Radiochemical procedures, detection

The NAA measurements on the paper samples were made at the Breazeale Nuclear Reactor Facility at the Pennsylvania State University with a TRIGA Mark III reactor at a flux of about 1013 n/cm2-sec. Samples were irradiated from 2 to 20 min and counted for 2000 sec, after a 90 min decay time for Ba and a 60 hr decay for Sb, Analyses were performed instrumentally, without radiochemical separation, using a 35cm3 coaxial Ge-Li detector and a 4096-channel pulse height analyzer. With these procedures, detection limits for Ba and Sb were 0.02ug and 0.001 ug, respectively. These sensitivities are comparable to those obtained by GA s radiochemical separation procedure, and are made possible by the use of the higher neutron output from the more powerful reactor and in combination with the higher resolution solid state detector... 

Radiochemical Purity. A sample is radiochemically pure at the time of counting if no other radionuclide is detected in it. As a general rule, the radiochemical procedure is chosen to separate from the radioanalyte all other radionuclides that are in the sample. Purification steps must be added to the usual procedure if the level of contaminant radionuclides is very high relative to the concentration of the radioanalyte. 

But the question remains as to what const outes no residue." Currie (] ) examined the corresponding problem of detection limits in radiochemical procedures and was frustrated by the differences in terminology and definitions which resulted in a range of three orders of magnitude for detection limits calculated for the same system. Figure 4, taken from his paper, shows the situation with respect to a specific radioactivity process. The horizontal lines indicate three specific levels L, "decision limit," the level a signal must exceed to permit a decision as to whether or not the result of an analysis indicates detection Lj), "detection limit," the level above which an analytical procedure can be relied upon to lead to detection and Lq, "determination limit," the level above... 

For Tc, the low-level energy jS-particles emitted (Emax = 292 keV) and the low concentrations in environmental samples represented an analytical challenge prior to the late 1970s. Based on a radiochemical procedure developed in 1969, the activity concentrations of Tc in waters ranged from 0.019 to 1.8Bql , and the detection limit was reached for 80% of the samples ( =150). For rainwater samples collected in 1967, the activity concentrations... 

The Cs concentration in foods, plants, com samples, etc., can be determined by direct measurement of radioactivity with a y-ray spectrometer. This method requires a higher Cs concentration than in the radiochemical procedure, and radioactive impurities whose y-rays have energies similar to that of cesium y-rays (such as Zr) can be present only in negligible amounts. The spectrometric method can be used either directly on the original sample or after concentrating Cs, depending on the detection efficiency and on the Cs concentration in the sample. 

A simple postirradiation radiochemical procedure has recently been developed at the Jozef Stefan Institute in Slovenia for the selective removal of iron from iron minerals and iron-based reference materials (Makreski et al. 2008, Jacimovic et al. 2008). Iron chlorocomplexes were extracted with di-i-propyl ether (DIPE) or i-amyl acetate (lAA). After Fe elimination, the distribution of 35-39 elements in the extraction systems was determined. Twelve to fourteen elements, particularly the lanthanides, could be detected with increased sensitivity, while Sb, Mo, Au, Se and Te were co-extracted with Fe, which prevented the determination of these elements. 

During the last decades methods such as Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES), Inductively Coupled Plasma Mass Spectroscopy (ICP-MS), and Resonance Ionization Mass spectrometry (RIMS) have decreased the need for selective radiochemical procedures. Many long-lived radionuclides today have lower detection limits if using, e.g., ICP-MS than if performing radiometric measurements with reasonable measuring times. At present, the half-life limit is a few hundred years, i.e., nuclides with longer half-hfe (e.g., Tc, Np, or Pu) should preferably be measured by ICP-MS and more short-... 

Chemical methods apply to the separation and purification of radioactive substances in the same way they apply to stable substances. A radiochemical separation is judged in terms of both the yield and the purity of the separated material this is particularly important when analyte concentrations are low or contaminant levels are high. Purity and fractional recovery are often evaluated with the tracer technique. Radiochemical and mass spectrometric detection methods are quite sensitive, and it is possible to work with trace amoimts of analyte. However, radiochemical procedures involving the presence of an isotopic carrier (which can also function as a tracer) are often simpler to design than are carrier-free procedures losses from adsorption on vessel walls or suspended particles may negatively affect the recovery of the analyte in a tracer-level sample. 

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. 

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. 

The development of solvent-impregnated resins and extraction-chromatographic procedures has enabled the automation of radiochemical separations for analytical radionuclide determinations. These separations provide preconcentration from simple matrices like groundwater and separation from complex matrixes such as dissolved sediments, dissolved spent fuel, or nuclear-waste materials. Most of the published work has been carried out using fluidic systems to couple column-based separations to on-line detection, but robotic methods also appear to be very promising. Many approaches to fluidic automation have been used, from individual FI and SI systems to commercial FI sample-introduction systems for atomic spectroscopies. 

Kucera et al. [161] performed a comparative analysis of three procedures for determining I in food. Their accuracy and precision were checked against reference materials based on food products. The limit of detection was 1 pg/kg. Instrumental NAA (INAA) and replicate sample INAA (RSINAA) were used in Portugal to test a wide assortment of food products for their total Se content [162], INAA and radiochemical NAA (RNAA) were also used to assess the quality of bottled water sold in Greece [163]. Elements such as U, Ba, La, Sb, Ca, Cr, Zn, K, As, Br, Se and... 

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. 

