离子交换过程中锂同位素分馏对锂同位素测试准确度的影响
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摘要
锂同位素被广泛应用于地球与行星科学各个领域,准确测定锂同位素比值是示踪各种自然过程的前提,但目前国际实验室报道的锂同位素标准物质测定值存在较大偏差,例如已报道的海水δ7Li测试值相差5‰。针对这一现状,本文基于离子交换理论基础,使用正态分布函数拟合淋出曲线,通过理论计算得到离子交换纯化过程造成的锂同位素分馏的理论值,该数值与MC-ICP-MS检测无关,但对锂同位素测试准确度有直接的影响。在此基础上,定义相对回收率(Rc)用于监测锂同位素分馏。基于本实验室分离纯化流程,通过理论计算得出,当Rc> 99. 8%时,可认为离子交换纯化过程中没有引起可观察到的锂同位素分馏,进而不影响MC-ICP-MS检测准确度。目前世界上各实验室主要通过绝对回收率或Rc来判断分离过程中是否发生同位素分馏。由于测试的空间电荷效应,绝对回收率易被高估,而> 99%的Rc并未全部达到理论计算得到的Rc,表明各实验室对同种标准物质测试结果的偏差极可能是由于离子交换纯化过程中锂同位素分馏导致的。本文提出,对于每一样品,只需要分别测量离子交换过程中接收区间及其前后一定区间溶液中锂含量,将得到的Rc值与其理论值比较,即可判断分离纯化过程中是否引起可观察到的锂同位素分馏。
BACKGROUND: Lithium isotopes are widely used in various fields of earth and planetary science. Accurate determination of the lithium isotopic ratio is the premise of tracing various natural processes. However,there are large deviations in the measured values of lithium isotopic reference materials reported by international laboratories.For example,the reported seawater δ7 Li test value differs by 5‰. To avoid this fractionation,laboratories employ their own empirical recovery values obtained from recovery tests rather than a theoretical one.OBJECTIVES: To investigate the reasons of this huge discrepancy and get an instructively theoretical value,so as to appraise Li fractionation during ion exchange and purification processes.METHODS: Based on the theory of ion exchange,this study uses the normal distribution function to fit the leaching curve,and theoretically calculates the theoretical value of lithium isotope fractionation caused by the ion exchange purification process,which is independent of MC-ICP-MS detection. However,it has a direct impact on the accuracy of lithium isotope testing. On this basis,the relative recovery is defined to monitor lithium isotope fractionation.RESULTS: The fitting calculations show that only when recovery is higher than 99. 8%,there would be no observable Li isotope fractionation caused by ion exchange and purification processes. At present,laboratories in the world mainly judge whether or not isotope fractionation occurs in the separation process by absolute recovery or relative recovery. Due to the space charge effect of the test,the absolute recovery is easily overestimated,and> 99% of relative recovery does not reach the theoretically calculated one,indicating that the deviation of the test results of the same reference material in each laboratory is most likely due to ion exchange.CONCLUSIONS: It is proposed that for each sample,it is only necessary to separately measure the lithium content in the receiving interval and the intervals before and after the ion exchange processes,and the obtained relative recovery value is compared with the theoretical value to determine whether observable lithium isotopic fractionation occurs during the separation and purification processes.
BACKGROUND: Lithium isotopes are widely used in various fields of earth and planetary science. Accurate determination of the lithium isotopic ratio is the premise of tracing various natural processes. However,there are large deviations in the measured values of lithium isotopic reference materials reported by international laboratories.For example,the reported seawater δ7 Li test value differs by 5‰. To avoid this fractionation,laboratories employ their own empirical recovery values obtained from recovery tests rather than a theoretical one.OBJECTIVES: To investigate the reasons of this huge discrepancy and get an instructively theoretical value,so as to appraise Li fractionation during ion exchange and purification processes.METHODS: Based on the theory of ion exchange,this study uses the normal distribution function to fit the leaching curve,and theoretically calculates the theoretical value of lithium isotope fractionation caused by the ion exchange purification process,which is independent of MC-ICP-MS detection. However,it has a direct impact on the accuracy of lithium isotope testing. On this basis,the relative recovery is defined to monitor lithium isotope fractionation.RESULTS: The fitting calculations show that only when recovery is higher than 99. 8%,there would be no observable Li isotope fractionation caused by ion exchange and purification processes. At present,laboratories in the world mainly judge whether or not isotope fractionation occurs in the separation process by absolute recovery or relative recovery. Due to the space charge effect of the test,the absolute recovery is easily overestimated,and> 99% of relative recovery does not reach the theoretically calculated one,indicating that the deviation of the test results of the same reference material in each laboratory is most likely due to ion exchange.CONCLUSIONS: It is proposed that for each sample,it is only necessary to separately measure the lithium content in the receiving interval and the intervals before and after the ion exchange processes,and the obtained relative recovery value is compared with the theoretical value to determine whether observable lithium isotopic fractionation occurs during the separation and purification processes.
