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摘要:
Sr、Nd、U等同位素体系被广泛应用于地球表生过程中年代测定及物源示踪等研究, 高效地分离这些同位素体系，对于推广这些同位素方法的应用具有重要现实意义。若要同时分析地质样品中Sr、Nd、U三种元素的同位素，现有方法往往需要消解两份样品，一份用于Sr-Nd而另一份用于U的分离提纯。这种方法不但增加了样品用量，而且需要多次蒸干溶液转换介质，既延长了分离流程也增加了样品被污染的风险。为了提高样品利用率和分析效率，本文通过将树脂柱串联改进了分离流程，提出一种仅需消解一份样品，便可同时提取Sr、Nd、U三种元素的新方法。本方法中Sr的分离采用Sr特效树脂，包含Nd在内的稀土元素(REE)的分离采用AG50W-X8树脂，U的分离采用UTEVA特效树脂。实验中将三种树脂柱串联，采用3mol/L硝酸淋洗液淋洗，同步进行平衡树脂、上样、洗杂志，避免了蒸干操作。分离后的淋出液使用电感耦合等离子体质谱仪(ICP-MS)测试元素含量。结果表明：U的回收率接近99.9%，Sr的回收率超过90%，Nd的回收率超过80%；同时三种树脂柱串联的分离流程，主要基体元素(K、Ca、Na、Ba、Fe、Rb等)的去除率均超过99%，降低了对Sr、Nd、U高精度同位素分析的干扰；REE中的Sm则可以通过后续使用Ln树脂等进一步去除。此外，本文还交换了Sr特效树脂和UTEVA树脂的位置，比对两种不同串联顺序对分离结果的影响，结果表明两种树脂柱串联顺序对目标元素的分离并无显著影响。使用该方法可以有效地实现Sr、Nd、U的分离，在减少操作步骤的同时节省约一半的样品用量，提高了同位素分析效率。
Abstract:BACKGROUND Uranium-series nuclides are one of the three major radioactive decay systems, which are suitable for studying various geological processes at different time scales. In addition, 87Sr/86Sr and 143Nd/144Nd isotopes (Sr-Nd isotopes for short) are two commonly used isotopes, for rock dating, chemical weathering assessment and tracing sediment sources. In this case, the combination of Sr-Nd-U isotopes can provide more comprehensively knowledge of the element cycle on the earth's surface and deepen our understanding of the sediment "Source to Sink"processes.
Most previous studies involving the Sr-Nd-U isotopes have been established by separating Sr-Nd and U isotopes respectively. In this way, the digestion operation must be performed twice, one for separating Sr-Nd and the other for separating U. Alternatively, if only one sample is digested for measuring all three isotopes, between each element separation, the residue must be dried and dissolved in another solution so as to start a new column work. The former increases the amount of samples, which is not conducive to analysis of precious and trace samples; the latter adds additional drying operation, which is time consuming and increases the risk of sample loss and contamination.
OBJECTIVES To establish a new Sr-Nd-U combined separation scheme. In this method, only one sample is dissolved, avoiding solution transfer between each separation, so as to reduce sample amount and improve the efficiency of separation and purification of Sr-Nd-U isotopes.
METHODS A new chromatographic scheme of separating Sr-Nd-U with one sample digestion using a tandem column scheme is presented. Three columns were overlain sequentially to separate Sr in Sr Spec column, Nd in AG50W-X8 column and U in UTEVA column. 3mol/L HNO3 was used to pre-condition, load the sample, and rinse the matrix. After rinsing the matrix, the tandem column was separated to 3 independent columns to elute the target elements (Sr, Nd, U) respectively.
As the connection sequence of different resin columns may interfere with the recovery of target elements, two different chromatographic schemes were compared. In Scheme 1, U column was placed on top of Sr column, while in Scheme 2 the positions of U column and Sr resin column were exchanged. In both schemes, the AG50W-X8 resin column was set at the bottom as the cationic resin can adsorb the most complex elements.
All separated elution was tested for element concentration using inductively coupled plasma-mass spectrometry (ICP-MS). The basalt standard sample (BCR-2) was used to examine the behavior and recovery of each element in the separation procedure.
RESULTS A total of 10 fractions were recovered from the tandem column scheme, among which fraction 1 represented the leachate recovered by loading samples and rinsing matrix from the tandem three columns. Fraction 2, 5 and 8 represented the leachate recovered by Sr Spec resin, AG50W-X8 resin and UTEVA resin respectively rinsing matrix after separation of the three columns. Fraction 3, 6 and 9 represented the leachate recovered by Sr, REE and U columns, which was Sr, Nd and U collection. Fraction 4, 7 and 10 represented the leachate from each resin recycle stage.
