Strontium-90 isotopes

Strontium isotopes, reported as the ratio Sr/ Sr, have been used primarily in the study of human movement in the past. There are, however, a number of other potential applications usually involving the sourcing or proveniencing of other kinds of objects and materials such as pottery, stone, textiles, or animals. 

The analysis proceeds through the sampling of tooth enamel to obtain the isotopic signal of the place of birth. The local isotopic signal can be determined in several ways in human bone from the individuals whose teeth are analyzed, from the bones of humans or archaeological fauna at the site, or from modem fauna in the vicinity. The local geological isotope signals of strontium have been constant over the last several 1,000 years. These isotope ratios have been reported for the rocks. 

Samples weighing 2-5 mg are then dissolved in 5-molar nitric acid. The strontium fraction is purified using ion-specific resin and eluted with nitric acid followed by water. This solution is loaded onto a titanium filament for placement in the instrument (Fig. 4.20). Isotopic compositions are obtained on the strontium fraction thermal ionization mass spectrometer (TIMS). This is a single focusing, magnetic sector instrument equipped with multiple Faraday collectors. Strontium is placed on a thin filament and measured. Sr/ Sr ratios are corrected for mass fractionation using an exponential mass fractionation law. Sr/ Sr ratios are reported relative to a value of 0.710250 for the NIST 987 standard (e.g., if the Sr/ Sr ratios for the standards analyzed with the samples average 0.710260, a value of 0.000010 is subtracted from the ratio for each sample). 

Strontium isotopes have also been used in the investigation of wood and maize (com) at the ancient site of Pueblo Bonito in Chaco Canyon by geologists in Arizona and Colorado. This ancient, multistory apartment complex housed approximately 1,000 people from ad 850-1200. The pine, fir, and spruce trees that pro- 

Conditions were too dry in Chaco Canyon for trees to grow to any size. Today pine, fir, and spruce grow only at higher elevation in the mountains around the site (Fig. 4.21). Comparison of strontium isotopes in the soils from the mountain area to the south and west of Pueblo Bonito with the timber at the site provided a good match (Fig. 4.22). These areas (Chuska and San Mateo), more than 80 km (50 miles) to the west, were the source of the wood beams at Chaco Canyon (Fig. 4.21). 

Strontium incorporated into the teeth, for example, becomes immobilized after formation of the teeth. The relative amoxmts of the isotopes of strontium in the teeth is, therefore, related to the average strontium isotope composition that an animal ingested in food or water while growing. Thus teeth retain a kind of record of both early exposure to the isotopes of strontium in the water and food ingested during their growing, early life. In the 

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These chemical effects become important in medicine because living systems operate mostly through the reactions of enzymes, which catalyze all sorts of metabolic reactions but are very sensitive to small changes in their environment. Such sensitivity can lead to preferential absorption of some deleterious isotopes in place of the more normal, beneficial ones. One example in metabolic systems can be found in the incorporation of a radioactive strontium isotope in place of calcium. 

Rubidium-87 emits beta-particles and decomposes to strontium. The age of some rocks and minerals can be measured by the determination of the ratio of the mbidium isotope to the strontium isotope (see Radioisotopes). The technique has also been studied in dating human artifacts. Rubidium has also been used in photoelectric cells. Rubidium compounds act as catalysts in some organic reactions, although the use is mainly restricted to that of a cocatalyst. 

Strontium isotopes. Strontium isotopic compositions ( Sr/ Sr) of anhydrite, gypsum and barite from Kuroko deposits are summarized in Fig. 1.45 (Farrell et al., 1978 Honma and Shuto, 1979 Farrell and Holland, 1983 Yoneda et al., 1993 Yoneda and Shirahata, 1995). Sr/ Sr values of anhydrite and gypsum are slightly lower than that of seawater, suggesting that most of the strontium was derived from seawater, but a small amount of... 

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Inherent in all these methodologies, which measure either absolute Sr levels or strontium isotope ratios in mineralized tissue, is the assumption that diagenesis has not altered the signal since death. This has been a matter of some considerable debate (e.g., Nelson et al. 1986), but the consensus of current opinion amongst practitioners is that the repeated acid-washing procedures used remove any diagenetic mineral, because it has a higher... 

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