A Study on Memory Effects in Lithium and Boron Isotope Analysis Using MC-ICP-MS
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摘要:
锂(Li)和硼(B)同位素是地质作用过程中良好的示踪剂，被广泛应用于岩石起源、矿床成因和环境演化等领域。但锂、硼在MC-ICP-MS仪器分析中的“记忆效应”明显，不同实验室已报道的MC-ICP-MS分析中锂、硼背景占信号比变化范围大(0.01%～5%)，所采用的背景控制方法和效果也各不相同，因此给锂、硼同位素的准确测定带来困难。为研究MC-ICP-MS锂、硼同位素记忆效应及其抑制方案，提高测试的稳定性，本文参考前人研究成果，设计不同背景清洗方案，并对各种国际标样(IRMM-016、JG-2、ERM-AE121、ERM-AE122、NASS-7)和实验室内部标准(Alfa Li、Alfa B)进行长期测试，检验实验方案的长期重现性。结果表明：仅使用0.3%氯化钠溶液清洗背景可以显著降低锂背景信号，从20mV下降至4mV，并保证7Li背景值在24h内低于5mV，实验室内部标准溶液Alfa Li的δ7Li长期测试外精度为0.13‰(2SD，n=73)。氟化钠、氨水等清洗液并不能显著降低本研究所使用仪器的硼背景，因此选择使用灵活的空白扣除方法来保证数据稳定性。实验室内部标准溶液Alfa B的δ11B长期测试外精度为0.19‰(2SD，n=60)。本文锂、硼同位素国际标样的测试结果与前人数据在误差范围内一致，证明了实验结论的可靠性。
要点(1)MC-ICP-MS测定锂、硼同位素组成时记忆效应明显，导致不同批次测试的分析精度差异大，数据重现性较差。
(2)使用0.3%氯化钠溶液清洗锂背景时，锂同位素分析稳定性最佳；灵活的空白扣除方法适合硼同位素的准确测定。
(3)使用指定的背景清洗方案，国内外标样锂、硼同位素测试精度分别可达0.2‰和0.3‰。
HIGHLIGHTS(1) The memory effects of Li and B in MC-ICP-MS are obvious, which results in a poor data reproducibility among different measurement batches.
(2) When the Li background is rinsed with 0.3% NaCl solution, the stability of Li isotope analysis is the best. The flexible blank deduction method is suitable for accurate determination of B isotopes.
(3) After using the suggested background cleaning method, the detection accuracy of international standards of Li and B isotopes can reach 0.2‰ and 0.3‰, respectively.
Abstract:Lithium (Li) and boron (B) isotopes are excellent tracers in geological processes. In order to study and eliminate the memory effects of lithium and boron element in isotopic measurements using MC-ICP-MS, different background rinsing protocols were designed with reference to previous research. The δ7Li and δ11B values of different types of reference materials were tested to evaluate the long-term reproducibility of the measurements using designated rinsing protocols for at least six months. The results show that using only 0.3% NaCl solution to clean the background for 60s can significantly reduce the lithium background signal and ensure the 7Li background signal is less than 5mV within 24h. The long-term external precision of δ7Li values for Alfa Li, an in-house standard, was 0.13‰ (2SD, n=73). Rinsing solutions, such as NAF and NH3·H2O could not significantly reduce the boron background of the instrument. However, the flexible blank subtracting method was used for precise determination of δ11B values. The long-term external precision of δ11B values for Alfa B was 0.19‰ (2SD, n=60). The average δ7Li and δ11B values of different types of reference materials were tested using these rinsing protocols, and the results were well consistent with reported data, supporting the applicability of the conclusions. The BRIEF REPORT is available for this paper at http://www.ykcs.ac.cn/en/article/doi/10.15898/j.ykcs.202310260167.
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Keywords:
- memory effect /
- lithium /
- boron /
- MC-ICP-MS /
- 0.3% NaCl solution /
- blank deduction
BRIEF REPORTSignificance: MC-ICP-MS is commonly used to determine the composition of lithium and boron isotopes. However, data quality is often limited due to significant memory effects caused by deposition or adhesion of Li and B into the instrument. The rinsing solutions, including NaCl solution for Li and ammonia or NaF solutions for B, were tried, to eliminate memory effects by previous work[12,16,27-28,32]. However, the sensitivity and background of Li and B elements in different instruments are different, such as in Neptune or Nu MC-ICP-MS, the memory reduction method should be retested in a new instrument. In this paper attention to the Li and B memory effects and their reduction method in the isotopic measurements using Nu Sapphire MC-ICP-MS was given. The results show that using only 0.3% NaCl solution to clean the background can significantly reduce the lithium background signal, and ensure that the 7Li background value is less than 5mV within 24h. The flexible blank deduction technique can be used for precise determination of δ11B values.
Methods: 0.1%(V/V) NaCl, 0.3% NaCl, 2.5% NaCl and 5% NaCl solutions were introduced for 1min to rinse the background of Li before a daily batch run. The background of Li after a normal 2% HNO3 rinsing sequence and the internal precision (SE) of 7Li/6Li when testing 200ng/g Li sample solutions were recorded for several hours. For B isotopes measurement, water, 0.1% ammonia and 0.06mg/g NaF solutions were introduced for 1min or 2min to control the B memory effects. The background of B after a normal 2% HNO3 rinsing sequence were recorded when testing an 80ng/g B sample solution. The blank subtraction method with different frequencies was also used to control the memory effects of B. The δ7Li and δ11B values of standard materials and their external precisions (2SD) were calculated to ensure the accuracy and long-term stability of the measurements using a different memory reduction method.