引文
[1] Meija J,Coplen T B,Berglund M,et al. Atomic weights of the elements 2013(IUPAC Technical Report)[J]. Pure and Applied Chemistry,2016,88(3):265-291.
[2] Meija J,Coplen T B,Berglund M,et al. Isotopic compositions of the elements 2013(IUPAC Technical Report)[J]. Pure and Applied Chemistry,2016,88(3):293-306.
[3]苟龙飞,金章东,贺茂勇.锂同位素示踪大陆风化:进展与挑战[J].地球环境学报,2017,8(2):89-102.Gou L F,Jin Z D,He M Y. Using lithium isotopes traces continental weathering:Progresses and challenges[J].Journal of Earth Environment,2017,8(2):89-102.
[4] Tomascak P B. Developments in the Understanding and Application of Lithium Isotopes in the Earth and Planetary Sciences[M]//Johnson C M,Beard B L,Albarède F. Geochemistry of Non-traditional Stable Isotopes:Reviews in Mineralogy and Geochemistry.Washington D C:Mineralogical Society of America,2004,55:153-195.
[5] Magna T,Day J M D,Mezger K,et al. Lithium isotope constraints on crust-mantle interactions and surface processes on Mars[J]. Geochimica et Cosmochimica Acta,2015,162:46-65.
[6] Pogge von Strandmann P A E,Vaks A,Bar-Matthews M,et al. Lithium isotopes in speleothems:Temperaturecontrolled variation in silicate weathering during glacial cycles[J]. Earth and Planetary Science Letters,2017,469:64-74.
[7] Ionov D A,Doucet L S,Pogge von Strandmann P A E,et al. Links between deformation,chemical enrichments and Li-isotope compositions in the lithospheric mantle of the Central Siberian Craton[J]. Chemical Geology,2017,475:105-121.
[8] Pogge von Strandmann P A E,Frings P J,Murphy M J.Lithium isotope behaviour during weathering in the Ganges Alluvial Plain[J]. Geochimica et Cosmochimica Acta,2017,198:17-31.
[9] Chan L H. Lithium isotope analysis by thermal ionization mass spectrometry of lithium tetraborate[J]. Analytical Chemistry,1987,59(22):2662-2665.
[10] Lin J,Liu Y,Hu Z,et al. Accurate determination of lithium isotope ratios by MC-ICP-MS without strict matrixmatching by using a novel washing method[J]. Journal of Analytical Atomic Spectrometry, 2016, 31(2):390-397.
[11]苟龙飞,金章东,邓丽,等.高效分离Li及其同位素的MC-ICP-MS精确测定[J].地球化学,2017,46(6):528-537.Gou L F,Jin Z D,Deng L,et al. Efficient purification for Li and high-precision and accuracy determination of Li isotopic compositions by MC-ICP-MS[J]. Geochimica,2017,46(6):528-537.
[12] Gou L F,Jin Z D,Deng L,et al. Effects of different cone combinations on accurate and precise determination of Li isotopic composition by MC-ICP-MS[J]. Spectrochimica Acta Part B,2018,146:1-8.
[13] Tomascak P B,Carlson R W,Shirey S B. Accurate and precise determination of Li isotopic compositions by multi-collector sector ICP-MS[J]. Chemical Geology,1999,158:145-154.
[14] Choi M S,Ryu J S,Park H Y,et al. Precise determination of the lithium isotope ratio in geological samples using MC-ICP-MS with cool plasma[J]. Journal of Analytical Atomic Spectrometry,2013,28:505-509.