In either Scheme 1 or Scheme 2, most matrix elements (high content of K, Ca, Na, Mg, Al, Fe, Ti and P and low content of Rb, Hf and Th) were mainly concentrated in fraction 1. The elution rate of Na, Ti, Rb and Hf was up to 99%. The elution rate of K and Ca was slightly lower at about 85% and the elution rate of Fe was about 56%. Sr was mainly concentrated in fraction 3, which contained only a small amount of P and Ba. Nd was mainly concentrated in fraction 6, which also contained both Sm and Ce. U was mainly concentrated in fraction 9, which only contained a very small amount of P and Pb. The column recovery was almost 99.9% for U, 90% for Sr and over 80% for Nd.
The removal rate of major matrix elements (K, Ca, Na, Ba, Fe, Rb, etc.) exceeded 99%, which reduced interference with high-precision isotope analysis of Sr, Nd, and U. The recovery and purity of Sr, Nd, U were all quite high. A very small amount of P and Ba in fraction 3 had no interference with Sr isotopes (87Sr/86Sr). The Rb which was isomorphism of Sr was removed completely. With regard to Sm and Ce in fraction 6, previous studies had shown that 142Ce could not interfere with Nd isotope (143Nd/144Nd), and Sm could be further separated by Ln resin, so as not to affect Nd isotopic test. Fraction 9 contained nearly 100% U with no other elements.
The sequence of resin column splicing is a crucial consideration which may impact the element separation. Hence, the position of Sr Spec column and UTEVA column was exchanged to compare the influence of different column sequences on eluting target elements. Both Scheme 1 and Scheme 2 can effectively wash off most of the matrix elements, and the target elements Sr, Nd and U can be efficiently adsorbed on the resin. There is no significant difference on target element separation between the two different column sequences. This indicates that Sr Spec and UTEVA resins do not interfere with each other on the target elements.
CONCLUSIONS The new chromatographic scheme of separating Sr-Nd-U with one sample digestion using a tandem column scheme can be used to quickly and efficiently separate Sr, Nd and U elements from silicate rock samples. The recovery rate for U, Sr and Nd is 99.9%, 92.5% and 82.1%, respectively, which meet the requirements of subsequent isotope analysis. This Sr-Nd-U combined separation method can be used to reduce the sample consumption by about 50%, which is beneficial to the analysis of precious and trace samples. Meanwhile, as no solution transfer is needed between each column separation, this method can also save time for column work and increase the efficiency of chemical separation. A new idea for Sr-Nd-U multi-isotope separation is provided. If the recovery of Pb in the fraction 4 of this chromatographic scheme can be improved in further studies, the application of this new method may be expanded to more fields in the future.
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Key words:
- Sr /
- Nd /
- U /
- tandem resin column /
- column recovery /
- isotopic separation /
- inductively coupled plasma-mass spectrometry
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表 1 Sr、Nd、U同位素联合过柱分离流程
Table 1. Procedure of combined Sr, Nd, U isotopes separation
流程 分离步骤 树脂柱 淋洗试剂 试剂总体积
(mL)馏分
编号①
预清洗本底清洗 Sr特效树脂 3mol/L硝酸 6 - 超纯水 6 - 3mol/L硝酸 5 - 超纯水 5 - 3mol/L硝酸 5 - 超纯水 5 - AG50W-X8
树脂6mol/L盐酸 15 - 超纯水 5 - UTEVA特效
树脂3mol/L硝酸 4 - 3mol/L盐酸 4 - 3mol/L盐酸 4 - 超纯水 4 - ②
三柱串联平衡树脂 - 3mol/L硝酸 3 - 上样 3mol/L硝酸 2 1 洗杂质 3mol/L硝酸 6 ③
三柱分离洗杂质 Sr特效
树脂3mol/L硝酸 9 2 收集Sr 超纯水 4 3 回收树脂 6mol/L盐酸+超纯水 11 4 洗杂质 AG50W-X8
树脂2.5mol/L盐酸 4 5 收集REE 6mol/L盐酸 10 6 回收树脂 6mol/L盐酸 5 7 洗杂质 UTEVA特效
树脂3mol/L盐酸 6 8 收集U 1mol/L盐酸 4 9 回收树脂 超纯水+3mol/L硝酸 +3mol/L盐酸 +1mol/L盐酸 20 10 
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