多頻道感應耦合電漿質譜儀之高精度奈克級鉛同位素分析及關渡平原案例探討

Abstract

自然界中鉛同位素組成的差異可作為其來源的地球化學示蹤劑並被廣泛應用於地球科學研究，包含岩石學、古海洋學、定年學等。本研究建立了一套包含離子交換樹酯分離純化及運用多頻道感應偶合電漿質譜儀進行高精度鉛同位素組成量測的分析方法。新的離子交換樹酯分離方法可從各種樣本中淬取出90%以上的鉛，化學空白值僅10-20 pg。利用乾式進樣系統Aridus與高效率銳角錐提高多頻道感應偶合電漿質譜儀的靈敏度並使用鉈同位素 (203Tl-205Tl) 校正質量分化效應，使用八個法拉第杯同時接收鉛、鉈、汞同位素進行質量分化校正與干擾修正。在樣本量僅有2-10 ng的條件下，仍可達成同位素比例測量精準度100-300 ppm (2σ)的高精度分析。重複分析國際標準品SRM 981之長期再現性可達 ± 200 ppm， SRM 981、SRM982、USGS AGV-2 和BCR-2之測量值與國際其他質譜術量測數值相吻合。本技術之優勢在於低樣本量的高精度分析能力，可在樣本鉛含量有限的情況下得到高精度同位素比值，並適用於不同環境樣本。目前此分析技術已開始應用於土壤、河水與珊瑚等環境標本。

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.