Separation of Sr, Nd, and U from Geological Samples Using Tandem Resin Column
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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。
要点(1) 将树脂柱串联，采用相同淋洗液同步进行平衡树脂、上样、洗杂质，避免了蒸干操作。
(2) UTEVA特效树脂和Sr特效树脂的上下串联顺序，对Sr、Nd、U元素的回收率并无显著影响。
(3) 多次重复回收利用树脂可能导致树脂柱失效，影响对目标元素的吸附能力，需要及时更换新树脂。
HIGHLIGHTS(1) The resin columns are connected in series, and the same eluent is used to balance the resin, load the sample, and wash the impurities simultaneously, avoiding the operation of evaporation to dryness.
(2) Reversing the column positions, UTEVA column and Sr column, has no effect on Sr, Nd and U separation and the column recovery.
(3) Multiple reuse of resin can lead to resin column lapse, affecting the adsorption capacity of the target element, thus new resins should be used in a timely manner.
Abstract:BACKGROUNDUranium-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.
OBJECTIVESTo 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.
METHODSA 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.
RESULTSA 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.
CONCLUSIONSThe 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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Keywords:
- Sr /
- Nd /
- U /
- tandem resin column /
- column recovery /
- isotopic separation /
- inductively coupled plasma-mass spectrometry
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铀系核素是自然界三大放射性衰变体系之一，其衰变长链中不同半衰期的核素，适用于不同时间尺度地质过程的研究[1]。近年来随着测试技术的发展，铀系同位素已被广泛应用于海洋[2]、河流[3]、黄土[4]、冰川[5]等沉积物的表生地球化学研究，为沉积物搬运、流域风化和土壤侵蚀等过程提供了有效的定量约束。除了铀同位素体系，87Sr/86Sr和143Nd/144Nd同位素(简称Sr-Nd同位素)也是常见的两种同位素组合，由于它们在地表的独特行为[6-7]，而成为岩石定年[8-9]、化学风化评估[10]和沉积物来源示踪[11-13]等研究的有效手段。鉴于单一同位素指标的多解性和不确定性，近些年多同位素体系联合示踪的方法逐渐兴起，这对同位素测试分析提出了更高的要求。以234U/238U和Sr-Nd同位素为例，这些同位素指标的结合可以从多角度更全面地指示沉积物在地表的循环过程，有助于深化对沉积物“从源到汇”过程的理解[14-15]。
