測試項目：Pb同位素比值分析

測試對象：巖石、土壤、沉積物、海水、地下水

測試周期：45-90個工作日，可提供樣品測試加急服務(wù)。

送樣要求：

完成標(biāo)準(zhǔn)：前處理在超凈室100級超凈臺內(nèi)進(jìn)行，保證監(jiān)測空白及樣品無污染，標(biāo)樣和重復(fù)樣在允許誤差范圍內(nèi)。
標(biāo)樣數(shù)據(jù)：

方法描述:
12.1全巖Pb同位素比值分析
全巖Pb同位素前處理和測試由武漢上譜分析科技有限責(zé)任公司完成。
前處理流程：
前處理在配備100級操作臺的千級超凈室完成。樣品消解：（1）將200目樣品置于105℃烘箱中烘干12小時；（2）準(zhǔn)確稱取粉末樣品50-200mg置于Teflon溶樣彈中；（3）先后依次緩慢加入1-3ml高純HNO₃和1-3ml高純HF；（4）將Teflon溶樣彈放入鋼套，擰緊后置于190℃烘箱中加熱24小時以上；（5）待溶樣彈冷卻，開蓋后置于140℃電熱板上蒸干，然后加入1ml HNO₃ 并再次蒸干；（6）用1.0M HBr溶解樣品，待上柱分離?；瘜W(xué)分離：用離心機將樣品離心后，取上清液上柱。柱子填充AG樹脂。用1.0M HBr淋洗去除基體元素。最終用6.0M HCl將Hf從柱上洗脫并收集。收集的Hf溶液蒸干后等待上機測試。
儀器測試流程：
Pb同位素分析采用德國Thermo Fisher Scientific 公司的MC-ICP-MS（Neptune Plus）。儀器配備9個法拉第杯接收器。²⁰⁴（Pb+Hg）、²⁰⁶Pb、²⁰⁷Pb、²⁰⁸Pb、²⁰³Tl、²⁰⁵Tl和²⁰²Hg同時被7個接收器接收。其中²⁰²Hg被用于監(jiān)控并校正²⁰⁴Hg對²⁰⁴Pb同位素的同質(zhì)異位素干擾，²⁰⁴Hg/²⁰²Hg天然比值采用0.2301。MC-ICP-MS采用了Jet+X錐組合和干泵以提高儀器靈敏度。根據(jù)樣品中的Pb含量，50 µl/min-100 µl/min兩種微量霧化器被選擇使用。Alfa公司的Pb單元素溶液被用于優(yōu)化儀器操作參數(shù)。Pb國際標(biāo)準(zhǔn)溶液NIST 981 （200µg/L）作為質(zhì)量監(jiān)控和外部校正標(biāo)樣，²⁰⁸Pb的信號一般高于6V。每個標(biāo)樣和樣品中加入一定量的Tl溶液（NBS SRM 997），²⁰⁵Tl信號控制在4-5之間。Tl拖尾對²⁰⁴Pb的干擾沒有被觀察到。數(shù)據(jù)采集由8個blocks組成，每個block含10個cycles，每個cycle為4.194秒。Pb同位素的儀器質(zhì)量分餾采用假內(nèi)標(biāo)指數(shù)法則校正（Russell et al. 1978）：

公式中i和j指示同位素質(zhì)量數(shù)，R_m和R_T分別代表樣品的測試比值和參考比值（推薦值），f指儀器質(zhì)量分餾因子。²⁰⁵Tl/²⁰³Tl被用于計算Pb的質(zhì)量分餾因子（2.38714，NIST SRM 997證書值）。由于Tl的分餾行為與Pb分餾行為存在一定差別，因此NIST 981被用作外標(biāo)對數(shù)據(jù)進(jìn)行二次校正。實驗流程采用兩個Pb同位素標(biāo)樣（NIST 981和AlfaPb）之間插入7個樣品進(jìn)行分析。全部分析數(shù)據(jù)采用專業(yè)同位素數(shù)據(jù)處理軟件“Iso-Compass”進(jìn)行數(shù)據(jù)處理（Zhang et al., 2020）。NIST 981的^20xPb/²⁰⁴Pb的外部測試精度達(dá)到0.03%（2RSD）。NIST 981的推薦值采用²⁰⁸Pb/²⁰⁴Pb=36.7262±31，²⁰⁷Pb/²⁰⁴Pb=15.5000±13，²⁰⁶Pb/²⁰⁴Pb=16.9416±13（n=119，Baker et al. 2004）。
BCR-2（玄武巖）被選擇作為流程監(jiān)控標(biāo)樣。BCR-2的^20xPb/²⁰⁴Pb分析測試值為²⁰⁸Pb/²⁰⁴Pb=38.736±17，²⁰⁷Pb/²⁰⁴Pb=15.628±3，²⁰⁶Pb/²⁰⁴Pb=18.756±10 （2SD, n=22），與推薦值²⁰⁸Pb/²⁰⁴Pb=38.725±22，²⁰⁷Pb/²⁰⁴Pb=15.621±4，²⁰⁶Pb/²⁰⁴Pb=18.753±8（Zhang and Hu 2020）在0.03%誤差范圍內(nèi)一致。數(shù)據(jù)表明，本實驗流程可以對樣品進(jìn)行有效分離，分析準(zhǔn)確度和精密度滿足高精度的Pb同位素分析。
本測試方法適用Pb含量>2ppm的巖石樣品，保證樣品^20xPb/²⁰⁴Pb測試內(nèi)精度（2RSE）0.002%~0.025%），測試準(zhǔn)確度優(yōu)于0.03%。Pb含量低于2ppm的巖石樣品，測試精度和準(zhǔn)確度會受到影響，影響程度受樣品Pb含量控制。低Pb樣品分析請事先咨詢技術(shù)人員，確保樣品分析質(zhì)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