多偵測器感應耦合電漿質譜儀之精準鈾含量及同位素比值測量技術

Abstract

本研究利用多偵測器感應耦合電漿質譜儀發展了一套精準的鈾同位素比值及含量測量技術，精確度可達萬分比等級。我們使用四個法拉第杯同時測量鈾同位素, 233U, 234U, 235U, 及236U。利用233U-236U 雙示蹤劑有效校正質量分化效應以及計算鈾濃度。複雜之背景干擾，主要因不同238U 訊號、真空狀況及基質而改變外，亦235U 拖尾所影響，故同位素之背景值必須個別校正。若以簡單的指數行為描述背景頻譜，對233U, 234U 及236U的訊號強度，將造成萬分比至千分比的錯估。以進樣速率為每分鐘50 ml，測量鈾濃度2 ppm 約2-6 分鐘，僅需200-600 ng U 的標本量，δ234U 和鈾含量的測量精準度可達± 0.1-0.5‰ (2σ)。分析國際及實驗室之標準品，長時間之δ234U 及鈾含量測定重現性分別可達0.12‰及0.47‰，且與其他質譜術量測數值相吻合。重複分析國際標準品NBL-112A 和HU-1 的δ234U 平均結果分別為-37.10 ± 0.03‰ (2σm, n = 17) 和-0.17 ± 0.05‰ (2σm, n = 5)。自然界具有不同基質之各種珊瑚及洞穴石筍碳酸鹽樣品的δ234U 及鈾濃度測量值與其他千分比準度之質譜術所得數值一致。此分析技術將可應用至鈾系定年法，古氣候學，及岩石地球化學之相關研究。

Techniques used for precise and accurate determination of natural uranium isotopic ratios and concentration with sub-permil precision have been developed on a multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS), Thermo Electron NEPTUNE. Uranium isotopes, 233U, 234U, 235U, and 236U, were statically measured in 4 Faraday collectors. A 233U-236U double spike method was employed to correct mass bias and determine uranium concentration. The behavior of complicated spectral background is attributed to tailing characteristics of 235U and 238U ion beams as well as vacuum condition and matrix effect. Individual background correction should be applied to the measured isotopes, 233U, 234U, and 236U, or a bias of sub-permil to permil level could be caused by using a simple exponential function correction. By introducing 2 ppm uranium with a uptake rate of 50 ml/min for 2-6 minutes, a sample size of 200-600 ng U is needed to offer a within-run precision of 0.1-0.5‰ for d234U and uranium concentration. Measurements made on reference materials and in-house standards demonstrate that an external reproducibility of 0.12‰ can be achieved for d234U and 0.47‰ for uranium concentration. Repeated measurements of NBL-112A and HU-1 give averaged d234U values of -37.10 ± 0.03‰ (2σm, n = 17) and -0.17 ± 0.05‰ (2σm, n =5), respectively, within error of accepted values. MC-ICP-MS data of d234U and uranium concentration for different carbonates, including corals and speleothems with various matrices, are in agreement with those measured by other mass spectrometric methods with a permil-level precision. This technique can be applied to the exploration of the frontiers in diverse fields of Earth Sciences, such as U series chronology, paleoclimatology, and geochemistry.