Radiochemical techniques analysis

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. 

Almost all known physical methods of analysis have been used to study the constitution of phosphorus compounds. Among the most successful and widely used today are (1) XRD, (2) nuclear magnetic resonance spectroscopy (NMR), (3) infra-red spectroscopy (IR) and (4) chromatography. Emission spectra (visible, ultra-violet and x-ray), mass spectra, electron spin resonance (ESR), and radiochemical techniques are, however, becoming increasingly important. Many other techniques have also been employed, but their success in some instances has been limited to very narrow fields of application. Sensitivities and detection limits are often matrix dependent. Some terms which are currently used to represent a selection of available techniques, are listed in Table 14.4. 

In radiochemical activation analysis (RAA), the various techniques of activation analysis (AA), i.e., neutron activation analysis (NAA), photon activation analysis (PAA), and charged particle activation analysis (CPAA) are combined with radiochemical separation procedures with the intention of extending the capabilities offered by the purely instrumental methods. 

Neutron activation analysis, including nondestructive as weh as radiochemical techniques, has been successfully used for the determination of trace elements in metals, especially high purity metals. In the past, radiochemical separations were extensively used for the removal of matrix interferences. Later this procedure was extended to the group separation of trace elements. Radiochemical methods are also applied to the accurate and sensitive determination of single difficult-to-determine elements. 

Many years ago, the US National Academy of Sciences, within the National Research Council Nuclear Science Series, created a useful series of monographs on Radiochemistry and Radiochemical Techniques. At a time when much of activation analysis involved chemical separation, these were invaluable. They are now on the Internet at http //lib-www.lanl.gov/radiochemistry/elements.htm. 

The inherent sensitivity of these isotopic methods is especially attractive when a specific reaction is to be studied in detail. Furthermore, the formation of a radioactive product enables isotope dilution analysis to be applied to establishing the specificity of the reaction. A wide range of substrate and enzyme concentrations can be used and it is often possible to measure low enzymatic activity in crude homogenates without the need for extensive purification of the enzyme. This undoubtedly has constituted one of the major practical advantages of the radiochemical techniques. 

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. 

Alpha counting is done with an internal proportional counter or a scintiUation counter. Beta counting is carried out with an internal or external proportional gas-flow chamber or an end-window Geiger-MueUer tube. The operating principles and descriptions of various counting instmments are available, as are techniques for determining various radioelements in aqueous solution (20,44). A laboratory manual of radiochemical procedures has been compiled for analysis of specific radionucHdes in drinking water (45). Detector efficiency should be deterrnined with commercially available sources of known activity. 

The apphed pretreatment techniques were digestion with a combination of acids in the pressurized or atmospheric mode, programmed dry ashing, microwave digestion and irradiation with thermal neutrons. The analytical methods of final determination, at least four different for each element, covered all modern plasma techniques, various AAS modes, voltammetry, instrumental and radiochemical neutron activation analysis and isotope dilution MS. Each participating laboratory was requested to make a minimum of five independent rephcate determinations of each element on at least two different bottles on different days. Moreover, a series of different steps was undertaken in order to ensure that no substantial systematic errors were left undetected. 

For the analysis of americium in water, there is a broad array of sample preparation and detection methodologies that are available (see Table 7-2). Many of the common and standardized analytical methodologies typically include the minimization of sample volume, purification through co-precipitation, anion exchange column chromatography, and solvent extraction techniques followed by radiochemical detection of americium in the purified sample. Gross alpha analysis or liquid scintillation are common... 

We have recently modified U7) one of the several radiochemical methods (U5) which have been used for surface electrochemistry investigations in order to characterize adsorption on well-defined, single crystal electrodes. Below, we will describe the technique and identify some challenging issues which we will be able to address. The proposed method is sensitive to a few percent of a monolayer at smooth surfaces, is nondestructive and simple to use. The radiochemical measurements can be made with all compounds which can be labelled with reasonably long-lived, preferably g- emitting radioisotopes. We believe this technique will fulfill the quantitative function in in situ surface analysis as Auger spectroscopy currently does in vacuum, ex situ characterization of electrodes. 

Finally, radiochemical methods of analysis may be used to follow rates of detritiation. This method is particularly useful for very slow reactions (where it is impractical to collect data for any appreciable extent of reaction) as an initial rate approach may then be employed. Separation difficulties, at least for aqueous solutions, may be overcome by using the freeze-drying method or the more recent countercurrent dialysis and Sephadex gel filtration techniques. ... 

Long-lived radionuclides occur at extremely low concentrations, especially in environmental samples, therefore several authors have proposed matrix separation and enrichment of the analytes before analysis.21,24,26,3 39 Radiochemical methods often require very careful and time consuming separation and enrichment processes and measurement procedures of a-, (3- and -emitting radioactive species at the trace and ultratrace level using conventional radioanalytical techniques 40-43 Trace/matrix separation, which is performed offline or online in order to avoid possible isobaric interferences, matrix effects and to reduce the detection limits for the determination of long-lived radionuclides, is also advantageous before ICP-MS measurements as the most widely applied mass spectrometric technique. 

Procedures for the determination of 11 elements in coal—Sb, As, Br, Cd, Cs, Ga, Hg, Rb, Se, U, and Zn—by neutron activation analysis with radiochemical separation are summarized. Separation techniques include direct combustion, distillation, precipitation, ion exchange, and solvent extraction. The evaluation of the radiochemical neutron activation analysis for the determination of mercury in coal used by the Bureau of Mines in its mercury round-robin program is discussed. Neutron activation analysis has played an important role in recent programs to evaluate and test analysis methods and to develop standards for trace elements in coal carried out by the National Bureau of Standards and the Environmental Protection Agency. 

Very little work has been carried out on radiochemical derivatization for analysis of trace amounts of materials. The technique has the advantage of being both selective and sensitive. Die main advantage is that the sample background does not cause interference in the detection as it does in most other methods and which necessitates some degree of clean-up. Also, the reactions used are those for normal derivatization procedures, the only difference being that the reagent is radiolabeled and that appropriate precautions are required for radioactive substances. The few methods described below illustrate the application of this technique. 

Neutron activation analysis techniques are frequently used for trace element analyses of coal and coal-related materials (Weaver, 1978). Precision of the method is 25%, based on all elements reported in coal and other sample matrices. Overall accuracy is estimated at 50%. Neutron activation analysis utilizing radiochemical separations (NAA-RC) is employed by investigators when the sensitivity for a particular element or group of elements is inherently low or when spectral interference for a given element in a specific matrix is too great to be detected adequately. This situation was more prevalent before the advent of Ge(Li) spectrometry when only low-resolution Nal(TI) detectors were available. 

Egorov, O., Grate, J. W., and Ruzicka, J., Automation of radiochemical analysis by flow injection techniques Am-Pu separation using TRU-resin sorbent extraction column, J. Radioanal. Nucl. Chem., 234, 231-235, 1998. 

This laboratory experiment describes the preparation of a vegetation sample (e.g., grass) for radiochemical analysis. The sample is dried and ashed. In Part 12A, the ash is fused with sodium hydroxide and sodium carbonate to bring it into solution. An alternative method in Part 12B uses a microwave-assisted digestion technique with nitric and hydrofluoric acid. The prepared sample is suitable for radionuclide analysis, notably for radio-strontium or plutonium. 

Although the yield of the separation Is extremely low, l.e., estimated as 6.88 x 10 In the separation of Zr, method is useful as a good qualitative radiochemical analysis technique for a mixture of radioactive isotopes, because it does not require prior knowledge for the addition of inactive carriers as do most other radiochemical procedures. ... 

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