Data and Results: The fluctuation of sensitivity of 7Li and internal precision (SE) of 7Li/6Li caused by Na were limited when 0.3% NaCl solution was introduced for 60s. In the meanwhile, the 7Li background signal decreased from 20mV to 4mV and remained a low level within 24h. Thus, a washing process included the following steps: 0.3% NaCl solution was introduced for 60s at intervals of about 24h, the background intensity determined (zero test) using 2% HNO3 blank was subtracted before a daily batch run and 2% HNO3 (2min) was used to rinse the background between each standard and samples. The δ7Li values of reference materials and their external precisions (2SD) were obtained in six months to ensure the accuracy and long-term stability of the data. The δ7Li values of IRMM-016, USTC-Li, Alfa Li and JG-2 with respect to L-SVEC were 0.12‰±0.07‰ (n=50), −19.3‰±0.12‰ (n=56), 13.7‰±0.13‰ (n=73), 0.13‰ ±0.11‰ (n=12), respectively.
The 11B background signal could not be effectively reduced when the background rinsing solution was replaced with pure water, acidified NaF, ammonia and dilute nitric acid. To ensure the accuracy of the test, the background deduction method is flexibly selected, that is, each blank is deducted once between each standard and samples within the first 3h of the B isotope test sequence. Typical washing and testing process included the following steps: 120s 2% HNO3 wash–60s wash solution uptake–30s 2% HNO3 zero test–120s 2% HNO3 wash–60s sample uptake–150s sample or standard measurement. The zero-testing process could be lessened to once every nine samples testing after 3h of the batch run. After six months of reference materials testing, the δ11B values of ERM-AE121, ERM-AE122, Alfa B and NASS-7 were
19.78‰±0.31‰ (n=64), 39.54‰±0.33‰ (n=36), −4.66‰±0.19‰ (n=60) and 40.03‰±0.33‰ (n=35), respectively. The results were in good agreement with reported data, which indicates that excellent accuracy and precision can be achieved for Li and B isotope measurements using these designated rinsing protocols. -
锂有6Li、7Li两个稳定同位素，两者16.7%的相对质量差导致锂在自然界发生明显的同位素分馏［1-2］，不同地质储库间的锂同位素分馏可达60‰以上(δ7Li)［3］。硼有10B和11B两个稳定同位素，由于硼同位素较高的地球化学反应活性和较大的相对质量差(9.1%)，使其在不同地质储库中的同位素组成差异可高达70‰(δ11B)［4-5］。因此，锂和硼同位素是良好的地球化学示踪工具，在岩石、环境及矿床地球化学领域应用广泛［1-2，4-7］，也是分析地球化学研究的重要方向［8-18］。
通常使用热电离质谱(TIMS)［8-9，19］或多接收电感耦合等离子体质谱(MC-ICP-MS) [10-16，19］测定锂、硼同位素组成。TIMS测试过程相对复杂耗时，仪器质量分馏校正复杂。而MC-ICP-MS分析流程相对高效，仪器分馏校正简单(样品标样交叉法，SSB)，且进样系统可灵活切换，可与激光剥蚀系统联用从而获得微区同位素组成信息［20-23］，是目前最常见的锂、硼同位素分析技术。MC-ICP-MS对锂同位素测试精度变化范围大(0.2‰～1.0‰)［10-12，24-25］，对硼同位素测试精度为0.1‰～1.0‰［13-14 ，16，26］。在MC-ICP-MS测试过程中，样品进入离子源后，锂元素易在离子源的低温区域中沉积并在后续的分析中重新释放出来，导致分析结果产生偏差［15］。硼在稀酸溶液中以硼酸的形式存在，易挥发并易黏附在进样系统上从而导致背景持续升高。因此锂、硼在MC-ICP-MS分析时同位素“记忆效应”明显，影响测试稳定性。前人报道的锂背景变化范围大(20～200mV)［12］，采用的“记忆效应”应对方案也有所不同。如蔺洁等(2016)［12］在Neptune MC-ICP-MS(美国ThermoFisher公司)湿法模式测试过程中仅用稀酸清洗背景时，7Li背景可高达110mV(背景占比约1.2%)，而使用5%氯化钠溶液清洗1min后 7Li背景信号降低至1.5～2mV，维持3h且可以改善锂浓度和酸浓度不匹配引起的基体效应。