硅藻氧同位素比值测定的分步氟化处理方法

Stepwise Fluorination Method for Determination of Oxygen Isotopic Ratio of Diatoms

利用硅酸盐氧同位素制备系统,以BrF5为氟化剂,对硅藻试样和石英标样进行精细的分步氟化研究,揭示硅藻壳体不同层位氧同位素的变化特征。采用逐步氟化实验,确定了四龙湾玛珥湖沉积硅藻样品在去除25%～28%的外层氧后再完全氟化,IRMS(质谱仪)测定的δ18O集中度最高。将氟化流程简化为两步,对应的δ18O差别为0.3‰,测量精度优于±0.5‰。本研究确定了硅藻样品分步氟化方法:第一步,引入0.392 aK mL(a,样品质量(mg);K=0.18)的BrF5(25℃)气体,在550℃下反应1 h;第二步,系统恢复真空后,引入理论量5倍的BrF5,在550℃下反应3 h,产出的氧转化为CO2进行质谱测定,获得硅藻δ18O特征值。

Determination of the δ18O value of diatom can obtain the initial information of its oxygen isotopic composition,and also has favorable application prospects in paleoenvironment,paleontology,petroleum exploration and environmental prediction.We carried out eight-step fluorination experiments with BrF5 reagent on diatom samples and quartz standard to reveal variation of oxygen isotopic composition in different steps.The eight-step fluorination determined the proportion of surface oxygen to be 25%-28%.The δ18O values obtained from the above processes were all around（35.34±0.15）‰.Then we simplified the eight-step fluorination to two-step fluorination.First,we removed the surface oxygen of specific sedimentary diatoms,and then totally fluoridated the samples.The difference of δ18O values from eight-step and two-step fluorination experiments on the same samples was only 0.3‰.The accuracy of measurement has reached or preceded the repeated measurement error of ±0.5‰ in the international studies.On the basis of many conditional experiments,the stepwise fluorination methods on the diatoms were established.For diatom samples with weight of a mg,the 0.392 aK mL（K=0.18） BrF5 were loaded in the first step,and heated to 550 ℃ for 1 h;and then five times of theoretical quantity of BrF5 were loaded in the second step after the system regain vacuum,and heated to 550 ℃ for 3 h.The released oxygen were converted to CO2,and then analysed by MAT-252 to obtain δ18O value of diatom samples.