Proof that a lysine residue has been modified can be readily obtained because pyridoxyl derivatives of lysine possess characteristic white-blue fluorescence (Ronchi et al. 1969). In addition, they have a distinctive absorption maximum at 325 nm with of 9710 cm (Fisher et al. 1963). Finally, a radiochemical label can be introduced by reducing the pyridoxal-5-phosphate protein complex with tritium-labelled sodium borohydride. The peptide containing the derivatized lysine can therefore be detected either by fluorimetry, spectrophotometry or radiochemical techniques following routine procedures of proteolytic digestion and fractionation. Acid hydrolysis in 6 N HCl for 24 hr of peptides containing pyridoxal-5-phosphate lysine yields pyridoxyl-lysine since phosphate esters are readily hydrolyzed under these conditions. Pyridoxyl-lysine is eluted between lysine and histidine from a 55 cm column of Beckman 50 resin with 0.15 M citrate buffer pH 5.28. 

Radiochemical and INAA procedures for the determination of the low level trace elements in human fivers established the limits of detection of seven critical low level trace elements (Cr, As, Se, Mo, Ag, Sn, 122Sb and 124Sb). For silver, the value 0.7 pg kg-1 is given354. These limits represent concentrations at which a sample would generate a detectable signal355. 

All immunoassay procedures are based on the original discovery by Berson and Yalow that low concentrations of antibodies to the antigen hormone insulin could be detected radiochemically by their ability to bind radiolabeled ( I) insulin. The determination of unknown concentrations of antigen, then, is based on the fact that radiolabeled antigen and unlabeled antigen (from the sample or a standard) compete physiochemically for the binding sites on the antibodies (radioimmunoassay, RIA). This is illustrated in Figure 24.5. 

The total effective analytical efficiency (product of the recovery efficiency and the detection efficiency) is calculated based on the difference in analytical response obtained by the analysis of the spiked and unspiked samples. This approach provides a reliable method for remote, matrix matched, instrument calibration. Automated standard addition can be used for each sample or batch of samples. The automated radiochemical analysis procedure is rapid, with a total analysis time of 12.5 min per sample. The total analysis time for the standard addition measurement is 22 min (including analysis of both unspiked and spiked samples). For low-level samples, a much longer counting time can be expected. 

Laboratory measuring equipment used in radiochemical analytical procedures should show a high radiation sensitivity, high detection efficiency, low background, and high stability. 

One of the main differences between radiochemical analytical procedures and classical analytical methods is that the element (and particularly its radioisotope) to be determined is present in the sample in minor to trace amounts. Separation of radionuclides is performed with the aid of a suitable carrier. Generally, the carrier is a stable isotope (or a suitable compound) that is added to the radioactive compound in a small but detectable amount and has identical chemical properties. An isotopic carrier, i.e., a stable isotope of the element in question, is most frequently used. Both the radioactive isotope and the carrier must be in the same chemical form. The isotopic carrier is irreversibly mixed with the radioactive compound and cannot be separated from it again by chemical means. Such a carrier can therefore be used only when a lower specific activity is sufficient for the subsequent operations. For example, barium or lead can serve as carriers when... 

Today, activation analysis procedures rely almost exclusively on the detection of gamma rays for simultaneous qualitative identification and quantitative recording of the decay of the activation products. Exceptions are in the use of purely or predominantly -emitting activation products, such as or Si, in conjunction with radiochemical separation to achieve specificity (see O Sect. 30.5). INAA procedures utilize a variety of gamma-ray spectrometry systems the principal components of which are illustrated in O Fig. 30.3. In this section, the components and characteristics of gamma-ray spectrometers used for INAA will be briefly described. More technical details can be found in the excellent monographs available (Knoll 2000 Gilmore 2008 Debertin and Helmer 1988) and in O Chap. 48 of Vol. 5 on radiation detectors. 

Chemistry used in the recovery of plutonium from irradiated fuel must provide a separation from all these elements, other fission and activation products, and the actinides (including a large amount of unburned uranium), and still provide a complete recovery of plutonium. The same issues apply to the recovery of uranium from spent thorium fuel. Most of the processes must be performed remotely due to the intense radiation field associated with the spent fuel. As in the enrichment of uranium, the batch size in the later steps of the reprocessing procedure, where the fissile product has become more concentrated, is limited by the constraints of criticality safety. There is a balance between maximizing the yield of the precious fissile product and minimizing the concentrations of contaminant species left in the final product These residual contaminants, which can be detected at very small concentrations using standard radiochemical techniques, provide a fingerprint of the industrial process used to recover the material. 

In a two-cycle Purex procedure, the recoveries of both Pu and U are on the order of 99.9% the Pu/U separation factor is 10 . The literature reports that plutonium decontamination factors from the fission products are 10 , and that the only long-lived impurities detected in the final product are Zr, Tc, and ° Ru. However, the authors experience has been that the light rare earths (mainly La and Ce), thorium, neptunium, and the trivalent actinides (Am and Cm), which exhibit some degree of complexation in nitrate media (Guseva and Tikhomirova 1979), are present in a gram-sized plutonium sample at concentrations that are detectable by radiochemical means. 

Chemical test reagents can be applied also to thin-layer chromatograms which contain radioactive substances. In addition, various procedures for detection and quantitative determination of labelled compounds can be employed. Both chemical and radiometric methods of detection should always be used on the same thin-layer chromatogram this yields information about both the chemical and radiochemical purity of the sample studied. 

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