Trace element with radiochemical separations

Trace Elements in Coal by Neutron Activation Analysis with Radiochemical Separations... 

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. 

For a trace element concentration to be certified by NBS, it must be determined by at least two independent methods, the results of which must agree within a small experimental error range of 1% to 10%, depending on the nature of the sample and the concentration level of the element. Such accuracy in determining some trace elements for certification of coal SRM is achieved most easily by NAA with radiochemical separation. Scientists at NBS have extensively tested a neutron activation method that involves a combustion separation procedure on coal as well as on several other matrices to be certified as standard reference materials. The procedures they have thus developed to determine mercury (12), selenium (13), and arsenic, zinc, and cadmium (14) are outlined in a following section on methods for determining specific elements in coal. 

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... 

The advent of efficient high-resolution gamma-ray detectors during the past decade has nearly eliminated the tedious radiochemical separation of each trace element from all others which was once necessary, although group separations are often a powerful aid to specificity and sensitivity. If the matrix itself is the major contributor to the activity (as with Ge or most III-V compounds), safety precautions may be necessary to protect the chemist against radiation. Many procedures have been published for matrix removal and for the separation of trace constituents (29.3.2.30). Ga, Ge,... 

An accuracy of 0.2 ng has been reached in the analysis of biological and environmental samples containing about 4 ng Au on irradiation in the thermal neutron flux of 1 x 1013 n cm 2 s 1 during 6 days652 and subsequent radiochemical separation. 4.7 ppb and 1.9 ppb of Au was detected653 by INAA in two human kidney stones (one oxalate and one phosphate). There are indications that the trace element content of hair correlates with the body stores of these elements, particularly with those of bone654. The INAA of hair samples taken from osteoporosis patients showed 2.32 1.32 ngg 1 content of Au in osteoporotic hair and 3.57 1.90 ngg-1 in normal hair655. 

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). 

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). 

A radiochemical separation has three important advantages compared with a common chemical separation (1) Inactive carriers can be added for the elements to be separated (B and D). This avoids the difficulties of a chemical separation at the trace level. (2) Reagent impurities (or blanks) do not influence the detection limit capabilities of the analytical method. (3) Separations may not be quantitative and even not reproducible (see below). 

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... 

Reverse IDA (indirect IDA, or dilution with inactive isotopes) is particularly important in organic analysis and biochemistry [24], [32] to test for radiochemical purity and stability of labeled compounds. It is often used in radiochemical nuclear AA for the separation of activated element traces from a variety of interfering radionuclides in the analytical sample, for determination of the isolated activity, in comparison with that of a reference sample (i.e., to determine the radiochemical yield according to Eq. 10). 

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