| Summary: | 碩士 === 國立臺灣大學 === 地質科學研究所 === 96 === Diverse isotopic ratios of Pb have widely been used as geochemical tracers in the Earth Sciences, such as geochemistry, paleoceanography and chronology. We had developed a chromatographic separation technique with a cation exchange resin, Sr-Spec, to purify Pb from different environmental samples, including water, carbonate, terrestrial clay minerals, and igneous rock. The procedural blank is as low as 10-20 pg of Pb and the chemistry yield is higher than 90 %. A multi-collector ICP-MS (MC-ICP-MS), Thermo Electron Neptune, equipped with a high efficiency desolvation nebulization system, Cetac Aridus, was used to measure Pb isotopic compositions. An X-skimmer cone was used to enhance the instrumental transmission 1.5-2 times. With a sample uptake rate of 40 μL/min, an ion beam intensity of 208Pb > 1 volt can be generated for 5-10 ng/ml of Pb. A Faraday-cup protocol in static mode and an exponential-law mass bias normalization method to a 205Tl/203Tl value of 2.3889 were used. Isobaric interference of 204Hg on 204Pb was evaluated by monitoring ion beam intensity of 202Hg. Data acquisition time is 7-10 minutes, providing a higher sample throughput (2-3 samples per hour) than TIMS techniques. Measurements made on international reference material show (1) the 2-sigma long-term reproducibility of 200 ppm for 207Pb/206Pb and 208Pb/206Pb with 2-10 ng of Pb and (2) the accuracy is within error of accepted value. The key advantage of this established technique is to offer a possibility of analyzing Pb isotopic composition in nanogram quantity, mainly for sample with limited Pb content. This technique has been applied to diverse environmental samples, including soil, water, rice and coral.
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