Radiochemical analysis separation methods

A slightly different example is the separate determination of rates of reaction of nC and 14C labeled methyl iodide with N,N-dimcthyl-/>-toluidine as illustrated in Fig. 7.3. Again the method takes advantage of the convenience of radiochemical analysis. If, as likely, the KIE of interest is ki2/ki4, it can be obtained to sufficiently good approximation by applying a modified Swain-Schaad rule, ln[ki2/ki4]/ln[kn/ki4] = [(12/14)/(11/14)]1/2 obtained from the law of the geometric mean (see Section 10.5). 

Another consideration when choosing a detector is whether it is important to preserve the separated analytes, either for use or for further analysis. Some methods, such as evaporative laser scattering detection and mass spectrometry, destroy the sample during the measurement. Other methods, such as fluorescence or radiochemical detection, may require chemical labeling of the analytes ... 

Laboratory robotics represents an attractive approach for the automation of sample preparation and separation steps in radiochemical analysis, and for many years, such methods have been routinely used by laboratories serving the analytical needs of the International Atomic Energy Association.64 68-72 However, there are currently a limited number of published studies containing technical details on the radiochemical separations and how they were automated. Accordingly, the remainder of this chapter will focus on fluidic approaches. 

Although best known for simple serial assays in homogeneous solution, such as colorimetric reactions, FI methods have also been developed that perform separations or utilize solid phases.33 34,42 73 -76 The use of solid-phase separation columns in FI or SI systems for radiochemical analysis gathered momentum in the 1990s. 

By the late 1990s and into the 2000s, a number of additional groups became involved in automated fluidic separations for radiochemical analysis, especially as a front end for ICP-MS. Published journal articles on fluidic separations for radio-metric or mass spectrometric detection are summarized in Tables 9.1 through 9.5. The majority of such studies have used extraction chromatographic separations, and these will be the main focus of the remainder of this chapter. Section 9.4 describes methods that combine separation and detection. Section 9.5 describes a fully automated system that combines sample preparation, separation, and detection. 

Conventional radiochemical analysis of nuclear process or waste samples in the laboratory entails three primary activities sample preparation, radiochemical separation, and detection. Each of these activities may entail multiple steps. The automated fluidic methods described above, typically also carried out in the laboratory, link separation and detection. Sample preparation has, in many cases, been carried out first by manual laboratory methods. 

Uranium in nature may be measured either radiometrically or chemically because the main isotope - 238U - has a very long half life (i.e., relatively few of its radioactive atoms decay in a year). Its isotopes in water and urine samples usually are at low concentrations, for which popular analytical methods are (1) radiochemical purification plus alpha-particle spectral analysis, (2) neutron activation analysis, (3) fluorimetry, and (4) mass spectrometry. The radiochemical analysis method is similar in principle to that of the measurement of plutonium isotopes in water samples (Experiments 15 and 16). Mass spectrometric measurement involves ionization of the individual atoms of the uranium analyte, separation of the ions by isotopic mass, and measurement of the number of separated isotopic ions (see Chapter 17 of Radioanalytical Chemistry text). 

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. ... 

The EPA developed two methods for the radiochemical analysis of uranium in soils, vegetation, ores, and biota, using the equipment described above. The first is a fusion method in which the sample is ashed, the silica volatilized, the sample fused with potassium fluoride and pyrosulphate, a tracer is added, and the uranium extracted with triisooctylamine, purified on an anion exchange column, coprecipitated with lanthanum, filtered, and prepared in a planchet. Individual uranium isotopes are separately quantified by high resolution alpha spectroscopy and the sample concentration calculated using the yield. The second is a nonfusion method in which the sample is ashed, the siUca volatilized, a tracer added, and the uranium extracted with triisooctylamine, stripped with nitric acid, co-precipitated with lanthanum, transferred to a planchet, and analyzed in the same way by high resolution a-spectroscopy (EPA 1984). 

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. 

Gibbons D and Lambie DA (1971) Radiochemical methods of analysis. In WBson CL and WBson DW, eds. Comprehensive analytical chemistry, Vol 2C, Electrical methods, physical separation methods, pp. 130—205. Elsevier, Amsterdam. 

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. 

Solvent extraction enjoys a favored position among separation techniques because of its ease, simplicity, speed, and wide scope. Separation by extraction can usually be accomplished in a few minutes using a simple pear-shaped separatory funnel (Fig. 20.1), and is applicable both to trace-level impurities and to major constituents. Furthermore, inorganic constituents are often separated in a form suitable for direct analysis by spectrophotometric, atomic absorption, radiochemical, or other methods. 

A text with a scope similar to this book is Radiochemical Methods in Analysis (Coomber 1975). The text contains such relevant chapters as Separation methods for inorganic species and The use of tracers in inorganic analysis. A chapter titled Determination of radioactivity present in the environment contains information geared toward sample collection. 

Radiochemical analysis Inaccurate aliquots of sample Inaccurate aliquots of reagent Inaccurate reagent preparation Reagent instability Contaminabon Method not followed Inappropriate method Incomplete purification Separation step not fully functional Strange product appearance (amount, form, color) Low or excessively high yields... 