使用离子交换树脂对目标元素分离提纯是最常见的同位素富集分离方法，其原理是使用不同浓度种类的淋洗介质，改变树脂柱对元素的吸附能力来分离提纯目标元素。前人已经利用不同离子交换树脂以及不同淋洗流程建立了许多Sr-Nd同位素[16-18]以及U同位素[19-21]的独立分离方法，而对于同步分离多种元素的联合分离方法，目前主要集中于分离Sr、Nd、Pb、Hf等元素[22-24]。但对Sr、Nd、U元素的分离，大多数学者仍然是消解两份样品，一份用于分离Sr-Nd而另一份用于分离U[25]；或者是消解一份样品，中间通过蒸干样品转换介质，从而进行两次过柱分离[26]。前者增加了样品用量，不利于对珍贵样品(如大洋沉积物、极地钻孔等)的研究；后者增加了额外的蒸干操作，不仅耗费时间也增加了样品损失和受污染的风险。
本文在前人研究提出的分离方案基础上[27-29]，尝试建立一种Sr-Nd-U联合分离提取的新方案。通过将三种树脂柱串联组合，用相同酸介质来平衡树脂联合过柱，随后将树脂柱分开，对三种树脂柱分别淋洗得到Sr、Nd、U三种元素。该方法仅需溶解一份样品，避免了多次蒸干溶液转换介质的步骤，在减少样品用量的同时提高了三种元素分离提纯的效率。
1. 实验部分
1.1 仪器及工作条件
为了防止污染，本实验中所有样品的化学处理过程均在千级洁净度的超净分析室内进行，元素分离纯化所用的通风橱为百级工作台。分离流程所得各馏分，取1mL稀释约10倍后使用Agilent 7900电感耦合等离子体质谱仪(ICP-MS，美国Agilent公司)测试主量和微量元素含量，用于监控流程中元素回收率情况。仪器基本工作条件为：射频功率1600W，等离子气流速15L/min，载气流速0.86L/min，辅助气流速0.21L/min，采样深度8mm，雾化器为微流石英雾化器，采样时间200ms/mass。所有实验均在海洋地质国家重点实验室(同济大学)完成。
1.2 实验材料与主要试剂
Sr-Nd同位素的分离使用Sr特效树脂(50~100μm，法国Triskem公司)以及AG50W-X8阳离子交换树脂(100~200目，美国Bio-Rad公司)；U同位素的分离使用UTEVA特效树脂(100~150μm，法国Triskem公司)。本实验均采用7mm内径聚丙烯材质的树脂柱进行装填，Sr特效树脂的装填高度为1.6cm，UTEVA特效树脂的装填高度为4.2cm，AG50W-X8树脂的装填高度为5.6cm。
为有效监控样品的回收率，本实验使用玄武岩标准物质BCR-2(美国地质调查局，USGS)为实验样品，该标准物质是较常用的岩石Sr、Nd、U同位素测定标准。
所用浓硝酸、浓氢氟酸和浓盐酸(优纯级，国药集团)在使用前均通过Savillex的酸蒸馏装置进行两次亚沸蒸馏纯化。实验用水经Millipore净化系统纯化，出水的电阻率不高于18.2MΩ·cm。
1.3 实验方法
1.3.1 样品消解
准确称取50mg粉末状标准物质BCR-2，转移到聚四氟乙烯(TeflonTM)材质的消解罐中。加入浓硝酸和浓氢氟酸各1mL，盖紧消解罐盖子后放入配套的钢套内，拧紧后一同置于190℃烘箱中加热48h。消解后的样品置于电热板上蒸干，加入1mL浓硝酸重溶后再次蒸干。加入2mL 30%的硝酸，再次将消解罐放入钢套内，拧紧后放入190℃的烘箱中加热12h。最后将消解完全的溶液转移至15mL可溶性聚四氟乙烯(PFA)接样罐中，置于电热板上蒸干。过柱分离前，向装有样品的接样罐中加入2mL的3mol/L硝酸，密封后置于115℃电热板上加热2h，保证样品完全溶解[30-31]。
1.3.2 化学分离
本文所采用的Sr-Nd-U联合过柱分离方案，参考了前人的Sr-Nd分离和U分离纯化流程[21, 27-30, 32]，并加以综合和改进。前人研究表明，在3mol/L硝酸环境中，Sr特效树脂能够高效吸附Sr[33-34]，同样环境下AG50W-X8阳离子树脂也能高效吸附Nd[27, 35]，UTEVA特效树脂则能高效吸附U[21, 28]，同时都可以使用3mol/L硝酸进行洗杂质(但用量不同)。基于以上树脂特性，考虑将UTEVA特效树脂柱、Sr特效树脂柱以及AG50W-X8树脂柱串联，采用3mol/L硝酸同步进行三根树脂柱的“平衡树脂”、“上样”和“洗杂质”的操作。通过以上操作实现了仅消解一份样品，通过一次过柱，便可以使目标元素吸附于不同树脂柱上。相比需要消解两份样品的传统做法，采用本文改进的方法仅需消解一份样品，减少了约一半的样品用量。
