| 摘要: |
| 为了采用129I进行地质定年研究，该类样品中的129I/127I原子比值通常低于10-12，因此低的129I流程空白是分析地质定年样品的前提条件之一。在全球范围内，大气中129I的水平呈现显著变化趋势，129I/127I原子比值范围在10-10 到10-6。然而，在样品制备过程中，是否大气中的129I会影响流程空白尚未可知。本研究调查了三种常用的129I分析方法，包括直接沉淀法、溶剂萃取法和管式燃烧法（分别以压缩空气和氧气作为载气）来比较流程空白中的129I/127I比值。研究结果表明：在最佳实验室条件下，流程空白均被控制在较低的水平，可用于分析各种环境和地质样品。另外，通过研究溶液的长时间储存，发现当0.4 mol ∙ L-1NaOH溶液储存一年以上时，129I / 127I比值比新配制的NaOH溶液有所提高，但基本在实验室正常本底2×10-13以内。采用压缩空气作为载气的管式燃烧法比氧气为载气时具有明显提高的129I / 3I比值。研究表明样品和试剂的储存，以及制样过程中与大气的交换程度均会影响流程空白中129I的水平。因此在分析超低129I含量的地质样品时，固体样品中碘的分离应采用低本底的分析方法（如以纯气体作为载体的管式燃烧法），如有必要还可在低大气129I水平的实验室进行实验。 |
| 关键词: 碘-129 流程空白 129I的地质定年 |
| DOI：10.7515/JEE201605010 |
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| 基金项目:科技部基础性工作专项项目和创新方法工作专项项目（2015FY110800）；国家自然科学基金项目（11605207，41271512） |
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| Influence of atmospheric 129I level on procedural blanks in analysis of ultra-low 129I geological samples |
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ZHANG Luyuan, CHEN Ning, HOU Xiaolin, LIU Qi, FAN Yukun, XING Shan1,2
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1. State Key Laboratory of Loess and Quaternary Geology, Shaanxi Key Laboratory of
Accelerator Mass Spectrometry Technology and Application, Xi’an AMS center, Institute of Earth Environment,
Chinese Academy of Sciences, Xi’an 710061, China;2. Center for Nuclear Technologies,
Technical University of Denmark, Risø Campus, Roskilde 4000, Denmark
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| Abstract: |
| Background, aim, and scope For the purpose of geological dating using 129I, low procedural background is a prerequisite for analysis of ultra-low 129I geological samples generally below the level of 10-12 of 129I /127I atomic ratio. Significant variation of atmospheric 129I level is found in a wide range of 10-10 to 10-6 of 129I/127I atomic ratio on a global scale depending on the sources of 129I, however, whether 129I level in air could affect procedural background during sample preparation is crucial, but unclear. Materials and methods This study performed three conventional 129I analytical methods including direct precipitation of iodine, solvent extraction with CCl4, as well as combustion in a Pyrolyser furnace to compare 129I/127I ratio in the procedural blanks. In the combustion method, four experimental conditions were conducted under air or oxygen as carrier gas with or without system heating. Finally, 129I along with carrier iodine was precipitated as AgI, which is then dried, mixed with niobium powder and pressed in copper holder as target. 3 MV Tandem Accelerator Mass Spectrometer (AMS) was used to measure 129I in the target. Results 129I/127I atomic ratios in the procedure blanks are ranging from 1.48×10-13 to 28.5×10-13. Slightly increase of 129I background level was observed when NaOH solid reagent and solution stored over one year in contrast to the newly opened and prepared alkaline solution. In the three methods, the results also showed that the lowest 129I/127I ratio was generated by direct precipitate method, while the highest 129I background level was from the combustion method with condensed air as carrier gas. In particular, the 129I/127I ratio of combustion method with oxygen as carrier gas was 2—4 times higher than the direct precipitation. Most notably, 129I/127I ratio of combustion method with condensed air as carrier gas has significantly raised one order of magnitude compared to the direct precipitation, especially the first preparation for both experimental conditions of room temperature and heating. Discussion The results suggest procedural blanks are well controlled in our laboratory. The increase in 129I/127I ratio of procedural blanks prepared with long time stored NaOH solution is attributed to the exposure of alkaline solution into the air during frequent usage, because it could easily adsorb and react with molecular iodine in the air. 129I/127I ratio in solvent extraction method (1.75×10-13 in average) was slightly higher than the lowest 129I level, which indicates solvent extraction could only introduce negligible iodine during sample preparation. Obviously elevated 129I/127I ratio up to 10-12 was observed for sample combustion with condensed air as carrier gas in comparison with oxygen, which is likely caused by iodine adsorption onto the plastic transfer pipe of condensed air and higher atmospheric 129I level in Denmark where the experiment was conducted, rather than 129I residue in the combustion furnace system. Conclusions These results suggest 129I level of procedural blank could be influenced by storage of samples and reagents, as well as by the degree of exchange with ambient air during sample preparation. This work shows that the procedural blank can be well controlled using the methods, direct precipitate, solvent extraction and combustion with oxygen as carrier gas, except combustion method with condensed air as carrier gas. The results suggest interfusion of air during sample preparation could increase the blank 129I/127I ratio depending on the blending extent. Ultralow 129I level geological samples, especially aqueous samples should be carefully stored and analyzed as soon as possible in case of atmospheric iodine adsorption. Selection of sample preparation methods needs to minimize the mixing of sample with air, especially in high atmospheric 129I regions. Recommendations and perspectives In the aspect of sample preparation, geological samples for dating can be categorized to two types, high 127I concentration samples and low 127I concentration samples. Since the target source and AMS measurement need sufficient stable 127I amount, the former samples can be prepared by direct precipitation after simple samples pretreatment (e.g. oilfield brine by filtration through filter paper), while the latter samples have to be treated by relatively complex and long procedures for either extraction of sufficient stable 127I or addition of stable iodine carrier. On the basis of the observation in this study, therefore, it is recommended that analysis of ultra-low 129I geological samples should adopt low background analytical method (combustion with pure gas as carrier gas) and performed in low ambient 129I level laboratory if necessary. The iodine carrier-free or small iodine carrier addition method developed in recent years provide a great potential for geological dating of low 129I and low 127I samples. |
| Key words: Iodine-129 procedural blank geological dating of 129I |
