Thai T. Phan

High throughput method for Sr extraction from variable matrix waters and 87Sr/86Sr isotope analysis by MC-ICP-MS

Natural isotope tracers, such as strontium (Sr), can facilitate the tracking of brine migration caused by CO2 injection in carbon storage sites and assist in identifying the origin of formation waters associated with oil and gas exploration. However, it might be necessary to analyze tens of samples with complex chemical compositions over a short period to identify subsurface reactions and respond to unexpected fluid movement in the host formation. These conditions require streamlined Sr separation chemistry for samples ranging from pristine groundwaters to those containing high total dissolved solids, followed by rapid measurement of isotope ratios with high analytical precision. Here we describe a method useful for the separation of Sr from energy related geofluids and the rapid measurements of Sr isotopic ratios by MC-ICP-MS. Existing vacuum-assisted Sr separation procedures were modified by using inexpensive disposable parts that also eliminate cross contamination. These improvements will allow an operator to independently prepare samples for Sr isotope analysis using fast, low cost separation procedures and commercially available components. We optimized the elution chemistry by adjusting acid normality and elution rates to provide better separation of Sr from problematic matrices (e.g. Rb, Ca, Ba, K) associated with oilfield brines and formation waters. The separation procedure is designed for high sample throughputs that are ready for immediate Sr isotope measurements by MC-ICP-MS. Precise Sr isotope results can be achieved by MC-ICP-MS with a throughput of 4 to 5 samples per hour. Fluids from a range of geologic environments analyzed by this method yielded results within the analytical uncertainty of 87Sr/86Sr ratios previously determined by standard column separation and TIMS. This method provides a fast and effective way to use isolate Sr in a variety of geologic fluids for isotopic analysis by MC-ICP-MS.

Direct determination (without chromatographic separation) of lithium isotopes in saline fluids using MC-ICP-MS: establishing limits on water chemistry

Produced waters from petroleum and geothermal reservoirs contain large amounts of cations including lithium, and while lithium isotopes are time-intensive to measure in the traditional way, they have the potential to reveal information about fluid and solute origins and diagenesis. We tested the effect of added cations that dominate in produced waters (Na, Ca, Mg) on accuracy and precision of lithium isotope measurements by MC-ICP-MS in Li-isotope standard solutions without chromatographic separation. Repeated measurement of Li-isotope standards with no added matrix demonstrate high 2 SD reproducibility: LSVEC (RM8545) had δ7Li of 0.04 ± 0.74‰ (n = 19); IRMM16 had δ7Li of 0.07 ± 1.2‰ (n = 11); Li7N had δ7Li of 30.07 ± 0.12‰ (n = 11); Li6N had δ7Li of −8.04 ± 0.58‰ (n = 8). Replicates of standards with matrix and of diluted produced waters had 2 SD reproducibility smaller than ±1.8‰ and ±2.0‰, respectively. Results showed that Na/Li (weight ratio) up to about 500 (analysed aliquot <20 mg kg−1 Na) and Ca/Li or Mg/Li up to about 250 (analysed aliquot <10 mg kg−1 Ca or Mg) do not diminish δ7Li accuracy or precision, so long as Li concentration in the measured solution is greater than 20 μg kg−1. Because produced waters are chemically more complex than the added-matrix isotope standards we tested initially, we also compared δ7Li in produced waters from which the Li was chromatographically separated, as is the convention, with diluted produced waters. Results show marked Li isotope fractionation occurred in those samples in which Na/Li ranged from 500 to almost 10 000. Geothermal waters and Na–Ca–Cl-type produced waters, therefore, are fluids that are most likely to have cation/Li ratios that will make them amenable to direct determination of δ7Li with only sample dilution. We also found that two aliquots of the Li-isotope standard, LSVEC (RM8545), one obtained in 1985 and one in 2012, have δ7Li that differs by about 2‰.