Strontium isotopic ratios are widely used as tracers in geological processes and as indicators of provenance in an archaeological context.
A significant fraction of the Earth’s 87Sr is not primordial, but is produced by the decay of 87Rb. Geochemically, Strontium follows Calcium, while Rubidium follows Potassium; thus, the geological variability of Sr isotope ratios in rocks is a function of both their age and chemical composition, particularly, their Ca/K ratios. Since these parameters can vary widely between different geological environments, and because different geological processes can fractionate rock chemistry in predictable ways, Sr isotopic ratios serve as tracers regarding the origins of igneous rocks. Also of interest to earth scientists is the Sr isotopic composition of seawater, which has varied through geologic time. In this case, variability is set up by the competing rates of sea-floor production, and weathering of continental landmasses. Marine organisms which produce calcium carbonate from seawater will inherit the Sr isotopic composition of the global ocean at that point in time and can be useful for dating sediment from the sea floor.
Archeologists use the isotope ratios of strontium to determine residential origins and migration patterns of ancestral humans. The human body incorporates Sr by way of diet. Since Sr isotope ratios in soils, rocks, and waters vary widely in nature, and are not appreciatively fractionated by biologic processes, the assumption is that the isotope values for strontium in bone and tooth enamel will reflect those in the portion of the biosphere in which an individual lived. Thus, strontium isotope composition provides links to the land where food was grown or grazed.
Depending on the needs of the researcher, our lab extracts Sr by acid digestion; we can digest materials completely, or we can leach readily available material via partial extraction. Sr is then purified by standard cation exchange methods using small-volume Eichrom Sr-specific resin columns, thus minimizing the amount of reagents used and minimizing the reagent contribution to blank levels. We use only high-purity reagents and all work is done in a controlled cleanroom environment.
All Sr isotope measurements are done on a nuPlasma II mass spectrometer fitted with 16 faraday cups and 5 ion counters. 87Sr/86Sr ratios are corrected for mass bias, the presence of Rb, and for isobaric interference of Kr impurities in the Ar gas. We run NBS 987 during all sample runs as a reference check.
We currently only offer solution analyses, using a CETAC dry desolvating system to increase sensitivity; we are in the process of acquiring a laser-ablation setup, and we will announce when this is up and running.