Determination of Copper Isotope Composition of Soil Reference Materials by MC-ICP-MS
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摘要:
近年来，铜同位素在表生环境和生物地球化学中的应用越来越广泛，尤其是土壤的铜同位素组成可以示踪环境污染物来源及生物地球化学过程。目前，对土壤铜同位素进行研究时，主要以硅酸岩标准物质为标样来衡量土壤样品铜同位素测定的准确性和精确性。但土壤与硅酸岩中铜、基质离子及有机质的含量等存在很大差异（如：硅酸岩中的铜含量>80μg/g，一些土壤中的铜含量很低， < 20μg/g），将硅酸岩标准物质作为标样来监测土壤样品的数据质量缺乏代表性。为了弥补这一缺陷，本文精确测定4个国家土壤标准物质（GBW07443、GBW07425、GBW07427、GBW07389）的铜同位素组成，并将其作为检验土壤样品铜同位素测定过程中的标准。实验中采用高温高压反应釜消解样品，利用AG MP-1M树脂进行纯化，全流程空白 < 2ng，回收率≥ 98%，通过多接收器电感耦合等离子体质谱仪（MC-ICP-MS）采用标样-样品-标样间插法进行仪器分馏校正，δ65Cu的长期测试外精度优于0.05‰（n=306，2SD）。GBW07443、GBW07425、GBW07427和GBW07389的铜同位素组成分别为-0.04‰±0.04‰（n=9，2SD）、-0.07‰±0.05‰（n=12，2SD）、-0.06‰±0.04‰（n=12，2SD）、-0.02‰±0.06‰（n=12，2SD）。这些土壤标准物质的铜同位素组成均位于0附近，大致为自然界土壤铜同位素比值变化范围（-0.5‰~+0.5‰）的中间值，且样品容易获得，其化学和铜同位素组成均一，适合作为监控土壤铜同位素化学及质谱分析数据可靠性的标准物质。
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关键词:
- 土壤标准物质 /
- 铜同位素 /
- AG MP-1M树脂 /
- 标样-样品-标样间插法 /
- 多接收器电感耦合等离子体质谱法
Abstract:BACKGROUND In recent years, copper isotopes have been widely applied in supergene environments and biogeochemical processes, acting as novel tracers for soil pollution and biogeochemical cycles during pedogenesis. To date, Cu isotope studies on natural soils have commonly analyzed basalt geostandards for monitoring data quality. However, the contents of copper, matrix ions, and organic matter in the soil and silicate rock are very different. For example, the copper content in silicate rock is >80μg/g, and the copper content in some soils is very low, that is, < 20μg/g. The use of silicate standard materials as reference samples to monitor the data quality of soil samples lacks representativeness.
OBJECTIVES To provide new soil standards for high-precision Cu isotope analysis, this study reports high-precision copper isotope data for four soil reference materials (GBW07443, GBW07425, GBW07427, and GBW07389), expressed as δ65Cu relative to NIST SRM 976, which were measured using a multi-collector inductively coupled plasma-mass spectrometer (MC-ICP-MS).
METHODS Soil samples were completely dissolved in high-pressure bombs in a muffle furnace. Complete separation of Cu from the matrices was obtained using a strong anion exchange resin (AG MP-1M). The instrument mass bias was corrected using the standard sample-standard method.
RESULTS The long-term external reproducibility was higher than ±0.05‰ (n=306, 2SD). Cu isotopic compositions of the four soil reference materials, GBW07443, GBW07425, GBW07427, and GBW07389, from the China National Bureau of Standards were -0.04‰±0.04‰ (n=9, 2SD); -0.07‰±0.05‰ (n=12, 2SD); -0.06‰±0.04‰ (n=12, 2SD); and -0.02‰±0.06‰ (n=12, 2SD), respectively.