为了消除本底，各树脂柱在上样前，分别进行预清洗。洗柱后，将三种树脂柱串联(图 1)，同步进行平衡树脂、上样和洗杂质操作。随后将三柱分开，Sr特效树脂用于分离Sr，AG50W-X8阳离子交换树脂用于分离REE，UTEVA特效树脂用于分离U。以上每一步分别接取淋出液，留待测试分析，具体流程见图 1和表 1。其中馏分1表示三柱串联阶段上样、洗杂质所得淋出液；馏分2、5、8分别表示三柱分离后Sr特效树脂、AG50W-X8树脂、UTEVA特效树脂单独洗杂质所得淋出液；馏分3、6、9则表示接收Sr、REE、U的淋出液；馏分4、7、10表示各树脂回收阶段所得淋出液。
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图 1 Sr、Nd、U同位素联合分离流程示意图步骤1：对三根树脂柱分别进行预清洗，降低本底。步骤2：三柱串联，同步进行平衡树脂、上样、洗杂质。步骤3：三柱分离单独淋洗接取目标元素。Figure 1. Schematic diagrams of the combined Sr, Nd, U isotopes separation procedure.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 不同树脂柱自上而下的连接顺序，可能会对目标元素的回收产生相互干扰。由于富集REE的阳离子树脂吸附的元素种类最为复杂，因此本文将其置于最底部。为了比较UTEVA特效树脂柱和Sr特效树脂柱上下组合顺序对元素分离效果的影响，本文设计了两种树脂柱串联方案。方案一将UTEVA特效树脂置于Sr特效树脂上方，自上而下组合顺序为：UTEVA特效树脂柱、Sr特效树脂柱、AG50W-X8树脂柱(图 1)。方案二将UTEVA特效树脂柱和Sr特效树脂柱位置交换，自上而下组合顺序为：Sr特效树脂柱、UTEVA特效树脂柱和AG50W-X8树脂柱(图 1)。
1.3.3 淋出液中主量和微量元素含量分析
取适量淋出液分别转移到对应的离心管中，用3%硝酸稀释到合适浓度后用ICP-MS进行主量和微量元素测试。主量和微量元素测试采用多元素混标进行标线校准，分析过程中采用标样BCR-2进行校正。同时前后各测量一个3%硝酸空白样品，监测环境本底，仪器测试误差小于5%。全流程空白测试结果显示U本底值小于0.042ng，Nd本底值小于0.081ng，Sr本底值小于0.409ng。依据所有馏分中测得各元素的含量，计算出各元素占全流程总接收量的百分比即为该元素的回收率。
2. 结果与讨论
2.1 树脂柱串联法对Sr-Nd-U的纯化效果
高精度同位素测试对样品中元素的纯度有较高要求，基体元素以及待测同位素的同质异位素会对测试产生很大干扰[36-37]。本实验采用玄武岩标准样品BCR-2作为测试样品进行分析，每步操作的淋出溶液通过ICP-MS进行测试，根据各步骤所得馏分测试结果计算过柱流程中的元素回收率，见图 2和图 3。馏分1为三柱串联上样、洗杂质所得淋出液；馏分2、3、4为分离Sr环节所得淋出液；馏分5、6、7为分离REE环节所得淋出液；馏分8、9、10为分离U环节所得淋出液。
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图 2 方案一中Sr-Nd-U在不同馏分中的回收率Figure 2. Recoveries of Sr-Nd-U in different fractions in procedure Ⅰ. The Sr and U recoveries are over 90%, while the Nd recovery is only 43.6% and need further separation with Sm.![]()
