Geological-Earth-Based Thermal Ionization Mass Spectrometry for Direct Determination of Sr Isotope in Natural Waters

The Sr isotope is an important tracer in environmental science and hydrogeochemistry. It is helpful to understand the regional hydrogeochemistry, the weathering rate of rock in the basin, and the groundwater by measuring the Sr isotope ratio (87Sr/86Sr) in different water reservoirs. Water-rock interactions and other important geochemical processes, therefore Sr isotope has a wide range of application prospects in the above research areas. Thermal ionization mass spectrometer (TIMS) is the most accurate technique for Sr isotope analysis. However, the TIMS measurement requires high purity of the sample. In order to overcome matrix element and allotopic interference, traditional techniques require the use of cationic resin (AG50W or The Sr Spec) exchange technique separates the high-purity Sr component from the water sample and tests it. The tedious sample preparation requires a large amount of experimental resources (resin, reagents, manual operation) and preparation time, which greatly restricts the wide application of this method.

Li Chaofeng, a senior engineer at the Solid Isotope Laboratory of the Institute of Geology and Geophysics, Chinese Academy of Sciences, and his collaborators have established a TIMS method for the direct determination of Sr isotope ratios in water samples through a large number of experimental conditions. This method requires no sample preparation and requires only simple sample concentration. Can be tested, significantly improving the efficiency of the experiment and reducing the cost of the experiment. This method needs to grasp three key technical points: 1. Strictly control the amount of sample (2 ~ 4 ng Sr) to reduce the inhibition of the Sr+ signal by the matrix element; 2. Due to the small sample size, the high sensitivity of 1012 ohms with high sensitivity of the assembly is adopted. The Faraday cup of the amplifier receives the 87Sr and 86Sr signals. 3. The Re-band is used as the filament material to obtain the best Sr+ sensitivity while significantly suppressing the emission of trace Rb+ in the water sample, eliminating the potential 87Rb homotopy pair 87Sr interference. In order to verify the applicability and accuracy of the method, they also studied and analyzed different types of natural water samples (river, lake, drinking water, rainwater, and snow water) with large changes in matrix composition and Sr element concentration, which were mostly obtained. Good analytical accuracy (0.003 to 0.005%, 2 RSE), but water samples with a high Ca/Sr ratio (≥700), can not be satisfactorily tested due to Ca's strong suppression of Sr signal.

This method was recently published in the Analytical Chemistry (Li et al. Direct High-Precision Measurements of the Sr-87/Sr-86 Isotope Ratio in Natural Water without Chemical Separation Using Thermal Ionization Mass Spectrometry Equipped with 10(12) Omega Resistors. Analytical Chemistry, 2015, 87: 7426-7432).

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