快速量測皮克級珊瑚骨骼稀土族元素及其應用

Abstract

本研究利用磁場式感應耦合電漿質譜儀(ICP-SF-MS)，建立一套測量珊瑚骨骼內稀土族元素之分析技術，其偵測極限可達皮克等級(picogram)。我們使用乾式霧化器導入系統，以增強訊號、改進偵測極限，和降低樣品需求量。本方法所量測的同位素包括46Ca、89Y、138Ba及各稀土族元素；稀土族元素和鈣的比值，是經由離子訊號強度比值(REE/Ca)直接計算而得。每四個樣品穿插一個基質成份相近的標準溶液，校正儀器的質量分化和比值飄移。本方法的之測量準確度可達到 0.4-1.4% ，標準溶液的長期重現性可達 0.8-1.6% (2σ)。 我們分析一顆於2005年，在南海西側、位於越南中部Son Tra島所採集到的珊瑚標本，發現其稀土族元素在1991年夏季至秋季，產生異常偏高值，鑭鈣比值(La/Ca)高達平常年的30-40倍；鐿鈣比值(Yb/Ca)為平常年的2-4倍。推論造成該年夏季南海海域珊瑚大規模死亡的原因，與當年6月15日鄰近的菲律賓皮那土波火山(Mt. Pinatubo)大爆發所帶來大量火山灰有關，造成珊瑚生態系統死亡。 利用磁場式感應耦合電漿質譜儀所建立之珊瑚骨骼內稀土族元素之分析技術，最大優點在於不需經過繁複之化學處理步驟，可直接量測出極低含量（皮克等級）之稀土族元素。本方法的標本需求量僅為 10 μg，不僅可以提供高解析度之稀土族元素分析，將來可進一步應用於不同環境樣本，例如海綿、石灰華、石筍等之分析上，做為研究古環境之變化之極佳代用指標(proxy)。

Direct measurement of picogram-quantity REE levels in coral samples on an inductively coupled plasma sector field mass spectrometer, ICP-SF-MS, Thermo Electron ELEMENT2, has been developed. A dry introduction system, Cetac ARIDUS, and an ESI micro-nebulizer with an uptake rate 80 μl/min were used to enhance sensitivity, improve detection limit and reduce sample size. A peak hopping mode on a single secondary electron multiplier was used to measure the ion beams of 46Ca, 89Y, 138Ba, 139La, 140Ce, 141Pr, 146Nd, 147Sm, 153Eu, 160Gd, 159Tb, 163Dy, 165Ho, 166Er, 169Tm, 172Yb, and 175Lu. REE/Ca ratios were calculated directly from intensities of ion beams, using external matrix-matched synthetic standards to correct for instrumental ratio drifting and mass discrimination. Every four samples were bracketed with one standard. Internal 2-sigma precision is 0.4-1.4% for all REE/Ca ratios. Replicate measurements made on standards and samples show external precisions of 0.8-1.6% (2σ) can be achieved. The developed was applied to determine REE records of a living coral Porites, collected from Son Tra island (16°12’59.4”N, 108°11’57.1”E), central Vietnam, western South China Sea (SCS) in 2005. The 1991 REE anomalies suggest that a massive mass coral mortality event in the summer and fall of 1991 in the SCS might be enhanced by the volcanic ash fallout following the eruption of Mount Pinatubo on June 15, 1991. This established technique can provide a direct measurement of carbonate REE isotopic composition in picogram quantities without chemical separation steps. The requirement of a small sample size of 10 μg carbonate offers high spatial resolution REE records not only for corals, but for other different natural slow-growth-rate carbonates, such as sclerosponges, tufa and speleothems.