High throughput Sr isotope analysis using an automated column chemistry system

A new method has been developed for rapidly measuring 87Sr/86Sr isotope ratios using an autosampler that automates column chemistry for Sr purification. The autosampler, a SC2 DX with FAST2 valve block, produced by Elemental Scientific, Inc., utilizes a pair of six-way valves, a vacuum, and a peristaltic pump to load a sample from an autosampler tube onto the Eichrom Sr Resin in the separation column. The autosampler then elutes the sample from the column directly into the spray chamber of the mass spectrometer. Measurements are made on a Thermo-Finnigan Neptune ICP-MS. Sample-blank pairs require approximately 30 minutes for analysis. Normal throughput for the system is 24 samples and 11 standards per day. Adjustment of the pump speed allows for rapid loading of the column followed by a 3-minute data acquisition period with no fractionation of the Sr being eluted from the column. All data are blank-, interference-, and normalization-corrected online using 86Sr/88Sr = 0.1194. Analytical precision on a typical 66 ng/g analysis is ±0.00003 (2σ SE). Reproducibility of the SRM987 Sr standard (66 ng/g) over the course of a typical sequence is ±0.00004 (2σ SD, n=11). For comparison, offline column separation of the SRM987 Sr standard (66 ng/g) was conducted and measured using the same instrument method, yielding a reproducibility of ±0.00004 (2σ SD, n=7). The long-term average of the SRM987 standard (10-200 ng/g) utilizing the online column chemistry method is 0.71027 ± 0.00010 (2σ SD, n=239). A small memory effect has been measured by alternating spiked samples (87Sr/86Sr = 0.67465) with the SRM987 standard. The bias measured in this test (87Sr/86Sr +0.00006) slightly exceeds the 2σ standard reproducibility for a typical run with sample and standard concentrations near 66 ng/g, but is within the 2σ long-term reproducibility of the method. The optimal concentration range for the offline column chemistry system is 50-250 ng/g Sr. Sample concentrations above 250 ng/g Sr must be diluted to avoid the risk of overloading the detectors on the instrument. Samples with concentrations below 50 ng/g can be run, but measurement precision and accuracy decreases greatly at lower concentrations. Typical river and lake waters do not require dilution and can be analyzed following filtering and acidification. This new online column chemistry method measures 87Sr/86Sr isotope ratios with an accuracy and precision similar to that of offline column chemistry methods at a rate exceeding 100 samples per week. This method is ideally suited for Sr tracer and provenance applications, but it cannot currently be applied to geochronology or 88Sr/86Sr stable isotope problems. Presently the method is being applied to determine the spatial distribution of 87Sr/86Sr isotope ratios for a global terrestrial surface water dataset.