而使用Nu Plasma系列仪器的湿法模式测试锂同位素时，短期(2min)稀酸清洗后的背景一般在20mV左右(背景占比约0.5%)，控制“记忆效应”影响的方法一般为长时间稀酸清洗和每个样品测试前扣除背景值。对于硼同位素分析，已报道的MC-ICP-MS分析中硼的背景值变化范围一般为40～100mV，背景占比可高至12%［16，27-28］。前人应用多种方法来抑制硼元素背景的升高，包括向喷雾室充氨气，采用直接进样系统(Direct injection nebulizer)减少样品停留时间，增加背景清洗时间，以及改变洗液类型(稀硝酸、盐酸、氢氟酸、氨水、甘露醇和氟化钠等)［16，27-28］。例如，He等(2019)［16］使用Neptune MC-ICP-MS测试时，2%硝酸清洗5min后，背景信号仍有100mV(背景占比12%)；酸化的氟化钠溶液清洗1min，11B信号从1000mV降至3mV左右；Cai等(2021)［14］ 使用Nu PlasmaⅡ MC-ICP-MS测试硼同位素时，背景信号约有42mV(背景占比约4.5%)，使用稀酸空白扣除法进行数据计算来保证测试准确度。
综上所述，不同类型的MC-ICP-MS，包括Nu Plasma系列仪器和ThermoFisher公司的Neptune(Plus)仪器，它们在进样系统、离子源和接口的设计上差异大，导致锂、硼灵敏度和背景差异大。已发表的锂、硼背景清洗方法(氯化钠和氟化钠等)能否在Nu Plasma系列仪器上实现应用，是否需要优化实验方案以获得高质量数据，是值得探索的问题。本文研究了Nu Sapphire (Nu Plasma公司) MC-ICP-MS仪器测试锂、硼同位素的记忆效应，并设计不同的背景抑制实验和空白扣除方案，以建立高精度和高重现性的锂、硼同位素测试方法。针对锂使用不同浓度的氯化钠溶液作为清洗液，检验了其对7Li背景的控制效果及对测试精度的影响；对于硼同位素，对比了使用氨水、氟化钠溶液和稀酸作为清洗液的效果，结合灵活的空白扣除方法，选择最佳的稳定的测试方案，通过计算国际标样测试数据的长期重现性(2SD)说明实验方案的可靠性。
1. 实验部分
1.1 仪器
本实验在东华理工大学核资源与环境国家重点实验室完成。测试使用的仪器为Nu Sapphire MC-ICP-MS；进样系统和源区包括ESI2DX自动进样器、雾化器(玻璃材质和PFA材质)、玻璃冷却雾室、石英炬管和镍锥。Nu Sapphire在传输区域设计有“高能量”通道，与Nu Plasma Ⅲ相似，加速电压为6000V；“低能量”通道加设偏转透镜，改变离子束路径进入六极杆碰撞反应池，加速电压为4000V。仪器配备16个法拉第杯、1个离子计数器(SEM)和1个戴利检测器(Daly)。测试采用静态(Static)分析模式，锂同位素测试时使用法拉第杯L6和H9(1011Ω)接收7Li和6Li；硼同位素测试时采用法拉第杯L6和H6(1012Ω)同时接收10B和11B。Nu Sapphire MC-ICP-MS仪器基本配置见表1。
表 1 Nu Sapphire MC-ICP-MS仪器基本配置Table 1. Basic configuration of the Nu Sapphire MC-ICP-MS instrument工作参数 实验条件 工作参数 实验条件 射频功率 1300W 炬管 石英炬管 加速电压 6000V，高能模式 雾化器气体(Ar)压力 ～30psi 冷却气(Ar)流速 13L/min 锥 镍锥，湿锥 辅助气(Ar)流速 0.9～1.0L/min 接收器设置 H6-11B；L6-10B
H9-6Li；L6-7Li雾化器及流速 玻璃雾化器，PFA雾化器，100/50μL/min 分辨率 低分辨 进样方式采用湿法(Wet plasma)。通过测定2%(V/V)硝酸中锂和硼信号进行空白扣除。对于锂、硼同位素分析，1个测试序列(Cycle)包括30个测试点，每个点测试时间为5s，样品信号采集总时间150s。
锂、硼同位素组成以相对于标准物质的δ值表示，表达方式如下：
δ7Li=[(7Li/6Li)sample/(7Li/6Li)standard−1]×1000‰
式中：(7Li/6Li)sample代表样品7Li/6L比值的测定值；(7Li/6Li)standard表示与样品相邻的前后两次标准样品7Li/6Li比值测定值的平均值。锂同位素标准物质为碳酸锂NIST L-SVEC。
δ11B=[(11B/10B)sample/(11B/10B)standard−1]×1000‰
式中：(11B/10B)sample代表样品11B/10B比值的测定值；(11B/10B)standard表示与样品相邻的前后两次标准样品11B/10B比值测定值的平均值。硼同位素标准物质为硼酸NIST SRM951。
仪器质量分馏通过SSB法校正。100ng/g 锂同位素标准溶液测试内精度通常优于0.05‰(RSE)，δ7Li外精度一般优于0.2‰；80ng/g 硼同位素标准溶液测试内精度通常优于0.08‰(RSE)，δ11B测试外精度一般优于0.3‰。锂同位素灵敏度为～20V/(μg/g)，硼同位素灵敏度为～8V/(μg/g)。
1.2 标准物质和主要试剂
国际上常用的锂同位素标准物质：NIST L-SVEC(NIST SRM8545)和IRMM-016。L-SVEC(碳酸锂)作为外标，推荐值为7Li/6Li=12.0192±0.0289［29］；IRMM-016作为监控标样。标液制备：将大约50mg的L-SVEC粉末溶解在2mL浓硝酸中，蒸发至干，并用2%(V/V)硝酸稀释，产生约20μg/g储备溶液，上机前用2%硝酸匹配到合适浓度。使用相同的方法制备20μg/g IRMM-016原液。本实验中用到锂同位素内部标液还包括USTC-Li(中国科学技术大学实验室内部标准溶液，参考值δ7LiL-SVEC=−19.3‰)；阿法埃莎(中国)化学有限公司光谱纯等离子体标准溶液Alfa Li。使用2%硝酸将USTC-Li、Alfa Li(1000μg/g)稀释，形成100ng/g溶液。此外，选择岩石粉末国际标样JG-2(花岗岩)作为方法验证标样。以上标准物质参考值见表2。
表 2 标准物质锂同位素组成测定值与参考值Table 2. The measured and reported δ7LiL-SVEC values of standards标准物质 δ7LiL-SVEC参考值
(‰)δ7LiL-SVEC测试值
(‰)2SD