CONCLUSIONS The δ65Cu values of these reference materials were close to zero and corresponded to the intermediate values of natural soils. Moreover, the sample was easy to obtain, and the experimental results showed uniformity in its chemical and copper isotopic compositions, making it suitable as a standard material for monitoring the reliability of the soil copper isotope chemistry and the mass spectrometry data.
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表 1 GBW07443、GBW07425、GBW07427和GBW07389土壤标准物质化学成分参考值
Table 1. Reference values of components in GBW07443, GBW07425, GBW07427 and GBW07389
标准物质编号 各成分含量(%) 各成分含量(μg/g) SiO2 Al2O3 CaO MgO Na2O K2O TFe2O3 Ti Co Cu GBW07443
(水稻土)- - 1.48 - - 2.8 7.03 - 19.5 47 GBW07425
(辽河平原土壤)69.42 13.14 1.33 1.2 1.98 2.7 4.21 0.39 11.6 21.4 GBW07427
(华北平原土壤)64.88 11.76 5 2.05 1.86 2.27 4.11 0.38 11.3 21.6 GBW07389
(泛滥平原沉积物)59.68 12.62 6.91 2.24 1.62 2.4 4.73 0.37 13 25 注：原始数据来自中国国家标准局官网：http://www.crmrm.com/。GBW07443是土壤形态成分分析标准物质(水稻土)，缺乏部分全岩成分数据。 
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表 2 土壤样品的铜分离纯化方法(改自Liu等，2014[33])
Table 2. Protocols of ion-exchange chromatographic conditions of Cu (Modified from Liu, et al, 2014[33])
试剂 体积(mL) 纯化步骤 AG MP-1M (100~200目) 2 加树脂 8mol/L盐酸+0.001%双氧水 10 调环境 样品+8mol/L盐酸+0.001%双氧水 1 上样 8mol/L盐酸+0.001%双氧水 9 淋洗基质离子 8mol/L盐酸+0.001%双氧水 28 收集铜
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表 3 MC-ICP-MS仪器主要工作参数
Table 3. Working conditions of MC-ICP-MS instrument
工作参数 实验条件 工作参数 实验条件 工作电压 2000V 冷却气流速 16L/min 射频功率 1250W 辅助气流速 1.0L/min Cu灵敏度 50V/(mg·mL-1) 雾化气流速 1.0L/min 截取锥 镍锥(X) 法拉第接收杯L3 63Cu 分辨率 低(LR) 法拉第接收杯L1 65Cu 进样速率 100μL/min 积分时间 4.097s 样品载气 Ar 测试block(cycle) 3(30)
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表 4 本文报道的土壤标准物质铜同位素组成(相对于NIST SRM 976)
Table 4. Analytical results of Cu isotope composition of soil reference materials (relative to NIST SRM 976)
标准物质编号 土壤类型 Cu含量(μg/g) δ65Cu(‰) 2SD Ti/Cu n 测定值 平均值 测定值 平均值 GBW07443
(GSF-3)水稻土 47 -0.04 -0.03
-0.06-0.04 0.03 0.04
0.040.04 0.005 0.004
0.0019 GBW07425
(GSS-11)辽河平原土壤 21.4 -0.05 -0.04
-0.10 -0.09-0.07 0.02 0.07
0.04 0.070.05 0.028 0.020
0.021 0.01012 GBW07427
(GSS-13)华北平原土壤 21.6 -0.04 -0.06
-0.07 -0.06-0.06 0.02 0.05
0.05 0.040.04 0.029 0.004
0.005 0.01012 GBW07389
(GSS-33)泛滥平原沉积物 25 -0.02 -0.02
-0.01 -0.04-0.02 0.07 0.03
0.07 0.060.06 0.011 0.033
0.001 0.02912 注：表中Ti/Cu值代表三次化学纯化后溶液中Ti与Cu的摩尔比值。
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表 5 实验室标准溶液GSB Cu和NIST SRM 3114的铜同位素组成测定结果
Table 5. Analytical results of Cu isotope compositions of standard solution GSB Cu and NIST SRM 3114
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