Radiochemical separation techniques activation analysis

Although Meinke (603) points out that automation in analytical chemistry is most desirable to remove the drawbacks of radiochemical separations in activation analysis, many other analysts who use activation analysis for trace element determinations in biological materials continue effective research on separation systems for a single element or a small group of elements with similar chemical characteristics for example, the methods and techniques in the publication by Gorsuch (338) have been used by many analysts in their activation analysis determinations of trace elements. Other successful microchemical techniques used in activation analysis have been described by Pijck and Hoste (713), Sion, Hoste, and Gillis (858), Girardi and Merlini (331), and Smales and Mapper (864). 

Geochemists were some of the first researchers to realize the enormous benefits of ICP-MS for the determination of trace elements in digested rock samples. Until then, they had been using a number of different techniques, including neutron activation analysis (NAA), thermal ionization mass spectrometry (TIMS), ICP-OES, x-ray techniques, and GFAA. Unfortunately, they all had certain limitations, which meant that no one technique was suitable for all types of geochemical samples. For example, NAA was very sensitive, but when combined with radiochemical separation techniques for the determination of rare earth elements, it was extranely slow and expensive to run. TIMS was the technique of choice for carrying out isotope ratio studies because it offered excellent precision, but unfortunately was painfully slow. Plasma... 

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. 

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. 

Radiochemical methods of analysis are considerably more sensitive than other chemical methods. Most spectral methods can quantitate at the parts-per-mil-lion (ppm) level, whereas atomic absorption and some HPLC methods with UV, fluorescence, and electrochemical methods can quantitate at the parts-per-billion (ppb) levels. By controlling the specific activity levels, it is possible to attain quantitation levels lower than ppb levels of elements by radiochemical analyses. Radiochemical analysis, inmost cases, can be done without separation of the analyte. Radionuclides are identified based on the characteristic decay and the energy of the particles as described in detection procedures presented above. Radiochemical methods of analysis include tracer methods, activation analysis, and radioimmunoassay techniques. 

As mentioned before, two interlaboratory studies were organised prior to certification, involving ca. 15 laboratories using techniques such as cold vapour atomic absorption spectrometry, direct current plasma atomic emission spectrometry (DCP-AES), differential pulse anodic stripping voltammetry (DPASV), microwave plasma atomic emission spectrometry (MIP-AES), electrothermal atomic absorption spectrometry (ETAAS) and neutron activation analysis with radiochemical separation (RNAA). 

The availability of high flux thermal neutron irradiation facilities and high resolution intrinsic Ge and lithium drifted germanium (Ge(Li)) or silicon (Si(Li)) detectors has made neutron activation a very attractive tool for determining trace elemental composition of petroleum and petroleum products. This analytical technique is generally referred to as instrumental neutron activation analysis (INAA) to distinguish it from neutron activation followed by radiochemical separations. INAA can be used as a multi-elemental method with high sensitivity for many trace elements (Table 3.IV), and it has been applied to various petroleum materials in recent years (45-55). In some instances as many as 30 trace elements have been identified and measured in crude oils by this technique (56, 57). 

The application of microtron photon activation analysis with radiochemical separation in environmental and biological samples was described by Randa et al. (2001), and both flame and plasma emission spectroscopic methods are also widely used. A more recently developed technique is that of laser-excited atomic fluorescence spectrometry (LEAFS) (Cheam et al. 1998). 

The fundamental principle behind analysis by activation analysis is activation or excitation of an atomic nucleus by exposure to radiation such as neutrons, protons or high-energy photons with subsequent measurement of emitted sub-atomic particles or radiation. The most common aspect of the technique involves activation with neutrons in a nuclear reactor and measurement of delayed emitted gamma rays, denoted neutron activation analysis, either instrumental neutron activation analysis (INAA) or neutron activation followed by radiochemical separation (RNAA) in which the element of interest is chemically separated from the matrix after irradiation to provide for better, unimpeded counting. 