[15]赵悦,侯可军,田世洪,等.常用锂同位素地质标准物质的多接收器电感耦合等离子体质谱分析研究[J].岩矿测试,2015,34(1):28-39.Zhao Y,Hou K J,Tian S H,et al. Study on measurement of lithium isotopic composition for common standard reference materials using multi-collector inductively coupled plasma-mass spectrometry[J]. Rock and Mineral Analysis,2015,34(1):28-39.
[16] Nishio Y,Nakai S. Accurate and precise lithium isotopic determinations of igneous rock samples using multicollector inductively coupled plasma mass spectrometry[J]. Analytica Chimica Acta,2002,456:271-281.
[17] Pistiner J S,Henderson G M. Lithium-isotope fractionation during continental weathering processes[J]. Earth and Planetary Science Letters,2003,214:327-339.
[18] Bryant C J,McCulloch M T,Bennett V C. Impact of matrix effects on the accurate measurement of Li isotope ratios by inductively coupled plasma mass spectrometry(MC-ICP-MS)under ‘cold’plasma conditions[J].Journal of Analytical Atomic Spectrometry,2003,18:734-737.
[19] Jeffcoate A B,Elliott T,Thomas A,et al. Precise,small sample size determinations of lithium isotopic compositions of geological reference materials and modern seawater by MC-ICP-MS[J]. Geostandards and Geoanalytical Research,2004,28(1):161-172.
[20] Millot R,Guerrot C,Vigier N. Accurate and high-precision measurement of lithium isotopes in two reference materials by MC-ICP-MS[J]. Geostandards and Geoanalytical Research,2004,28(1):153-159.
[21] Bouman C,Elliott T,Vroon P Z. Lithium inputs to subduction zones[J]. Chemical Geology,2004,212:59-79.
[22] Romer R L,Heinrich W,Schr9der-Smeibidl B,et al. Elemental dispersion and stable isotope fractionation during reactive fluid-flow and fluid immiscibility in the Bufa del Diente aureole,NE-Mexico:Evidence from radiographies and Li,B,Sr,Nd,and Pb isotope systematics[J].Contributions to Mineralogy and Petrology,2005,149(4):400-429.
[23] Wunder B,Meixner A,Romer R L,et al. Temperaturedependent isotopic fractionation of lithium between clinopyroxene and high-pressure hydrous fluids[J].Contributions to Mineralogy and Petrology,2006,151:112-120.
[24]汪齐连,赵志琦,刘丛强,等.天然样品中锂的分离及其同位素比值的测定[J].分析化学,2006,34(6):764-768.Wang Q L,Zhao Z Q,Liu C Q,et al. Separation and isotopic determination of lithium in natural samples[J].Chinese Journal of Analytical Chemistry,2006,34(6):764-768.
[25] Rosner M,Ball L,Peucker-Ehrenbrink B,et al. A simplified,accurate and fast method for lithium isotope analysis of rocks and fluids,andδ7Li values of seawater and rock reference materials[J]. Geostandards and Geoanalytical Research,2007,31(2):77-88.
[26] Huang K F,You C F,Liu Y H,et al. Low-memory,small sample size,accurate and high-precision determinations of lithium isotopic ratios in natural materials by MC-ICPMS[J]. Journal of Analytical Atomic Spectrometry,2010,25:1019-1024.
[27]万红琼,孙贺,刘海洋,等.俯冲带Li同位素地球化学:回顾与展望[J].地学前缘,2015,22(5):29-43.Wang H Q,Sun H,Liu H Y,et al. Lithium isotopic geochemistry in subduction zones:Retrospects and prospects[J]. Earth Science Frontiers,2015,22(5):29-43.
[28]张群,秦礼萍.双稀释剂计算及校正方法[J].地球化学,2017,46(1):15-21.Zhang Q,Qin L P. The calculation and calibration of double-spike technique[J]. Geochimica,2017,46(1):15-21.
[29] Ko2ler J,Kucˇera M,Sylvester P. Precise measurement of Li isotopes in planktonic foraminiferal tests by quadrupole ICPMS[J]. Chemical Geology,2001,181:169-179.
[30] Magna T,Wiechert U H,Halliday A N. Low-blank isotope ratio measurement of small samples of lithium using multiple-collector ICPMS[J]. International Journal of Mass Spectrometry,2004,239(1):67-76.
