emitting gamma rays and, if necessary, other nuclear particles. The rate of gamma-ray emission is proportional to the analyte s initial concentration in the sample. For example, when a sample containing nonradioactive 13AI is placed in a nuclear reactor and irradiated with neutrons, the following nuclear reaction results. 

A form of internal standardization in which a radioactive form of the analyte serves as the internal standard. 

Isotope Dilution Another important quantitative radiochemical method is isotope dilution. In this method of analysis a sample of analyte, called a tracer, is prepared in a radioactive form with a known activity. Ax, for its radioactive decay. A measured mass of the tracer, Wf, is added to a sample containing an unknown mass, w, of a nonradioactive analyte, and the material is homogenized. The sample is then processed to isolate wa grams of purified analyte, containing both radioactive and nonradioactive materials. The activity of the isolated sample, A, is measured. If all the analyte, both radioactive and nonradioactive, is recovered, then A and Ax will be equal. Normally, some of the analyte is lost during isolation and purification. In this case A is less than Ax, and... 

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-... 

In comparison with most other analytical techniques, radiochemical methods are usually more expensive and require more time to complete an analysis. Radiochemical methods also are subject to significant safety concerns due to the analyst s potential exposure to high-energy radiation and the need to safely dispose of radioactive waste. 

Radiochemical methods of analysis take advantage of the decay of radioactive isotopes. A direct measurement of the rate at which a radioactive isotope decays may be used to determine its concentration in a sample. For analytes that are not naturally radioactive, neutron activation often can be used to induce radioactivity. Isotope dilution, in which a radioactively labeled form of an analyte is spiked into the sample, can be used as an internal standard for quantitative work. 

Isotopes of an element are formed by the protons in its nucleus combining with various numbers of neutrons. Most natural isotopes are not radioactive, and the approximate pattern of peaks they give in a mass spectrum can be used to identify the presence of many elements. The ratio of abundances of isotopes for any one element, when measured accurately, can be used for a variety of analytical purposes, such as dating geological samples or gaining insights into chemical reaction mechanisms. 

Many artificial (likely radioactive) isotopes can be created through nuclear reactions. Radioactive isotopes of iodine are used in medicine, while isotopes of plutonium are used in making atomic bombs. In many analytical applications, the ratio of occurrence of the isotopes is important. For example, it may be important to know the exact ratio of the abundances (relative amounts) of the isotopes 1, 2, and 3 in hydrogen. Such knowledge can be obtained through a mass spectrometric measurement of the isotope abundance ratio. 

In fact, most RIAs and many nonisotopic immunoassays use a competitive binding format (see Fig. 2). In this approach, the analyte in the sample to be measured competes with a known amount of added analyte that has been labeled with an indicator that binds to the immobilized antibody. After reaction, the free analyte—analyte-indicator solution is washed away from the soHd phase. The analyte-indicator on the soHd phase or remaining in the wash solution is then used to quantify the amount of analyte present in the sample as measured against a control assay using only an analyte-indicator. This is done by quantifying the analyte-indicator using the method appropriate for the assay, for example, enzyme activity, fluorescence, radioactivity, etc. 

Metallacarboranes. These are used in homogeneous catalysis (222), including hydrogenation, hydrosilylation, isomerization, hydrosilanolysis, phase transfer, bum rate modifiers in gun and rocket propellants, neutron capture therapy (254), medical imaging (255), processing of radioactive waste (192), analytical reagents, and as ceramic precursors. 

Tritium is readily detectable because of its radioactivity. Under certain conditions concentrations as low as 370 )-lBq/mL (10 //Ci/mL) can be detected. Most detection devices and many analytical techniques exploit the ioni2ing effect of the tritium P-decay as a principle of operation (62,63). 