Some relevant terms for activation analysis are activation analysis, neutron activation analysis (NAA), instrumental neutron activation analysis (INAA), neutron activation analysis with radiochemical separation (RNAA), photon activation, neutron capture prompt gamma activation analysis (PGAA), charged particle activation, autoradiography, liquid scintillation counting, nuclear microprobe analysis, radiocarbon (and other element) dating, radioimmunoassay, nuclear track technique, other nuclear and radiochemical methods. Briefly, the salient features of some of the more popular techniques are as follows ... 

Excellent, comprehensive treatments of the principles and fundamentals of nuclear activation analysis - including applications fundamentals - are found in the following five consecutive chapters in the first edition of Treatise on Analytical Chemistry Finston (1971a) (Radioactive and isotopic methods of analysis nature, scope, limitations, and interrelations) Finston (1971b) (Nuclear radiations characteristics and detection) Crouthamel and Heinrich (1971) (Radiochemical separations) Seaman (1971) (Tracer techniques) and Guinn (1971) (Activation analysis). A series of seven similarly comprehensive chapters appeared in the updated second edition Lieser (1986), (Fundamentals of nuclear activation and radioisotopic methods of analysis) Herpers... 

A number of investigators have used Nal(Tl) spectrometry after radiochemical separation to determine several elements in petroleum. Ordogh, et determined I, Br and V in Hungarian crudes by a nondestructive technique, and Palmai used a similar method to determine V, Na, Mn, and Al in products from refinig of crudes. Several Russian papers report the use of nondestructive activation analysis and Nal(Tl) gamma-ray spectrometry for such trace elements as V, Ni, Cu. Co and Na. Mast, et al. and Arroyo and Brune have used nondestructive techniques for the determination of V in crude oils. 

The section Radioactive Methods in volume 9 of the Treatise on Analytical Chemistry (Kolthoff and Elving 1971) discusses radioactive decay, radiation detection, tracer techniques, and activation analysis. It has a brief but informative chapter on radiochemical separations. A more recent text. Nuclear and Radiochemistry Fundamentals and Applications (Lieser 2001), discusses radioelements, decay, counting instruments, nuclear reactions, radioisotope production, and activation analysis in detail. It includes a brief chapter on the chemistry of radionuclides and a few pages on the properties of the actinides and transactinides. 

Either precipitation, solvent extraction, ion exchange, electrodeposition or volatilization, or a combination of these, are the techniques usually employed if chemistry is used in an activation analysis application. The Nuclear Science Series of the Subcommittee on Radiochemistry, National Academy of Sciences-National Research Ck)uncil (924) records many of the radiochemical separation methods used in activation analysis. The NAS-NRC Monograph, NAS-NRC-1351, (642) also provides useful information on radiochemistry and its applications. 

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. 

There are several configurations of this technique that include instrumental neutron activation analysis (INAA) where the sample is measured without any chemical treatment and radiochemical NAA (RNAA) where post-irradiation separation is done. 

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 second step in neutron activation analysis is the measurement of the induced radioactivity. A number of different techniques have evolved over the years to keep pace with changes in technology. The techniques can be divided into two groups radiochemical neutron activation analysis (RNAA), a destructive technique in which various elements are separated after the irradiation, and instrumental neutron activation analysis (INAA), a non-destructive technique in which the activity in the sample is measured directly, relying on differences in y-ray energy and half-life to discriminate the various nuclides. 

As shown in the lower pathway in f-igure 32-8. a destructive method requires that the analyte be separated from the other components of the sample prior to counting. If a chemical separation method is used, this technique is called radiochemical neutron activation. In this case a known amount of the irradiated sample is dissolved and the analyte separated by precipitation, extraction, ion exchange, or chromatography. The isolated material or a known fraction thereof is then counted for its gamma — or beta — activity. As in the nondestructive method, standards may be irradiated simultaneously and treated in an identical way. Equation. 32-21 is then used to calculate the results of the analysis. 

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