Radiochemical methods direct analysis

Three common quantitative applications of radiochemical methods of analysis are considered in this section the direct analysis of radioactive isotopes by measuring their rate of disintegration, neutron activation, and the use of radioactive isotopes as tracers in isotope dilution. 

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. 

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. 

Direct measurement of dietary zinc availability in humans requires development of the stable isotope tracer methodology. Several aspects of this integrated methodology are considered and briefly discussed. These are analytical isotopic measurement methodology, consequences of the finite precision of isotopic measurements, validation of in vivo measurements, and several aspects of biological labeling of human foods. It is shown that Radiochemical Neutron Activation Analysis provides a suitable method for accurate measurement of the stable isotopes Zn,... 

The essential apparatus for pressure measurement and analysis, and other important aspects such as furnaces and temperature control, are reviewed for thermal, photochemical and radiochemical systems. The latter two also involve sources of radiation, filters and actinometry or dosimetry. There are three main analytical techniques chemical, gas chromatographic and spectroscopic. Apart from the almost obsolete method of analysis by derivative formation, the first technique is also concerned with the use of traps to indicate the presence of free radicals and provide an effective measure of their concentration. Isotopes may be used for labelling and producing an isotope effect. Easily the most important analytical technique which has a wide application is gas chromatography (both GLC and Gsc). Intrinsic problems are those concerned with types of carrier gases, detectors, columns and temperature programming, whereas sampling methods have a direct role in gas-phase kinetic studies. Identification of reactants and products have to be confirmed usually by spectroscopic methods, mainly IR and mass spectroscopy. The latter two are also used for direct analysis as may trv, visible and ESR spectroscopy, nmr spectroscopy is confined to the study of solution reactions... 

The second general category of radiochemical analysis involves adding a radioactive substance to the sample, manipulating the sample by chemical or physical means, measuring the radioactivity, and ultimately calculating the amount of the component of interest. This category includes direct and inverse isotope dilution analysis, radiochemical titrations, and radiorelease methods of 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. 

Direct isotope dilution analysis is applied if an amount of an analyte cannot be separated quantitatively for analytical determination. A known amount of a radioactive isotope of the element of interest is added to the sample containing the analyte. Then a portion of the analyte is isolated in high purity from the sample. This separation step need not be quantitative. The mass and activity of the isolated portion are measured and used to calculate the amount of analyte in the original sample. There are several varieties known of this radiochemical method, e.g., reverse isotopic dilution. 

Since some detectors are unable to discriminate between radioisotopes, e.g. proportional counters, and others require a sample matrix clean-up, sample pretreatment is very often unavoidable. Thus, radionuclides are isolated before measurement to avoid the presence of other radionuclides and interfering elements, and therefore radiochemical methods usually include separation steps in their protocols. Apart from a limited number of radionuclides that can be analyzed by high-resolution gamma spectrometry, e.g. " Mn, °Co, " Cs, Cs, " Eu, Eu, and Sb, direct analysis is impossible, that is the sample needs to be previously chemically processed. Furthermore, if samples to be analyzed have low activities, e.g. environmental samples, a preconcentration procedure is mandatory. 

A special place is reserved for methods of activation analysis, involving slow and fast neutrons, charged particles, or photon.s, applied either directly or in combination with some type of radiochemical separation (Section 1.6.13). These methods quickly became almost indispen.sable, especially in extreme trace analysis of the ele-... 

This paper reports the results so far on tests of the usefulness of the programme imder more realistic conditions. The first phase involved spiking aqueous buffer solutions with mixtures of different activities of Sr-85, Sr-89 and Sr-90+Y-90. Y(OH)3 was then precipitated from these solutions to remove Y-90. The Sr nuclides were then concentrated by precipitating SrCOs. The SrCOs was dissolved and transferred to a scintillation vial, mixed with the usual cocktail and tracers then measured by LSC. Here, Sr losses are expected, spurious cross-contaminations in the laboratory are possible, and Y-90 grows in during the analysis and measurement procedure. In a second test phase, the laboratory participated in an interlaboratory comparison with raw milk spiked with Sr-89 and Sr-90 as well as potentially interfering nuclides 1-131, Ba-133, Cs-134 and Cs-137. A known activity of Sr-85 tracer was added directly to aliquots of sub-samples of the raw milk prior to radiochemical separation and preparation for LSC by a rapid method ll... 

Recent applications of photon activation to the analysis of heavy elements include the determination of Te, Sr, and Pb. Campbell and Steele measured Te in the presence of U by Ge(Li) spectrometry of the activated Te isotope [ Te(y,n) Te] with a half-life of 17 d. The method is useful because it avoids the complications arising in n.a.a. caused by fission of the uranium giving rise to direct and spectral interferences from fission products. The measurement of "Sr induced by Sr(), n) and Sr(y,y ) reactions using 30 MeV Bremsstrahlung has been applied to the analysis of Sr in sea-water at the 8 p.p.m. level this involved a radiochemical separation of the 2—8 h Sr. The (y,n) reaction of Pb has been used for the determination of Pb in milk powder.Measurement of the Pb isotope (/ 52 h) after a non-specific sulphate precipitation of the Pb is sufficient to attain a limit of detection of 0.5 //g. The activity was measured with a NaI(TI) detector after "Sr, which was also produced, had decayed. 

---