图 3 方案二中Sr-Nd-U在不同馏分中的回收率Figure 3. Recoveries of Sr-Nd-U in different fractions in procedure Ⅱ. The Sr and U recoveries are over 90%, while the Nd recovery is only 53.7% and need further separation with Sm.由图 2和图 3可知，无论是在方案一还是方案二中，大部分基体元素(含量较高的K、Ca、Na、Mg、Al、Fe、Ti、P以及含量较低的Rb、Hf、Th等)主要集中在馏分1中，即在上样后的洗杂质阶段就被淋洗出去。其中Na、Ti、Rb、Hf洗脱率高达99%；K、Ca洗脱率稍低，约85%；Fe洗脱率约为56%。此外本研究在三柱分离后，对各个树脂柱单独进行洗杂质操作，所得馏分编号为2、5、8，进一步洗脱了残余的少部分杂质如K、Ca、Ba、Fe。通过以上操作，成功去除了容易对后续目标元素测试产生影响的基体元素(Ca、Ba、Fe等)[24]。因此，两种树脂柱串联方案都可以有效地淋洗掉绝大部分的基体元素，并且目标元素Sr、Nd、U能够被高效地吸附在树脂柱上。
Sr元素主要集中于馏分3中，该馏分仅含有极少量的P、Ba元素。杂质中含量最高的Ba也不足2000ng，对Sr同位素主要产生干扰的Rb[17]则被完全去除，这验证了前人利用Sr特效树脂高效分离Sr、Rb的结果[27]。
U元素主要集中于馏分9，该馏分仅含有极少量的P和Pb元素，这也证实了Wang等(2013)[28]成功利用UTEVA特效树脂高纯度分离U的结果。
Nd元素主要集中于馏分6中，该馏分中还含有Sm和Ce。其中144Sm对144Nd产生干扰，142Ce则主要对142Nd产生干扰，因此Ce不会对143Nd/144Nd的测试产生明显影响[38-39]。同时杨岳衡等(2007)[40]通过测试一系列不同Ce/Nd比例的混合溶液，证实了虽然Ce的化学性质与Nd十分相似，难以在交换柱分离过程中分开，但高含量的Ce(Ce/Nd>3，一般地质样品Ce/Nd约为3)不会影响后续对Nd同位素(143Nd/144Nd)的测试[40]。当Sm的含量远小于Nd(Sm/Nd<0.04)[40]，也可以通过传统方法校正(利用147Sm/144Nd)得到Nd的含量，从而去除Sm的影响[24, 41-42]；而若Sm含量过高，前人也提出了多种解决方法[27, 35, 43]，通过Ln、P507等树脂都可以进一步纯化分离Nd、Sm和Ce。
2.2 树脂柱串联法对Sr-Nd-U的分离效果
方案一中Sr元素的回收率为92.5%，而U回收率接近99.9%。方案二中Sr元素的回收率为94.3%，U回收率高达99.8%。本文U的回收率很高，适用于U含量较低的大部分沉积物样品。Nd元素回收率较低，方案一中Nd回收率为43.6%，方案二为53.7%。
由图 2可知，串联上样洗杂质所获得的馏分1中，大约有6.8%的Nd淋失；收集REE阶段洗杂质获得的馏分5也有接近49.6%的Nd淋失。结合袁永海等(2018)[44]研究发现，AG50W-X8阳离子树脂经过10次回收再利用后，Nd元素的回收率会从90%下降到50%。通过对实验环境、材料、操作流程和仪器状态的检查核实，我们初步推测本文方法获得的Nd元素回收率较低，可能是因为AG50W-X8阳离子树脂经多次回收重复利用，使得树脂对Nd的吸附能力减弱所致。
2.3 Nd元素回收率的重新检验
为进一步查明Nd元素回收率偏低的原因，本文选用了新的AG50W-X8树脂重新充填制作色谱柱。按照方案一的流程，更新树脂后Sr的回收率为91.9%，U的回收率为99.3%，而Nd的分离回收率已经达到82.1%(图 4)。参考前人的回收率结果[18]，一般回收率超过70%便可以满足后续Nd同位素测试需要。该实验结果证实了在先前实验中Nd元素回收率偏低确实是由于AG50W-X8树脂失效导致，这也与前人结果相吻合。Zhu等[45]指出阳离子树脂柱使用超过6~7次后，目标元素回收率会低于60%；Li等[46]指出树脂柱过多地重复使用(超过5次)将会导致严重的交叉污染以及低回收率等现象。树脂柱残留的基体元素, 可能会与树脂本身络合进而破坏树脂结构，降低树脂柱对关键元素的吸附能力[35]。若树脂使用后采用适当的酸进行清洗(如参照表 1树脂回收流程)，可以有效地去除本底，延长树脂柱使用寿命。同时对于放射性Sr、Nd同位素，不要求全部回收[47]，因此本文的分离效果并不影响后续同位素的测试。
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图 4 更新AG50W-X8树脂后Nd的回收率Figure 4. Nd recovery using renewed AG50W-X8 resin. The updated recovery for Nd is increased to 82.1%.2.4 树脂柱的串联顺序对分离效果的影响