(‰)n IRMM-016 −0.17～0.40a 0.12 0.07 50 USTC-Li −19.30b −19.37 0.12 56 Alfa Li − 13.71 0.13 73 JG-2 0.15～0.32c 0.13 0.11 12 注：“−”表示无参考值。Alfa Li为实验室内部标准溶液，无参考值。a. IRMM-016的δ7LiL-SVEC参考值来源于GeoReM(Geological and Environmental Reference Materials)数据库及对应参考文献，仅统计MC-ICP-MS测试结果。b. USTC-Li 的δ7LiL-SVEC参考值由中国科技大学肖益林教授提供。c. JG-2的δ7LiL-SVEC参考值来源于Bouman等(2004)［30］、Jeffcoate等(2004)［15］、Li 等(2019)［31］、Lin等(2016)［32］和Zhu等(2020)［33］。Note:“−” indicates the absence of a reference value. Alfa Li serves as the internal standard solution in the laboratory，and no reference value is available. a. The δ7LiL-SVEC reference value of IRMM-016 is derived from the Geological and Environmental Reference Materials (GeoReM) database and corresponding references，considering only the MC-ICP-MS test results. b. The δ7LiL-SVEC reference value of USTC-Li is provided by Professor Xiao Yilin from the University of Science and Technology of China. c. The δ7LiL-SVEC reference values of JG-2 are derived from Bouman et al. (2004) [30］，Jeffcoate et al. (2004) ［15］，Li et al. (2019) ［31］，Lin et al. (2016) ［32］ and Zhu et al. (2020) ［33］. 国际上常用硼同位素标准物质：硼酸粉末NIST SRM951(NIST951a代替)、硼酸溶液ERM-AE121和ERM-AE122。NIST SRM951作为外标，推荐值为10B/11B=0.2473；ERM-AE121和ERM-AE122作为监控标样。标准溶液制备：称取约50mg的NIST951a硼酸粉末，溶解在超纯水中，制成约100μg/g储备溶液，用2%硝酸进行稀释，匹配到上机浓度，一般为80ng/g。ERM-AE121、ERM-AE122、阿法埃莎(中国)化学有限公司光谱纯等离子体标准溶液Alfa B使用同一批2%硝酸稀释到80ng/g备用。本实验用到硼同位素内部标液还包括Alfa B等离子体标准溶液1000μg/g。此外，选择海水标样NASS-7作为方法验证标样。以上标准物质参考值见表3。
表 3 标准物质硼同位素组成测定值与参考值Table 3. The determined values and reference values of the B isotope composition in the reference materials标准物质 δ11BNIST951参考值
(‰)δ11BNIST951a测试值
(‰)2SD
(‰)n ERM-AE121 19.54～20.33a 19.78 0.31 64 ERM-AE122 39.3～39.74a 39.54 0.33 36 Alfa B − −4.66 0.19 60 海水标样NASS-7 − 40.03 0.33 35 天然海水 39.98±0.35b − − − 天然海水 39.45～40.26b − − − 海水标样NASS-2 39.63～39.90c − − − 海水标样NASS-5 39.40～39.89c − − − 海水标样NASS-6 39.41～39.81c − − − 注：“−”表示无参考值。Alfa B为实验室内部标准溶液，无参考值。a. ERM-AE121和ERM-AE122的δ11BNIST 951参考值来自GeoReM数据库及对应参考文献。数据包括了样品相对于NIST SRM951和NIST SRM951a的参考值。b. 天然海水δ11BNIST 951参考值引自Chen等(2019)［27］及其中参考文献。c. NASS系列海水δ11BNIST 951参考值引自GeoReM数据库及对应参考文献。Note: “−” indicates the absence of a reference value. Alfa B is a laboratory standard solution without a designated reference value. a. The δ11BNIST 951 reference values of ERM-AE121 and ERM-AE122 are from the GeoReM database and corresponding references. The data include reference values of the sample with respect to NIST SRM951 and NIST SRM951a. b. The reference value of δ11BNIST 951 in natural seawater is cited from Chen et al. (2019) ［27］ and other references. c. The reference value of δ11BNIST 951 for NASS series seawater is sourced from the GeoReM database and corresponding references. 试剂：25℃时电阻率为18.2MΩ·cm的高纯水，由纯水机Milli-Q® EQ7000超纯水系统(法国Millipore 公司)制得。高纯度硝酸，由优级纯硝酸经Savillex TMDST-1000亚沸蒸馏系统(美国Savillex公司)二次蒸馏制得。其他试剂包括29%高纯度氨水，购自上海傲班科技有限公司(G3级，单元素浓度最高不超过1ng/g)；优级纯氯化钠粉末；优级纯氟化钠粉末。
实验过程中使用的PFA容器均经过高纯硝酸、盐酸、王水、超纯水反复加热浸泡清洗；高密度聚乙烯（HDPE）试剂瓶、移液枪头等一次性容器则使用稀硝酸反复加热浸泡清洗。实验中所有酸和标准样品均在千级清洁实验室的百级通风橱内制备，为最大限度地减少锂、硼空白污染以及交叉污染所带来的影响。
1.3 实验方法
为控制仪器测试时锂、硼同位素记忆效应对测试准确度的影响，本研究分别设计以下实验流程，以便于观察不同的背景控制方法对应的测试结果。
1.3.1 锂同位素记忆效应实验
