海水U、Th长寿命核素的高精密度MC-ICP-MS测定方法

Abstract

建立了适合海水u、TH同位素高精密度测量要求的MC-ICP-MS方法.确定了采用Mg(OH)2共沉淀法富集海水u、TH同位素时的最佳nH3·H2O浓度为0.14 MOl/dM3.采用CrM-145 u标准溶液,经质量分馏、电子倍增器增益、丰度灵敏度、仪器及试剂本底和记忆效应等校正后,历时3个月得235u/238u和234u/238u的准确度分别优于0.05%和0.08%,精密度分别优于0.12%和0.10%.将该方法应用于台湾海峡b9站位3份平行水样中u、TH同位素含量及比值的测定,得238u平均含量为3.05±0.05μg/kg,234u/238u平均值为(6.28±0.11)x10-5,u含量及其同位素比值的标准偏差均小于3%;232TH的平均含量为(1.66±0.04)x103Pg/kg,232TH/230TH平均值为(1.95±0.01)x105,TH含量及其同位素比值的标准偏差均小于2%.测定结果与海区的水文学条件及物质来源相吻合,证明了方法的可靠性.

A high precision method has been developed for analyzing uranium and thorium in seawater by MC-ICPMS in this study.The optimal ratio of NH3·H2O with acidified seawater of p H = 2( 0.14 mol / dm3) is quantified for Mg( OH)2coprecipitation protocol.The instrument and solvent background,abundance sensitivity,memory effects,mass fractionation and IC gains are properly corrected using CRM-145 U standard.Overall,the accuracy of235 U /238 U and234U /238 U are better than 0.05% and 0.08% with precisions better than 0.12% and 0.10%,respectively,in about 3 month measurements of CRM-145 U standard.This method has been successfully applied to analyze triplicate surface seawater samples from the Taiwan Strait.The average concentrations of238 U and232Th are3.05 ± 0.05μg / kg and( 1.66 ± 0.04) × 103 pg / kg,respectively,while the average ratios of234 U /238 U and232Th /230 Th are( 6.28 ± 0.11) × 10- 5and( 1.95 ± 0.01) × 105,respectively.The standard deviations are less than 3% for U isotopes and 2% for Th isotopes.The results are consistent with hydrological characteristics and isotopic provenance in the study area,which support the reliability of the method.