树脂柱的设置顺序是串联法的一个关键，各树脂柱在上下不同的位置，可能会改变不同元素的吸附行为，增加淋洗液的用量[24, 41]。本文通过改变Sr特效树脂柱和UTEVA特效树脂柱的摆放顺序设计了两种过柱方案，以此比较二者的分离效果。在方案一中Sr元素的回收率为92.5%，Nd的回收率为43.6%，U回收率接近99.9%。方案二中Sr元素的回收率为94.3%，Nd的回收率为53.7%，U回收率接近99.8%。本文首轮实验中方案一和方案二都以较高的回收率分离出高纯度的Sr和U元素。而对于Nd元素，虽然其回收率偏低，但回收的含量(495.8ng)也已经可以满足多次Nd同位素测试的需求。不过偏低的回收率仍然会对低含量Nd样品的分析测试造成困难，为此本文通过重新充填树脂柱证实了偏低的Nd回收率是AG50W-X8树脂失效引起的。综合以上分析，结合Sr和U的联合过柱回收率，本文认为此次实验中设计的两种不同树脂柱串联方式对Sr-Nd-U元素的回收率并无显著影响。
3. 结论
本文提出了一种只需一次溶样的三柱串联Sr-Nd-U联合分离方法，该方法可以快速高效地将Sr、Nd、U元素从硅酸盐岩样品中分离出来，其中U回收率最高接近99.9%，Sr回收率为92.5%，Nd回收率为82.1%，该回收率水平完全可以满足后续的同位素分析要求。
三柱串联Sr-Nd-U联合分离方法相比于传统的分离方法，减少了约一半的样品用量，有利于珍贵稀有样品(如大洋沉积物和极地样品)的分析；同时避免了蒸干转换介质操作，缩短了过柱时间，显著提高了化学分离效率。本文方法为Sr-Nd-U多同位素分离提供了新思路，未来可以尝试增加对流程中馏分4内Pb元素的回收，进一步拓展本文技术的应用范围。
致谢: 感谢同济大学海洋与地球科学学院马松阳硕士对本文撰写提供的指导和建议，同时感谢两位匿名审稿人提出的宝贵意见。 -
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图 1 Sr、Nd、U同位素联合分离流程示意图
步骤1：对三根树脂柱分别进行预清洗，降低本底。步骤2：三柱串联，同步进行平衡树脂、上样、洗杂质。步骤3：三柱分离单独淋洗接取目标元素。
Figure 1. Schematic diagrams of the combined Sr, Nd, U isotopes separation procedure.
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图 2 方案一中Sr-Nd-U在不同馏分中的回收率
Figure 2. Recoveries of Sr-Nd-U in different fractions in procedure Ⅰ. The Sr and U recoveries are over 90%, while the Nd recovery is only 43.6% and need further separation with Sm.
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图 3 方案二中Sr-Nd-U在不同馏分中的回收率
Figure 3. Recoveries of Sr-Nd-U in different fractions in procedure Ⅱ. The Sr and U recoveries are over 90%, while the Nd recovery is only 53.7% and need further separation with Sm.
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图 4 更新AG50W-X8树脂后Nd的回收率
Figure 4. Nd recovery using renewed AG50W-X8 resin. The updated recovery for Nd is increased to 82.1%.
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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