(1)每个样品测试前扣除2%硝酸空白
一个6Li/7Li数据的测试序列，包括2×60s的2%硝酸(清洗液)—80s的2%硝酸(空白提升)—30s的2%硝酸(空白测试)—2×60s的2%硝酸(清洗液)—80s的样品提升—150s的样品测试，总时间为580s。每个样品前增加空白测试实验，也相当于增加背景清洗时间(350s)，为正常酸洗的时间(120s)的三倍。记录锂同位素的背景变化，计算标准溶液测试的δ7Li值和外精度(2SD)。
(2)氯化钠洗液降低锂背景
称量适量优级纯氯化钠粉末溶于2%硝酸中，配制成0.1%、0.3%、2.5%和5%氯化钠溶液；在锂同位素测试序列开始前，使用以上浓度氯化钠溶液进样60s；观察短时间内锂同位素比值的内精度(SE)变化，记录24h内7Li元素背景值的变化，计算标准溶液测试的δ7Li值及其外精度(2SD)。
(3)标准物质测定
根据以上观测结果，选择合适的空白扣除方式，测试国际标样δ7Li值及其长期测试重现性(2SD)。
1.3.2 硼同位素记忆效应实验
(1)每个样品测试前扣除稀酸空白
与锂同位素比值测试相同，一个11B/10B的测定总时间580s。记录硼同位素的背景变化，计算标准溶液测试的δ11B值及其外精度(2SD)。
(2)改变背景清洗液类型
正常清洗液为2%硝酸，本实验分别尝试将洗液更换为纯水、1%硝酸、酸化的0.06mg/g 氟化钠溶液、0.1%氨水几种方案，观察11B背景信号的变化。
(3)标准物质测定
根据以上观测结果，选择合适的空白扣除方式，测试并计算国际标样δ11B值及长期重现性(2SD)。
2. 结果与讨论
2.1 锂同位素背景清洗方案的确定
对于其他金属(如镁、锌、铜)稳定同位素，使用2%硝酸清洗120s(60s+60s)，背景值能迅速降低到几个毫伏(～0.1ng/g)［34-35］。使用Neptune MC-ICP-MS测试锂同位素时7Li背景信号高达30～110mV，质谱仪背景会随着进样(正常运行)逐渐增加(1mV/h)，清洗3～5min也很难完全干净［12］。本研究使用的Nu Sapphire仪器测试中，7Li背景随着测试开始从几个毫伏逐渐上升到10～20mV(图1)，且随着测试的进行背景会累积增大。前人研究发现Li背景中同位素比值与样品同位素比值差异大，高背景时的稀酸空白扣除(on-peak-zero)会带来新的误差［15］ 。因此需要保证在Li的低空白水平测试。
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图 1 2%硝酸清洗条件下，7Li背景信号随着仪器连续测试时间的增长而逐渐升高Figure 1. The background signal of 7Li gradually increased with the continuous testing time when using 2% HNO3 rinse solution对于逐一扣空白的方式，其背景清洗和空白测试时间过长(350s)，会降低仪器测试效率和增大SSB法校正误差，在仪器或环境条件不稳定时尤为严重。Lin等(2016)［32］在Neptune MC-ICP-MS测试锂同位素过程中发现，使用5%氯化钠溶液清洗背景60s，7Li背景值从30～110mV显着降低到1.5～2mV，背景占信号比从1.2%下降至0.02%，并且没有发现锂同位素比值的明显漂移，同时可减轻基体效应等带来的影响。Nu Sapphire MC-ICP-MS灵敏度相对较低，正常稀酸清洗120s后的背景也偏低(10～20mV)，7Li背景占信号比约为0.3%。如图2在本仪器上使用5%氯化钠溶液清洗背景60s后，7Li背景能在10h内控制在5mV左右，清洗后的2h内灵敏度显著下降(下降率最大22%)，7Li/6Li比值测试内精度(SE)变差(最高～1‰)。因此，高浓度的氯化钠溶液对锂同位素测试有一定的负面影响，也可能使进样系统污染，增加维护成本，需要选择合适浓度的氯化钠溶液清洗Li背景。
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图 2 5%氯化钠溶液清洗背景60s后，在正常测试过程中样品7Li灵敏度(200ng/g)、7Li背景(a)和7Li/6Li 测试内精度(SE)(b)的变化Figure 2. The 7Li signal intensity of samples (200ng/g) and background (a), and internal precision (SE) of 7Li/6Li (b) during normal testing after cleaning the background with a 5% NaCl solution for 60s使用不同浓度氯化钠溶液作为背景清洗液时可以发现：当氯化钠浓度过低时(0.1%)对背景清洗效果相对较差(～6mV)，背景信号恢复过快，在8h后开始迅速回升；使用2.5%氯化钠溶液，高浓度Na离子对Li的电离产生较明显的基体效应，Li背景显著下降的同时灵敏度下降～12%，7Li/6Li比值内精度(SE)变差，从0.04%上升至0.1%以上，约1.5h后恢复正常；而选择在测试前进行60s的0.3%氯化钠溶液清洗，Li背景在24h内稳定在4mV左右，同时Na离子对Li灵敏度和测试内精度的影响时间和程度可控制至最低。一般灵敏度短期下降在10%以内，7Li/6Li比值内精度从0.04%上升至0.08%左右，1h内的内精度恢复至0.05%以内。且较低浓度氯化钠洗液进样还能避免对进样系统和采样锥的污染，因此在锂同位素测试过程中选择0.3%氯化钠溶液清洗背景(图3)。
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图 3 2.5%、0.3%和0.1% 氯化钠溶液清洗背景60s后，在样品正常测试过程中7Li背景值的变化Figure 3. The 7Li signal intensity of background after using 2.5%, 0.3%, and 0.1% NaCl rinse solutions for 60s2.2 硼同位素清洗方案的确定
硼同位素在MC-ICP-MS上的记忆效应比Li更严重。80ng/g硼同位素溶液上机测试时，采用120s稀酸清洗背景，2h后11B背景信号高达40mV，灵敏度占比约4.5%(图4)。测试序列的前3h内，每个11B/10B比值测试仅扣除同一背景值，δ11B测试偏差高达1‰。
