样品量差异对团簇同位素Δ_(47)测定的影响

EFFECTS OF DIFFERENT SAMPLE SIZE ON THE REPRODUCIBILITY OF CLUMPED ISOTOPE(Δ_(47))MEASUREMENTS

团簇同位素是地球化学领域的新兴技术,已成功应用于古温度重建研究。由于团簇同位素的自然丰度极低,目前碳酸盐中团簇同位素Δ_(47)测定所需要的样品量仍较大(约10 mg),因此如何在保证Δ_(47)测定值可靠的前提下,使用更少量的样品进行测定一直是各实验室努力的目标。本研究在福建师范大学稳定同位素中心,以实验室高纯CO_(2)气体(Gas-lab)为测试对象,基于Mat 253 Plus同位素比质谱仪双路测定模式,通过不同气体量条件下的对比测定,探讨了双路模式下样品量的差异对测量结果的影响。研究结果显示:在可变体容积储样器体积为100%、70%、30%和15%的设定气体量条件下,每个测定过程中Gas-lab的Δ_(47)的测定值变化范围分别为-0.047‰~0.059‰、-0.054‰~0.039‰、-0.067‰~0.083‰和-0.065‰~0.058‰,当气体量较小时,测定的Gas-lab的Δ_(47)值更为分散。因气体同位素质谱仪在不同信号强度时测定的同位素值存在差异,在气体量较少条件下测量过程中样品气和参考气离子束信号降低幅度增大,从而影响了Δ_(47)测定,使其Δ_(47)测定值呈现为更加离散的特征。为了提高数据的精确性,本文建议在测定过程中确保可变体容积储样器的体积不低于总体积的30%。

Carbon dioxide clumped isotope thermometry(Δ_(47)),focused on the formation temperatures of carbonate minerals,provides a new tool for investigating paleotemperature.Unlike the classic oxygen isotope paleothermometer,Δ_(47)is based solely on the degree of heavy isotope beyond an expected random distribution in carbonates.It requires no assumptions about the δ^(18)O of water from which carbonates grew.However,a significant challenge for Δ_(47)is the difficulty of the measurement due to the extremely low natural abundance and subtle variation with the temperature of clumped isotopologues.The measures typically require multiple analyses to obtain good precision,and most methods are limited by sample weight requirements of ca.10 mg.Sample size limitations are especially important while records of paleotemperature with high-resolution are in high demand in paleoseasonality reconstructions.Therefore,how to reduce the sample size without losing the reliability of Δ_(47)during the measurements has always been a topic of significant interest in all laboratories today.Here,we describe the performance of Δ_(47)values with four gas volume size types,corresponding to different sample sizes.For this approach,the Δ_(47)of laboratory high-purity CO_(2) gas(Gas-lab)with different volume in the bellow of dual inlet system was tested.All measurements were run using Thermo Scientific Mat 253 Plus isotope ratio mass spectrometer,which was widely used in most laboratories for clumped isotope thermometry,at the Stable Isotope Center of Fujian Normal University.The result showed that the Δ_(47)values of Gas-lab range from -0.047‰ to 0.059‰,-0.054‰ to 0.039‰,-0.067‰ to 0.083‰ and -0.065‰ to 0.058‰ respectively,and the amplitude of variation of Δ_(47)of Gas-lab is 0.106‰,0.093‰,0.150‰ and 0.123‰ respectively under 100%,70%,30%and 15% gas volume.It shows that the measured Δ_(47)value of single acquisition typically has poor precision and was more dispersed when the sample was small gas size.We conclude that the difference in the gas volume size can affect the accuracy and precision of Δ_(47)values.The isotope ratio was usually affected by the ion beam intensity,and the dispersion of Δ_(47)values from small gas volume size is consistent with the intensity difference between sample gas and the reference gas.Our results showed that the affection on Δ_(47)values from the change of the ion beam intensity was significant and would not be ignored.In order to improve the stability of Δ_(47)values,we thus conclude that a more than 30% size of the total bellow volume in dual inlet system was recommended in the laboratory.