Mass Spectrometer. The mass spectrometer is the principal analytical tool of direct process control for the estimation of tritium. Gas samples are taken from several process points and analy2ed rapidly and continually to ensure proper operation of the system. Mass spectrometry is particularly useful in the detection of diatomic hydrogen species such as HD, HT, and DT. Mass spectrometric detection of helium-3 formed by radioactive decay of tritium is still another way to detect low levels of tritium (65). Accelerator mass spectroscopy (ams) has also been used for the detection of tritium and carbon-14 at extremely low levels. The principal appHcation of ams as of this writing has been in archeology and the geosciences, but this technique is expected to faciUtate the use of tritium in biomedical research, various clinical appHcations, and in environmental investigations (66). 

The abundance of a trace element is often too small to be accurately quantihed using conventional analytical methods such as ion chromatography or mass spectrometry. It is possible, however, to precisely determine very low concentrations of a constituent by measuring its radioactive decay properties. In order to understand how U-Th series radionuclides can provide such low-level tracer information, a brief review of the basic principles of radioactive decay and the application of these radionuclides as geochronological tools is useful. " The U-Th decay series together consist of 36 radionuclides that are isotopes (same atomic number, Z, different atomic mass, M) of 10 distinct elements (Figure 1). Some of these are very short-lived (tj j 1 -nd are thus not directly useful as marine tracers. It is the other radioisotopes with half-lives greater than 1 day that are most useful and are the focus of this chapter. 

NAA is well suited for Si semiconductor impurities analysis. The sensitivity and the bulk mode of analysis make this an important tool for controlling trace impurities during crystal growth or fer monitoring cleanliness of various processing operations for device manufacturing. It is expected that research reactors will ser e as the central analytical facilities for NAA in the industry. Since reactors are already set up to handle radioactive materials and waste, this makes an attractive choice over installing individual facilities in industries. 

For the purposes of analytical chemistry, four kinds of monochromatic beams need to be considered. (The quotation marks are to remind the reader that the beams under discussion are not always truly monochromatic.) Three kinds of beams—those produced by Bragg reflection (4.9), filtered beams (4.6), beams in which characteristic lines predominate over a background that can be neglected— will be discussed later ( 6.2). The fourth kind of beam contains monochromatic x-rays that are a by-product of our atomic age and that promise to grow in importance they are given off by radioactive isotopes. These x-rays must not be confused with 7-rays (11.1), which also originate from radioactive atoms but which differ from x-rays because the transformation that leads to radiation involves the nucleus. 

This can result in a radioactive product from the A(n, t)A reaction where A is the stable element, n is a thermal neutron, A is the radioactive product of one atomic mass unit greater than A, and y is the prompt gamma ray resulting from the reaction. A is usually a beta and/or gamma emitter of reasonably long half-life. Where access to a nuclear reactor has been convenient, thermal neutron activation analysis has proven to be an extremely valuable nondestructive analytical tool and in many cases, the only method for performing specific analyses at high sensitivities... 

It is fruitless to attempt detailed study of a phenomenon whose products are not well identified. It is unfortunately frequently noted in the literature, especially in cases of column chromatography, that fractions are only identified as to the chemical operations which brought them to light. Fractions are identified, for example, only by the solvent used. Speculations as to the composition of the radioactive solutes in such solutions can seldom be really reliable, and the presence of an unexpected radioactive species is in such cases undetectable. It is also important in reading the literature to watch out for cases in which the chemical yields of the carriers have not been measured. Extensive decomposition can often occur on silica gel and alumina columns, especially when photosensitive or moisture sensitive compounds are used. For these reasons much of the information now existing in the literature must be regarded as only exploratory, awaiting the development of better analytical methods for separation, purification, identification and determination of the products —known or expected. 

This example assumes that RIA was chosen. The principle behind RIA is the competition between the analyte A and a radioactively tagged control C (e.g., a /-marked ester of the species in question) for the binding site of an antibody specifically induced and harvested for this purpose. The calibration function takes on the shape of a logistic curve that extends over about three orders of magnitude. (Cf. Fig. 4.38a.) The limit of detection is near the B/Bo = 1 point (arrow ) in the upper left corner, where the antibody s binding sites are fully sequestered by C the nearly linear center portion is preferrably used for quantitation. 

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