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图 4 2%硝酸清洗条件下，11B背景占信号比随着仪器连续测试时间的增长而逐渐升高Figure 4. The ratio of 11Bbackground/11Bsensitivity increased during the continuous test when using the 2% HNO3 rinse solution以每个测试前后都扣除背景的方式获得一个11B/10B数据的测试时间约为10min，会影响SSB法校正效果，导致分析外精度变差，且测试效率不高。如图5所示，结果表明将背景清洗液更换为纯水、酸化氟化钠、氨水与稀硝酸组合时，均无法有效地降低11B背景值。稀氨水短时间内可降低11B背景，但2%硝酸进液时11B背景迅速恢复，说明稀氨水只具有短暂的信号抑制作用，并不能快速清洗进样管道中残留的硼元素。虽然He等(2019)［16］在Neptune MC-ICP-MS测试硼同位素组成时发现1%硝酸酸化的氟化钠溶液(0.6mg/g)可在短时间内有效地降低11B背景值，且不影响仪器稳定性。但是两台仪器的进样系统和灵敏度不同，记忆效应也不同。并且氟离子可能破坏Nu Plasma系列MC-ICP-MS仪器的进样系统(玻璃雾化器和雾室)，因此不适合作为背景清洗液。
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图 5 在不同类型洗液组合下11B背景信号随时间的变化① PFA雾化器，1%硝酸酸化NaF溶液清洗1min，随后2%硝酸继续清洗；② PFA雾化器，纯水清洗1min后2%硝酸继续清洗；③ PFA雾化器，纯水清洗2min后2%硝酸继续清洗；④ 玻璃雾化器，0.1%氨水清洗1min后2%硝酸继续清洗；⑤ 玻璃雾化器，纯水清洗1min后2%硝酸继续清洗。Figure 5. The variations in 11B background signal over time under various combinations of lotions①100µL/min PFA nebulizer was used. The B background was rinsed by 0.6mg/g NaF in 1% HNO3 solution for 1min before a normal cleaning by 2% HNO3 solution; ②100µL/min PFA nebulizer was used. The B background was rinsed by water for 1min before a normal cleaning by 2% HNO3 solution; ③100µL/min PFA nebulizer was used. The B background was rinsed by water for 2min before a normal cleaning by 2% HNO3 solution; ④100µL/min glass nebulizer was used. The B background was rinsed by 0.1% (V/V) ammonia for 1min before a normal cleaning by 2% HNO3 solution; ⑤100µL/min glass nebulizer was used. The B background was rinsed by water for 1min before a normal cleaning by 2% HNO3 solution.2.3 锂硼同位素测试结果与长期稳定性
2.3.1 标准样品锂同位素测试结果与长期稳定性
为保证测试精度和准确度，结合背景扣除和背景清洗方法，选择在首次实验前进行60s的0.3%氯化钠溶液清洗背景，再继续使用2%硝酸清洗背景120s后开始正常的仪器调试和测试实验。实验过程中每获得9个7Li/6Li比值测试一次背景值。在此策略下一般能保持优于0.2‰的测试外精度(2SD)。经过长达半年的标准样品测定，获得包括IRMM-016、USTC-Li、Alfa Li和JG-2相对于L-SVEC的δ7Li值，分别为0.12‰±0.07‰ (n=50)、−19.37‰±0.12‰ (n=56)、13.71‰±0.13‰ (n=73)、0.13‰±0.11‰ (n=12)(图6)。其中JG-2(花岗岩)岩石粉末锂同位素纯化流程根据Li等(2019)［31］ 改进，测试结果与参考值在误差范围内一致(图6，表2)，表明测试方法的准确性。
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图 6 标准样品IRMM-016(a)、USTC-Li(b)、Alfa Li(c)和JG-2(d)相对于L-SVEC的δ7Li测定值及其参考值红色方框及其误差限表示参考值和参考值变化范围或其测试外精度(2SD)。样品误差表示单次测试的外精度(2SD)。Figure 6. The measured δ7LiL-SVEC of the standards and their reference values. The red box and its errors bars represent the reference value and the range of variation of the values or their external precisions (2SD). The sample error bars represent external precisions (2SD) of individual tests.2.3.2 标准样品硼同位素测试结果与长期稳定性
为保证测试精度和准确度，灵活选择背景扣除方式，即选择在硼同位素测试序列的前3h内，进行每个比值扣一次空白，获得一个11B/10B比值的流程包括2×60s的2%硝酸(清洗液)—80s 的2%硝酸(空白提升)—30s的2%硝酸(空白测试)—2×60s的2%硝酸(清洗液)—80s的样品提升—150s的样品测试，总时间为580s。背景稳定后，在测试开始前进行一次空白背景值(即230s背景测试)，随后每获得9个11B/10B比值再测定一次空白背景值。在此策略下一般能保持优于0.35‰的测试外精度(2SD)。经过长达半年的标准物质测定，ERM-AE121、ERM-AE122、Alfa B和NASS-7，其δ11B测定值分别为19.78‰±0.31‰(n=64)、39.54‰±0.33‰(n=36)、−4.66‰±0.19‰(n=60)、40.03‰±0.33‰ (n=35)(图7)。其中NASS-7海水标样的硼同位素纯化流程参考Cai等(2021)［14］，结果为5批次纯化后8次测试结果的平均值。标准物质测试结果在误差内与参考值一致(表3)，表明测试方法的准确性。
