在鈾的放射性序列（U-decay series）中的鈾同位素值與含量已被廣泛地應用在地球科學的相關研究中，其分析測量技術已有許多發展，例如使用α粒子計數儀（α Spectrometer）、熱游離質譜儀儀（Thermal Ionization Mass Spectrometer： TIMS）與高解析度感應耦合電漿質譜儀（High Resolution Inductively Coupled Plasma Mass Spectrometer： HR-ICPMS），而本論文工作乃著力於利用造價較TIMS與HR-ICPMS為“平民化”的四極桿式感應式耦合電漿質譜儀（Quadrupole Inductively Coupled Plasma Mass Spectrometer：Q-ICPMS）的分析技術及數據處理，以期求得能如同TIMS與HR-ICPMS相近之高精密度與高靈敏度的鈾同位素值與鈾濃度之測量。而此方法的突破將可達到快速、大量標本處理且高精密度及靈敏度的鈾同位素與濃度分析的優點，使鈾同位素及含量在地球科學研究主題及自然環境標本之應用上更臻寬廣。
本論文工作為達上述的目標，所採用之策略為（一）以微濃縮霧化進樣系統（micro-concentric nebulization introduction system）取代質譜儀之霧化器（nebulizer），以提高儀器靈敏度（sensitivity）並降低標本量；（二）運用測量值之標準偏差法過濾雜訊（standard deviation filter）提高測量精確度；（三）使用233U-236U雙示蹤劑標準添加法（double spike standard addition）作質量分化校正，並採用標準樣品穿插校正法（standard bracketing method）消除比值飄移現象（ratio drifting），以確立測量精確度。
利用一標準樣品NBL 112A（New Brunswick Laboratory 112A）依上列之分析策略進行分析測試，結果可知儀器靈敏度（偵測到離子數/進樣之總鈾量）可達0.4，訊號強度可提高60倍；因此單一樣品之測量僅需標本量微至30 ng的鈾，同時，測量時間僅需15到20分鐘。除NBL 112A外，本研究亦採用另外兩種標準樣品HU1（Harwell Uranite）及石筍碳酸鈣CAVE-1共同監視測量精密度與準確度。結果顯示，所有標準樣品中234U（表示鈾同位素值）之單一測量精密度（within-run precision，±2 R.S.D.）小於3.3，而[238U] （表示鈾濃度）之單一測量精密度為3.1。長期測量234U的精密度（between-run precision，±2 R.S.D.）可小於4.1，至於[238U]之長期測量精密度則小於2.4。將測得之標準樣品234U值施行” t測試”，結果發現本研究測量值於95%可信度區間內（confidence level）與標準樣品之公認值（112A、HU1與CAVE-1的234U組成分別為-36.94、0 與2453）在統計上無顯著差異。
將上列的分析測量策略應用於測量台灣南灣之海水、秘魯之石筍與蘇門達臘之珊瑚標本等的測量分析上，結果顯示當鈾標本量大於30 ng時，234U之平均單一測量精密度約為5，[238U] 之平均單一測量精密度約為5.5。將這些天然樣品234U測量值執行“paired t test”，發現與HR-ICPMS之測值於統計上並無顯著差異，顯示本論文工作所致力突破的Q-ICPMS進行的鈾同位素值與濃度的測量在自然環境上的標本測量準確度已達一般HR-ICPMS之水準。
綜合言之，於本論文工作所提之策略下，四極桿式感應式耦合電漿質譜儀在量測標本中鈾濃度與鈾同位素值時，所需之鈾標本量僅30 ng，且測量精密度可達到千分等級（permil level），已達到高靈敏度與高精密度的測量目標。此分析策略之成功，對於應用於古海洋的研究課題上將可提供一個快速、低標本量、高精準度的測鈾方法；此外，未來再將此技術與策略應用於釷同位素值的測量，將可使鈾釷同位素在古海洋的研究上有更寬廣的應用。

Uranium isotopic composition and concentration have been widely applied to various fields in the earth sciences. This work focused on the improvement of analytical technique for uranium isotopic ratio and concentration measurements by Quadrupole Inductively Coupled Plasma Mass Spectrometer (Q-ICPMS). To approach the goal of permil-level analytical precision with only ng-U sample sizes, three strategies were applied. First, the Q-ICPMS was connected to a micro-concentric nebulization (MCN) introduction system to amplify sensitivity and to reduce required sample size. Standard deviation filters were then used to remove outliers to improve precision and accuracy. Finally, the 233U-236U double spikes were added and then data were processed with the standard bracketing method to correct mass fractionation and ratio drifting and, therefore, higher precision and accuracy could be achieved.
The results showed that the overall sensitivity (ion detected/ atoms introduced) was about 0.3~0.4. Oxide and hydride levels are less than 0.3% and 2 ppm, respectively. Under measurement time of 15-20 minutes, U standards with 30 ng of U give within-run precisions better than 3.3 (±2 R.S.D.) for 234U and better than 3.1 (±2 R.S.D.) for [238U]. Replicate measurements made on standards reveal that between-run precision of 4.1 (±2 R.S.D.) for 234U and 2.4 (±2 R.S.D.) for [238U] can be achieved. Measurements of U isotopic ratios and concentrations in coral, sea water, and speleothem samples show averaged 234U and [238U] within-run precisions of 5 and 5.5, respectively. All measurements made by this work reveal no significant difference from the values measured by High Resolution ICPMS (Li et al., 2005; X. Wang unpublished data) in the 95% confidence interval.

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