Available Stable Isotopes of the Elements

Electronic Structure of Selected Elements Available Stable Isotopes of the Elements... 

Numerous trace elements are known to be nutritionally essential in man In order to assess the essentiality, dietary availability, and metabolic fate of these, means of labeling for subsequent identification are needed In animal studies, radioisotopes are often used for this purpose, but their use in human studies is generally contraindicated due to the radiation hazards An alternate method is to use stable isotopes of the elements, which overcomes this limitation A method will be described for conveniently measuring the stable isotopes of selenium, permitting their use as metabolic tags in tracer studies Using one stable isotope as the tracer and another as internal standard, one can quantitatively identify in a sample the tracer, natural (unenriched) selenium present with it, and total selenium Some of the kinds of information obtainable from metabolic tracer studies will be discussed ... 

Whether in the environment or in the human body, uranium will undergo radioactive decay to form a series of radioactive nuclides that end in a stable isotope of lead (see Chapter 3). Examples of these include radioactive isotopes of the elements thorium, radium, radon, polonium, and lead. Analytical methods with the required sensitivity and accuracy are also available for quantification of these elements in the environment where large sample are normally available (EPA 1980,1984), but not necessarily for the levels from the decay of uranium in the body. More sensitive analytical methods are needed for accurately measuring very low levels of these radionuclides. 

AMS does not determine the absolute concentration of isotopes rather an isotope ratio of a rare isotope to an abundant stable isotope of the same element is measured. There is no commercially available AMS instrument that measures all isotopes, such that most instruments are built to detect one or two elements only. Highly specialized, complex, and time-consuming sample preparation is often required prior to AMS analysis. AMS instruments for various radioisotope analyses have been reviewed in a recent publication [14], Here we would only discuss AMS instrument for 14C analysis, the most important radioisotope for biomedical applications. For 14C measurement, an AMS instrument usually consists of an ion source to generate negative ions, a low magnetic field for initial ion separation, a high-voltage... 

Isotope dilution This relatively new calibration method is occasionally used in specialized forensic applications. The method affords exceptional accuracy and precision, but is limited by the availability of suitable standards. Although the use of deuterated internal standards is sometimes equated with isotope dilution, the techniques are not the same. Isotope-dilution calibration is based on the addition of standards that contained enriched levels of stable isotopes of common elements such as C. The enriched standard, referred to as the spike, is added to the sample and allowed to reach equilibrium. The sample is then analyzed via mass spectrometry. Because the spike is chemically identical to the target analyte, the matrix correction is as good as that achieved with standard-addition calibration. However, until enriched stable isotope analogs of forensic analytes are available and affordable, isotope dilution will not be widely applied in forensic chemistry. 

The natural occurrence of the group-IIA elements ranges from common to rare e.g., Ca is 5th in the order of atomic abundance of the elements in the earth s crust. Mg is 7th, Ba and Sr are 21st and 22nd, respectively, and Be is 32nd . Radium is of extremely limited availability. It does occur naturally, although it has neither stable nor long-lived radioaetive isotopes it is found in association with U, since ll/2... 

A known activity of a second radionuclide may be added to the solution as a tracer for the radionuclide of interest when no stable carrier is available, or if measuring the tracer is more convenient than measuring the carrier. Preferably, both radionuclides are isotopes of the same element, e.g., Sr tracer for °Sr measurement. The initial count rate of the radionuclide of interest is calculated from its final count rate by Eq. (4.5a), taking wji and mr to be the initial and final net count rates, respectively, of the tracer. 

The meta-elements of Crookes anticipated the idea of isotopes. The most readily available sources of separated stable isotopes are lead 206 from uranium minerals and lead 208 from thorium minerals, and it is interesting enough that before 1910 several successful chemical separations of radioactive isotopes of the same element were reported, involving both thorium 230 (ionium) and lead 210 (radium D). In otir opinion, this can only be due to kinetically metastable chemical nonequivalency in the mixture, for instance, due to colloidal or oligomeric complexes. The valuable conclusion of this story is that the chemical similarity of trivalent rare earths is so striking that doubts have been expressed whether they deserved more than one place in the Periodic Table, a situation isotopes later had to accept. Such a doubt has never been expressed for any other elements, not even for a pair of elements like vanadium and chromium, which were confused at the time of their discovery (7). Nevertheless, studies based on the possibility of metaelements continued rather late for instance, Debierne attempted to separate neo-radium from conventional radium 226 and to perform nuclear reactions on charcoal cooled with liquid helium (77). 

---