测试项目：Pb同位素比值分析

测试对象：岩石、土壤、沉积物、海水、地下水

测试周期：45-90个工作日，可提供样品测试加急服务。

送样要求：

完成标准：前处理在超净室100级超净台内进行，保证监测空白及样品无污染，标样和重复样在允许误差范围内。
标样数据：

方法描述:
12.1全岩Pb同位素比值分析
全岩Pb同位素前处理和测试由武汉上谱分析科技有限责任公司完成。
前处理流程：
前处理在配备100级操作台的千级超净室完成。样品消解：（1）将200目样品置于105℃烘箱中烘干12小时；（2）准确称取粉末样品50-200mg置于Teflon溶样弹中；（3）先后依次缓慢加入1-3ml高纯HNO₃和1-3ml高纯HF；（4）将Teflon溶样弹放入钢套，拧紧后置于190℃烘箱中加热24小时以上；（5）待溶样弹冷却，开盖后置于140℃电热板上蒸干，然后加入1ml HNO₃ 并再次蒸干；（6）用1.0M HBr溶解样品，待上柱分离。化学分离：用离心机将样品离心后，取上清液上柱。柱子填充AG树脂。用1.0M HBr淋洗去除基体元素。最终用6.0M HCl将Hf从柱上洗脱并收集。收集的Hf溶液蒸干后等待上机测试。
仪器测试流程：
Pb同位素分析采用德国Thermo Fisher Scientific 公司的MC-ICP-MS（Neptune Plus）。仪器配备9个法拉第杯接收器。²⁰⁴（Pb+Hg）、²⁰⁶Pb、²⁰⁷Pb、²⁰⁸Pb、²⁰³Tl、²⁰⁵Tl和²⁰²Hg同时被7个接收器接收。其中²⁰²Hg被用于监控并校正²⁰⁴Hg对²⁰⁴Pb同位素的同质异位素干扰，²⁰⁴Hg/²⁰²Hg天然比值采用0.2301。MC-ICP-MS采用了Jet+X锥组合和干泵以提高仪器灵敏度。根据样品中的Pb含量，50 µl/min-100 µl/min两种微量雾化器被选择使用。Alfa公司的Pb单元素溶液被用于优化仪器操作参数。Pb国际标准溶液NIST 981 （200µg/L）作为质量监控和外部校正标样，²⁰⁸Pb的信号一般高于6V。每个标样和样品中加入一定量的Tl溶液（NBS SRM 997），²⁰⁵Tl信号控制在4-5之间。Tl拖尾对²⁰⁴Pb的干扰没有被观察到。数据采集由8个blocks组成，每个block含10个cycles，每个cycle为4.194秒。Pb同位素的仪器质量分馏采用假内标指数法则校正（Russell et al. 1978）：