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图 7 标准样品EPM-AE 121(a)、EPM-AE 122(b)、Alfa B(c)和NASS-7(d)相对于NIST951a的δ11B测定值及其参考值红色方框及其误差限表示参考值平均值和参考值变化范围。样品误差限表示单次测试的外精度(2SD)。Figure 7. The measured δ11BNIST951a of the standards and the reference values. The red box and its errors bars represent the reference value and the range of variation of the values or their external precisions (2SD). The sample error bars represent external precisions (2SD) of individual tests.3. 结论
主要研究了Nu Saphhire MC-ICP-MS在锂、硼同位素测试过程中的记忆效应。实验结果表明，选择在首次实验前进行60s的0.3%氯化钠进行锂背景清洗，再使用2%硝酸清洗背景120s后开始正常的仪器调试和测试实验，能显著降低锂同位素记忆效应的影响，并保证后续分析的灵敏度和分析内精度的稳定性，溶液标准物质δ7Li值外精度优于0.2‰。对于硼同位素测定，常见清洗液(包括氟化钠和氨水)并不能明显降低Nu Saphhire MC-ICP-MS的硼背景信号。而选择灵活的空白扣除方法，一般能保持δ11B优于0.35‰的测试外精度。锂、硼同位素标准物质的长期测试结果与参考值在误差范围内一致，证明了实验方案的可靠性。
虽然本文针对锂、硼同位素测定制定了可靠的分析方案，但是锂、硼同位素在不同仪器上的记忆效应机理仍待研究。尤其是硼同位素在天然样品测试中，硼酸的特殊化学性质可能导致复杂络合物产生，导致背景变化不稳定。此外，硼同位素上机测试的空白扣除方式还需要考虑样品前处理流程空白。因此，进一步探索背景清洗方案仍是获得高精度硼同位素数据最直接的方法。
致谢：中国科学技术大学肖益林教授提供了USTC-Li标准溶液及其δ7LiL-SVEC参考值，中国科学院地球环境研究所邓丽博士提供了标准溶液ERMAE121和ERMAE122，在此一致表示衷心感谢。
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图 1 2%硝酸清洗条件下，7Li背景信号随着仪器连续测试时间的增长而逐渐升高
Figure 1. The background signal of 7Li gradually increased with the continuous testing time when using 2% HNO3 rinse solution
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图 2 5%氯化钠溶液清洗背景60s后，在正常测试过程中样品7Li灵敏度(200ng/g)、7Li背景(a)和7Li/6Li 测试内精度(SE)(b)的变化
Figure 2. The 7Li signal intensity of samples (200ng/g) and background (a), and internal precision (SE) of 7Li/6Li (b) during normal testing after cleaning the background with a 5% NaCl solution for 60s
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图 3 2.5%、0.3%和0.1% 氯化钠溶液清洗背景60s后，在样品正常测试过程中7Li背景值的变化
Figure 3. The 7Li signal intensity of background after using 2.5%, 0.3%, and 0.1% NaCl rinse solutions for 60s
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图 4 2%硝酸清洗条件下，11B背景占信号比随着仪器连续测试时间的增长而逐渐升高
Figure 4. The ratio of 11Bbackground/11Bsensitivity increased during the continuous test when using the 2% HNO3 rinse solution
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图 5 在不同类型洗液组合下11B背景信号随时间的变化
① PFA雾化器，1%硝酸酸化NaF溶液清洗1min，随后2%硝酸继续清洗；② PFA雾化器，纯水清洗1min后2%硝酸继续清洗；③ PFA雾化器，纯水清洗2min后2%硝酸继续清洗；④ 玻璃雾化器，0.1%氨水清洗1min后2%硝酸继续清洗；⑤ 玻璃雾化器，纯水清洗1min后2%硝酸继续清洗。
Figure 5. The variations in 11B background signal over time under various combinations of lotions
①100µL/min PFA nebulizer was used. The B background was rinsed by 0.6mg/g NaF in 1% HNO3 solution for 1min before a normal cleaning by 2% HNO3 solution; ②100µL/min PFA nebulizer was used. The B background was rinsed by water for 1min before a normal cleaning by 2% HNO3 solution; ③100µL/min PFA nebulizer was used. The B background was rinsed by water for 2min before a normal cleaning by 2% HNO3 solution; ④100µL/min glass nebulizer was used. The B background was rinsed by 0.1% (V/V) ammonia for 1min before a normal cleaning by 2% HNO3 solution; ⑤100µL/min glass nebulizer was used. The B background was rinsed by water for 1min before a normal cleaning by 2% HNO3 solution.