[31] James R H,Palmer M R. The lithium isotope composition of international rock standards[J]. Chemical Geology,2000,166:319-326.
[32] Oi T,Odagiri T,Nomura M. Extraction of lithium from GSJ rock reference samples and determination of their lithium isotopic compositions[J]. Analytica Chimica Acta,1997,340:221-225.
[33] Schuessler J A,Schoenberg R,Sigmarsson O. Iron and lithium isotope systematics of the Hekla volcano,Iceland—Evidence for Fe isotope fractionation during magma differentiation[J]. Chemical Geology,2009,258:78-91.
[34]唐索寒,朱祥坤,蔡俊军,等.用于多接收器等离子体质谱铜铁锌同位素测定的离子交换分离方法[J].岩矿测试,2006,25(1):5-8.Tang S H,Zhu X K,Cai J J,et al. Chromatographic separation of Cu,Fe and Zn using AG MP-1 anion exchange resin for isotope determination by MC-ICPMS[J]. Rock and Mineral Analysis,2006,25(1):5-8.
[35] Seitz H M,Brey G P,Lahaye Y,et al. Lithium isotopic signatures of peridotite xenoliths and isotopic fractionation at high temperature between olivine and pyroxenes[J]. Chemical Geology,2004,212:163-177.
[36] Miller J M. Chromatography:Concepts and Contrasts[M].Hoboken:Wiley,2004:1-520.
[37] Tomascak P B,Magna T,Dohmen R. Methodology of Lithium Analytical Chemistry and Isotopic Measurements[M]//Advances in Lithium Isotope Geochemistry.Switzerland:Springer International Publishing,2016:5-18.
[38] Chan L H,Lassiter J C,Hauri E H,et al. Lithium isotope systematics of lavas from the Cook-Austral islands:Constraints on the origin of HIMU mantle[J]. Earth and Planetary Science Letters,2009,277:433-442.
[39] Houk R S,Schoer J K,Crain J S. Plasma potential measurements for inductively coupled plasma mass spectrometry with a centre-tapped load coil[J]. Journal of Analytical Atomic Spectrometry,1987,2:283-286.
[40]刘晔,柳小明,胡兆初,等. ICP-MS测定地质样品中37个元素的准确度和长期稳定性分析[J].岩石学报,2007,23(5):1203-1210.Liu Y,Liu X M,Hu Z C,et al. Evaluation of accuracy and long term stability of determination of 37 trace elements in geological samples by ICP-MS[J]. Acta Petrologica Sinica,2007,23(5):1203-1210.
[2] Meija J,Coplen T B,Berglund M,et al. Isotopic compositions of the elements 2013(IUPAC Technical Report)[J]. Pure and Applied Chemistry,2016,88(3):293-306.
[3]苟龙飞,金章东,贺茂勇.锂同位素示踪大陆风化:进展与挑战[J].地球环境学报,2017,8(2):89-102.Gou L F,Jin Z D,He M Y. Using lithium isotopes traces continental weathering:Progresses and challenges[J].Journal of Earth Environment,2017,8(2):89-102.
[4] Tomascak P B. Developments in the Understanding and Application of Lithium Isotopes in the Earth and Planetary Sciences[M]//Johnson C M,Beard B L,Albarède F. Geochemistry of Non-traditional Stable Isotopes:Reviews in Mineralogy and Geochemistry.Washington D C:Mineralogical Society of America,2004,55:153-195.
[5] Magna T,Day J M D,Mezger K,et al. Lithium isotope constraints on crust-mantle interactions and surface processes on Mars[J]. Geochimica et Cosmochimica Acta,2015,162:46-65.
[6] Pogge von Strandmann P A E,Vaks A,Bar-Matthews M,et al. Lithium isotopes in speleothems:Temperaturecontrolled variation in silicate weathering during glacial cycles[J]. Earth and Planetary Science Letters,2017,469:64-74.
[7] Ionov D A,Doucet L S,Pogge von Strandmann P A E,et al. Links between deformation,chemical enrichments and Li-isotope compositions in the lithospheric mantle of the Central Siberian Craton[J]. Chemical Geology,2017,475:105-121.