公式中i和j指示同位素质量数，R_m和R_T分别代表样品的测试比值和参考比值（推荐值），f指仪器质量分馏因子。²⁰⁵Tl/²⁰³Tl被用于计算Pb的质量分馏因子（2.38714，NIST SRM 997证书值）。由于Tl的分馏行为与Pb分馏行为存在一定差别，因此NIST 981被用作外标对数据进行二次校正。实验流程采用两个Pb同位素标样（NIST 981和AlfaPb）之间插入7个样品进行分析。全部分析数据采用专业同位素数据处理软件“Iso-Compass”进行数据处理（Zhang et al., 2020）。NIST 981的^20xPb/²⁰⁴Pb的外部测试精度达到0.03%（2RSD）。NIST 981的推荐值采用²⁰⁸Pb/²⁰⁴Pb=36.7262±31，²⁰⁷Pb/²⁰⁴Pb=15.5000±13，²⁰⁶Pb/²⁰⁴Pb=16.9416±13（n=119，Baker et al. 2004）。
BCR-2（玄武岩）被选择作为流程监控标样。BCR-2的^20xPb/²⁰⁴Pb分析测试值为²⁰⁸Pb/²⁰⁴Pb=38.736±17，²⁰⁷Pb/²⁰⁴Pb=15.628±3，²⁰⁶Pb/²⁰⁴Pb=18.756±10 （2SD, n=22），与推荐值²⁰⁸Pb/²⁰⁴Pb=38.725±22，²⁰⁷Pb/²⁰⁴Pb=15.621±4，²⁰⁶Pb/²⁰⁴Pb=18.753±8（Zhang and Hu 2020）在0.03%误差范围内一致。数据表明，本实验流程可以对样品进行有效分离，分析准确度和精密度满足高精度的Pb同位素分析。
本测试方法适用Pb含量>2ppm的岩石样品，保证样品^20xPb/²⁰⁴Pb测试内精度（2RSE）0.002%~0.025%），测试准确度优于0.03%。Pb含量低于2ppm的岩石样品，测试精度和准确度会受到影响，影响程度受样品Pb含量控制。低Pb样品分析请事先咨询技术人员，确保样品分析质量。
12.2 Scheme for Pb isotope ratio analyses using MC-ICP-MS
All chemical preparations were performed on class 100 work benches within a class 1000 over-pressured clean laboratory. Sample digestion: (1) Sample powder (200 mesh) were placed in an oven at 105 ℃ for drying of 12 hours; (2) 50-200 mg sample powder was accurately weighed and placed in an Teflon bomb; (3) 1-3 ml HNO₃ ａnd 1-3 ml HF were added into the Teflon bomb; (4) Teflon bomb was putted in a stainless steel pressure jacket and heated to 190 ℃ in an oven for >24 hours; (5) After cooling, the Teflon bomb was opened and placed on a hotplate at 140 ℃ and evaporated to incipient dryness, and then 1 ml HNO₃ was added and evaporated to dryness again; (6) The sample was dissolved in 1.0 mL of 1.0 M HBr. Column chemistry: After centrifugation, the supernatant solution was loaded into an ion-exchange column packed with AG resin. After complete draining of the sample solution, columns were rinsed with 1.0 M HBr to remove undesirable matrix elements. Finally, the Pb fraction was eluted using 6.0 M HCl and gently evaporated to dryness prior to mass-spectrometric measurement.
Pb isotope analyses were performed on a Neptune Plus MC-ICP-MS (Thermo Fisher Scientific, Dreieich, Germany) at the Wuhan Sample Solution Analytical Technology Co., Ltd, Hubei, China. The Neptune Plus, a double focusing MC-ICP- MS, was equipped with seven fixed electron multiplier ICs, and nine Faraday cups fitted with 10¹¹ Ω resistors. The faraday collector configuration of the mass system was composed of an array to monitor ²⁰⁴(Pb+Hg）、²⁰⁶Pb、²⁰⁷Pb、²⁰⁸Pb、²⁰³Tl、²⁰⁵Tl和²⁰²Hg. The large dry interface pump (120 m³ hr^-1 pumping speed) and newly designed X skimmer cone and Jet sample cone were used to increase the instrumental sensitivity. Pb single element solution from Alfa (Alfa Aesar, Karlsruhe, Germany) was used to optimize instrument operating parameters. An aliquot of the international standard solution of 100 μg L⁻¹ NIST 981 was used regularly for uating the reproducibility and accuracy of the instrument. Typically, the signal intensities of ²⁰⁸Pb⁺ in NIST 981 were > ~6.0 V. The Pb isotopic data were acquired in the static mode at low resolution. The routine data acquisition consisted of ten blocks of 10 cycles (4.194 s integration time per cycle). The total time of one measurement lasted about 7 min.
The exponential law, which initially was developed for TIMS measurement (Russell et al. 1978) ａnd remains the most widely accepted and utilized with MC-ICP-MS, was used to assess the instrumental mass discrimination in this study. Mass discrimination correction was carried out via normalization to a ²⁰⁵Tl/²⁰³Tl ratio of 2.38714 (the certified value of NIST SRM 997). All data reduction for the MC-ICP-MS analysis of Hf isotope ratios was conducted using “Iso-Compass” software (Zhang et al. 2020). Because of the difference of the mass bias behaviors between Pb and Tl, all measured ^20xPb/²⁰⁴Pb ratios of unknown samples were normalized to the well-accepted NIST 981 values of ²⁰⁸Pb/²⁰⁴Pb=36.7262±31，²⁰⁷Pb/²⁰⁴Pb=15.5000±13，²⁰⁶Pb/²⁰⁴Pb=16.9416±13（n=119，Baker et al. 2004）. One NIST 981 standard was measured every ten samples analyzed. Analyses of NIST 981 standard yielded external precisions of 0.03% (2RSD) for ^20xPb/²⁰⁴Pb ratios. In addition, the USGS reference materials BCR-2 (basalt) yielded results of ²⁰⁸Pb/²⁰⁴Pb=38.736±17, ²⁰⁷Pb/²⁰⁴Pb=15.628±3, ²⁰⁶Pb/²⁰⁴Pb=18.756±10 (2SD, n=22)，respectively, which is identical within error of 0.03% to their published values (Zhang and Hu 2020).
References
Russell, W.A., Papanastassiou, D.A., Tombrello, T.A., (1978). Ca isotope fractionation on the earth and other solar system materials. Geochim. Cosmochim. Acta, 42 (8), 1075–1090.
Zhang W., Hu Z.C. (2020). Estimation of isotopic reference values for pure materials and geological reference materials. At. Spectrosc. 2020, 41 (3), 93–102.
Zhang W., Hu Z.C., Liu Y.S. (2020). Iso-Compass: new freeware software for isotopic data reduction of LA-MC-ICP-MS. J. Anal. At. Spectrom., 2020, 35, 1087–1096.
Baker, J., Peate, D., Waight, T., Meyzen, C. (2004). Pb isotopic analysis of standards and samples using a 207Pb–204Pb double spike and thallium to correct for mass bias with a double-focusing mc-icp-ms. Chemical Geology, 211(3–4), 275-303