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图 6 标准样品IRMM-016(a)、USTC-Li(b)、Alfa Li(c)和JG-2(d)相对于L-SVEC的δ7Li测定值及其参考值
红色方框及其误差限表示参考值和参考值变化范围或其测试外精度(2SD)。样品误差表示单次测试的外精度(2SD)。
Figure 6. The measured δ7LiL-SVEC of the standards and their reference values. The red box and its errors bars represent the reference value and the range of variation of the values or their external precisions (2SD). The sample error bars represent external precisions (2SD) of individual tests.
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图 7 标准样品EPM-AE 121(a)、EPM-AE 122(b)、Alfa B(c)和NASS-7(d)相对于NIST951a的δ11B测定值及其参考值
红色方框及其误差限表示参考值平均值和参考值变化范围。样品误差限表示单次测试的外精度(2SD)。
Figure 7. The measured δ11BNIST951a of the standards and the reference values. The red box and its errors bars represent the reference value and the range of variation of the values or their external precisions (2SD). The sample error bars represent external precisions (2SD) of individual tests.
表 1 Nu Sapphire MC-ICP-MS仪器基本配置
Table 1 Basic configuration of the Nu Sapphire MC-ICP-MS instrument
工作参数 实验条件 工作参数 实验条件 射频功率 1300W 炬管 石英炬管 加速电压 6000V，高能模式 雾化器气体(Ar)压力 ～30psi 冷却气(Ar)流速 13L/min 锥 镍锥，湿锥 辅助气(Ar)流速 0.9～1.0L/min 接收器设置 H6-11B；L6-10B
H9-6Li；L6-7Li雾化器及流速 玻璃雾化器，PFA雾化器，100/50μL/min 分辨率 低分辨 
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表 2 标准物质锂同位素组成测定值与参考值
Table 2 The measured and reported δ7LiL-SVEC values of standards
标准物质 δ7LiL-SVEC参考值
(‰)δ7LiL-SVEC测试值
(‰)2SD
(‰)n IRMM-016 −0.17～0.40a 0.12 0.07 50 USTC-Li −19.30b −19.37 0.12 56 Alfa Li − 13.71 0.13 73 JG-2 0.15～0.32c 0.13 0.11 12 注：“−”表示无参考值。Alfa Li为实验室内部标准溶液，无参考值。a. IRMM-016的δ7LiL-SVEC参考值来源于GeoReM(Geological and Environmental Reference Materials)数据库及对应参考文献，仅统计MC-ICP-MS测试结果。b. USTC-Li 的δ7LiL-SVEC参考值由中国科技大学肖益林教授提供。c. JG-2的δ7LiL-SVEC参考值来源于Bouman等(2004)［30］、Jeffcoate等(2004)［15］、Li 等(2019)［31］、Lin等(2016)［32］和Zhu等(2020)［33］。Note:“−” indicates the absence of a reference value. Alfa Li serves as the internal standard solution in the laboratory，and no reference value is available. a. The δ7LiL-SVEC reference value of IRMM-016 is derived from the Geological and Environmental Reference Materials (GeoReM) database and corresponding references，considering only the MC-ICP-MS test results. b. The δ7LiL-SVEC reference value of USTC-Li is provided by Professor Xiao Yilin from the University of Science and Technology of China. c. The δ7LiL-SVEC reference values of JG-2 are derived from Bouman et al. (2004) [30］，Jeffcoate et al. (2004) ［15］，Li et al. (2019) ［31］，Lin et al. (2016) ［32］ and Zhu et al. (2020) ［33］.
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表 3 标准物质硼同位素组成测定值与参考值
Table 3 The determined values and reference values of the B isotope composition in the reference materials
标准物质 δ11BNIST951参考值
(‰)δ11BNIST951a测试值
(‰)2SD
(‰)n ERM-AE121 19.54～20.33a 19.78 0.31 64 ERM-AE122 39.3～39.74a 39.54 0.33 36 Alfa B − −4.66 0.19 60 海水标样NASS-7 − 40.03 0.33 35 天然海水 39.98±0.35b − − − 天然海水 39.45～40.26b − − − 海水标样NASS-2 39.63～39.90c − − − 海水标样NASS-5 39.40～39.89c − − − 海水标样NASS-6 39.41～39.81c − − − 注：“−”表示无参考值。Alfa B为实验室内部标准溶液，无参考值。a. ERM-AE121和ERM-AE122的δ11BNIST 951参考值来自GeoReM数据库及对应参考文献。数据包括了样品相对于NIST SRM951和NIST SRM951a的参考值。b. 天然海水δ11BNIST 951参考值引自Chen等(2019)［27］及其中参考文献。c. NASS系列海水δ11BNIST 951参考值引自GeoReM数据库及对应参考文献。Note: “−” indicates the absence of a reference value. Alfa B is a laboratory standard solution without a designated reference value. a. The δ11BNIST 951 reference values of ERM-AE121 and ERM-AE122 are from the GeoReM database and corresponding references. The data include reference values of the sample with respect to NIST SRM951 and NIST SRM951a. b. The reference value of δ11BNIST 951 in natural seawater is cited from Chen et al. (2019) ［27］ and other references. c. The reference value of δ11BNIST 951 for NASS series seawater is sourced from the GeoReM database and corresponding references.
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