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. 

Allan and Parthasarathy (11) and Kukula et al. (502) offer some specific radiochemical methods based on the use of solvent extraction separations. Ion exchange separation methods applied to activation analysis have been described by Aubouin and Laverlochere (40), Co mar and lePoec (193, 194), Cram and Brownlee (208), Eisner et al. (250), Hadzistelios and... 

Some investigators are finding activation analysis a useful method to determine trace elements, either in essential or nutrient forms or as contaminants, in various food products. Das et al. (217) have shown that many dairy products contain only nanogram amounts of Mn. Hingorani and Chandrasekaran (401), Moeller and Leddicotte (620) and Samuelsson (814) have determined the strontium content of activated milk by radiochemical separations. Molinski, el al. (623) used a rapid radiochemical separation method to determine nanogram concentrations of V in milk powder. Samuelsson (813) also determined submicrogram amoimts of copper in whole milk by activation analysis and Allaway and Cary (12) developed an activation analysis method to determine 0.1 ppm of Se in dried skim milk. 

Instrumental analysis is possible if spectral inter ference can be avoided by a proper choice of the incident energy or the measuring conditions (see Interferences). If not, the radionuclide B, formed from the analyte element A, has to be separated radiochemically from interfering radionuclide(s) D, formed out of interfering element(s) C. The latter case is called radiochemical analysis, in contrast to instrumental analysis for the former. This section deals with some major differences between radiochemical separation and common chemical separation, used for non-nuclear methods of analysis. 

In spite of the considerable increase in the availability of instrumental methods of activation analysis, this in no way decreases the need for efficient methods of chemical separation. The field of radiochemical separation methods was reviewed by Girardi and only the most significant general developments in recent publications will be discussed. Information about the very many specific separation methods for particular elements and matrices is easily retrieved using the comprehensive bibliographies. ... 

A number of the prooeradiometric determination of wanlum. The method of separation in these procedures, however, is applicable to radiochemical analysis and is, therefore, included. A number of papers and reports describe, in detail, procedures for the determination of uranium. These should be noted. The work of Rodden and Warf has frequently been mentioned in this paper. In addition to procedures for the precipitation, solvent extraction, volatilization, and electrodeposltlon of uranium, these authors have presented a number of selected procedures for the solution of ores and minerals and the separation and determination of uranium. Procedures for the analytical determination in naturally occurring materials have also been described by Hodden and Tregonning,- Grimaldi, May, Fletcher, and Tltcomb, 2Z. Sohoeller and Powell,and in the "Handbook of Chemical Determination of Uranium in Minerals and Ores." 2 The recent publication by Hoore on extraction with amines contains a collection of procedures., many of which have to do with the separation of uranium. 

Photoactivation analysis has also been used to determine fluoride in seawater [73]. In this method a sample and simulated seawater standards containing known amounts of fluoride are freeze-dried, and then irradiated simultaneously and identically, for 20 min, with high-energy photons. The half-life of 18F (110 min) allows sufficient time for radiochemical separation from the seawater matrix before counting. The specific activities of sample and standards being the same, the amount of fluoride in the unknown may be calculated. The limit of detection is 7 ng fluoride, and the precision is sufficient to permit detection of variations in the fluoride content of oceans. The method can be adapted for the simultaneous determination of fluorine, bromine, and iodine. 

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. ... 

Radioactivity of uranium can be measured by alpha counters. The metal is digested in nitric acid. Alpha activity is measured by a counting instrument, such as an alpha scintillation counter or gas-flow proportional counter. Uranium may be separated from the other radioactive substances by radiochemical methods. The metal or its compound(s) is first dissolved. Uranium is coprecipitated with ferric hydroxide. Precipitate is dissolved in an acid and the solution passed through an anion exchange column. Uranium is eluted with dilute hydrochloric acid. The solution is evaporated to near dryness. Uranium is converted to its nitrate and alpha activity is counted. Alternatively, uranium is separated and electrodeposited onto a stainless steel disk and alpha particles counted by alpha pulse height analysis using a silicon surface barrier detector, a semiconductor particle-type detector. 

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. 

The increase in accuracy afforded by a radiochemical separation is absolutely necessary in the determination by NAA of trace elements in the coals selected as standards. The fact that interferences from the coal matrix are removed by a radiochemical separation is the advantage of this method of analysis over such instrumental methods as x-ray fluorescence and emission spectroscopy. 

This article presents a comprehensive view of the present state-of-the-art of radiochemical separations for the following trace elements in coal Hg, Rb, Cs, Se, Ga, As, Sb, Br, Zn, Cd, and U. Most of the work on the determination of trace elements in coal is very recent. The accuracy of the analysis methods, nearly all newly developed, has been open to question because of the lack of standards and lack of knowledge of the range of concentrations for many trace elements in coal. Federal government laboratories have taken the lead in evaluating methods of analysis and in developing standards. By a round-... 

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