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

Abstract

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.