[8] Pogge von Strandmann P A E,Frings P J,Murphy M J.Lithium isotope behaviour during weathering in the Ganges Alluvial Plain[J]. Geochimica et Cosmochimica Acta,2017,198:17-31.
[9] Chan L H. Lithium isotope analysis by thermal ionization mass spectrometry of lithium tetraborate[J]. Analytical Chemistry,1987,59(22):2662-2665.
[10] Lin J,Liu Y,Hu Z,et al. Accurate determination of lithium isotope ratios by MC-ICP-MS without strict matrixmatching by using a novel washing method[J]. Journal of Analytical Atomic Spectrometry, 2016, 31(2):390-397.
[11]苟龙飞,金章东,邓丽,等.高效分离Li及其同位素的MC-ICP-MS精确测定[J].地球化学,2017,46(6):528-537.Gou L F,Jin Z D,Deng L,et al. Efficient purification for Li and high-precision and accuracy determination of Li isotopic compositions by MC-ICP-MS[J]. Geochimica,2017,46(6):528-537.
[12] Gou L F,Jin Z D,Deng L,et al. Effects of different cone combinations on accurate and precise determination of Li isotopic composition by MC-ICP-MS[J]. Spectrochimica Acta Part B,2018,146:1-8.
[13] Tomascak P B,Carlson R W,Shirey S B. Accurate and precise determination of Li isotopic compositions by multi-collector sector ICP-MS[J]. Chemical Geology,1999,158:145-154.
[14] Choi M S,Ryu J S,Park H Y,et al. Precise determination of the lithium isotope ratio in geological samples using MC-ICP-MS with cool plasma[J]. Journal of Analytical Atomic Spectrometry,2013,28:505-509.
[15]赵悦,侯可军,田世洪,等.常用锂同位素地质标准物质的多接收器电感耦合等离子体质谱分析研究[J].岩矿测试,2015,34(1):28-39.Zhao Y,Hou K J,Tian S H,et al. Study on measurement of lithium isotopic composition for common standard reference materials using multi-collector inductively coupled plasma-mass spectrometry[J]. Rock and Mineral Analysis,2015,34(1):28-39.
[16] Nishio Y,Nakai S. Accurate and precise lithium isotopic determinations of igneous rock samples using multicollector inductively coupled plasma mass spectrometry[J]. Analytica Chimica Acta,2002,456:271-281.
[17] Pistiner J S,Henderson G M. Lithium-isotope fractionation during continental weathering processes[J]. Earth and Planetary Science Letters,2003,214:327-339.
[18] Bryant C J,McCulloch M T,Bennett V C. Impact of matrix effects on the accurate measurement of Li isotope ratios by inductively coupled plasma mass spectrometry(MC-ICP-MS)under ‘cold’plasma conditions[J].Journal of Analytical Atomic Spectrometry,2003,18:734-737.
[19] Jeffcoate A B,Elliott T,Thomas A,et al. Precise,small sample size determinations of lithium isotopic compositions of geological reference materials and modern seawater by MC-ICP-MS[J]. Geostandards and Geoanalytical Research,2004,28(1):161-172.
[20] Millot R,Guerrot C,Vigier N. Accurate and high-precision measurement of lithium isotopes in two reference materials by MC-ICP-MS[J]. Geostandards and Geoanalytical Research,2004,28(1):153-159.
[21] Bouman C,Elliott T,Vroon P Z. Lithium inputs to subduction zones[J]. Chemical Geology,2004,212:59-79.
[22] Romer R L,Heinrich W,Schr9der-Smeibidl B,et al. Elemental dispersion and stable isotope fractionation during reactive fluid-flow and fluid immiscibility in the Bufa del Diente aureole,NE-Mexico:Evidence from radiographies and Li,B,Sr,Nd,and Pb isotope systematics[J].Contributions to Mineralogy and Petrology,2005,149(4):400-429.
[23] Wunder B,Meixner A,Romer R L,et al. Temperaturedependent isotopic fractionation of lithium between clinopyroxene and high-pressure hydrous fluids[J].Contributions to Mineralogy and Petrology,2006,151:112-120.
[24]汪齐连,赵志琦,刘丛强,等.天然样品中锂的分离及其同位素比值的测定[J].分析化学,2006,34(6):764-768.Wang Q L,Zhao Z Q,Liu C Q,et al. Separation and isotopic determination of lithium in natural samples[J].Chinese Journal of Analytical Chemistry,2006,34(6):764-768.
[25] Rosner M,Ball L,Peucker-Ehrenbrink B,et al. A simplified,accurate and fast method for lithium isotope analysis of rocks and fluids,andδ7Li values of seawater and rock reference materials[J]. Geostandards and Geoanalytical Research,2007,31(2):77-88.
[26] Huang K F,You C F,Liu Y H,et al. Low-memory,small sample size,accurate and high-precision determinations of lithium isotopic ratios in natural materials by MC-ICPMS[J]. Journal of Analytical Atomic Spectrometry,2010,25:1019-1024.
[27]万红琼,孙贺,刘海洋,等.俯冲带Li同位素地球化学:回顾与展望[J].地学前缘,2015,22(5):29-43.Wang H Q,Sun H,Liu H Y,et al. Lithium isotopic geochemistry in subduction zones:Retrospects and prospects[J]. Earth Science Frontiers,2015,22(5):29-43.
[28]张群,秦礼萍.双稀释剂计算及校正方法[J].地球化学,2017,46(1):15-21.Zhang Q,Qin L P. The calculation and calibration of double-spike technique[J]. Geochimica,2017,46(1):15-21.
[29] Ko2ler J,Kucˇera M,Sylvester P. Precise measurement of Li isotopes in planktonic foraminiferal tests by quadrupole ICPMS[J]. Chemical Geology,2001,181:169-179.
[30] Magna T,Wiechert U H,Halliday A N. Low-blank isotope ratio measurement of small samples of lithium using multiple-collector ICPMS[J]. International Journal of Mass Spectrometry,2004,239(1):67-76.
[31] James R H,Palmer M R. The lithium isotope composition of international rock standards[J]. Chemical Geology,2000,166:319-326.
[32] Oi T,Odagiri T,Nomura M. Extraction of lithium from GSJ rock reference samples and determination of their lithium isotopic compositions[J]. Analytica Chimica Acta,1997,340:221-225.
[33] Schuessler J A,Schoenberg R,Sigmarsson O. Iron and lithium isotope systematics of the Hekla volcano,Iceland—Evidence for Fe isotope fractionation during magma differentiation[J]. Chemical Geology,2009,258:78-91.
[34]唐索寒,朱祥坤,蔡俊军,等.用于多接收器等离子体质谱铜铁锌同位素测定的离子交换分离方法[J].岩矿测试,2006,25(1):5-8.Tang S H,Zhu X K,Cai J J,et al. Chromatographic separation of Cu,Fe and Zn using AG MP-1 anion exchange resin for isotope determination by MC-ICPMS[J]. Rock and Mineral Analysis,2006,25(1):5-8.
[35] Seitz H M,Brey G P,Lahaye Y,et al. Lithium isotopic signatures of peridotite xenoliths and isotopic fractionation at high temperature between olivine and pyroxenes[J]. Chemical Geology,2004,212:163-177.
[36] Miller J M. Chromatography:Concepts and Contrasts[M].Hoboken:Wiley,2004:1-520.
[37] Tomascak P B,Magna T,Dohmen R. Methodology of Lithium Analytical Chemistry and Isotopic Measurements[M]//Advances in Lithium Isotope Geochemistry.Switzerland:Springer International Publishing,2016:5-18.
[38] Chan L H,Lassiter J C,Hauri E H,et al. Lithium isotope systematics of lavas from the Cook-Austral islands:Constraints on the origin of HIMU mantle[J]. Earth and Planetary Science Letters,2009,277:433-442.
[39] Houk R S,Schoer J K,Crain J S. Plasma potential measurements for inductively coupled plasma mass spectrometry with a centre-tapped load coil[J]. Journal of Analytical Atomic Spectrometry,1987,2:283-286.
[40]刘晔,柳小明,胡兆初,等. ICP-MS测定地质样品中37个元素的准确度和长期稳定性分析[J].岩石学报,2007,23(5):1203-1210.Liu Y,Liu X M,Hu Z C,et al. Evaluation of accuracy and long term stability of determination of 37 trace elements in geological samples by ICP-MS[J]. Acta Petrologica Sinica,2007,23(5):1203-1210.