| 摘要 | 一定元素的同位素组成被认为是该元素特有的“指纹”，同位素组成测量是地球化学、生命科学、环境化学、地质科学和核科学等领域重要的研究手段。利用同位素技术开展生命过程，地球系统中的物理、化学、生物过程及其资源、环境与灾害效应，资源勘探，污染物溯源等方面的研究，既是该技术的前沿研究主题，也使相关领域的研究更加“精细量化”，从而在新的科学纵深揭示出更加清晰的规律。
1992年多接收电感耦合等离子体质谱仪(MC-ICP-MS)的问世，为同位素分析提供了一种强有力的技术手段，与传统的热电离同位素质谱相比，MC-ICP-MS具有测量速度快、操作简便、灵敏度高等优点。而且，由于等离子体源产生的高温，在理论上能测量所有的金属元素和一些非金属元素，并已很好地解决了一些高电离电位元素同位素测量的难题（如Se, Zn, Hf等），用MC-ICP-MS准确、精密测量各种元素同位素组成的方法正在逐渐得到发展和完善。目前，MC-ICP-MS比较成熟的方法主要是针对核和地质科学研究中应用较多的U, Pb, Sm, Nd, Sr, Hf, B, Li等，在硒和锌同位素测量方法学的研究还相对较少（尤其是硒），对一些测量中受各种干扰较为严重的、原子量小于80 的元素同位素的测量技术还有待进一步深入探索和研究。锌、硒元素不仅与人类健康息息相关，而且随着质谱分析技术的发展，使其在环境地球化学、生命科学等领域有着广泛的应用前景。准确测量生物、食品、环境、地质等样品中的锌、硒元素含量、各种形态及其同位素组成受到越来越多的关注。锌、硒同位素准确测量的方法学研究，不仅可以广泛应用于各相关领域，也为锌和硒同位素基、标准物质研制奠定技术基础，从而为锌、硒元素含量和同位素测量提供量值溯源保障。
本工作针对锌和硒元素同位素组成以及生物、环境等样品中成分量准确测量存在的问题，通过使用六极杆碰撞室MC-ICP-MS进行准确测量锌和硒元素同位素的技术研究，结合在化学计量研究中的长期实践及相关文献，从方法学角度和应用方面得出以下结论：
1.MC-ICP-MS仪器测量主要参数，如炬管轴向位置、载气流量、碰撞气流量、仪器稳定性等对测量结果影响很大，要获得高精度的测量结果，须优化和固定参数设置，保持仪器的稳定状态。在六极杆碰撞室MC-ICP-MS测量锌同位素时，高纯氩气碰撞气模式是较为理想的模式，64Zn/66Zn、67Zn/66Zn、68Zn/66Zn同位素丰度比测量精度达到0.002－0.008％，70Zn/66Zn 测量精度达到0.01％；在高纯氢气和氩气碰撞气按一定比例混合的模式下， 76Se/80Se、77Se/80Se、78Se/80Se、82Se/80Se同位素丰度比测量精度达到0.004－0.005%。
2.采用高纯、高浓缩64Zn和66Zn配制了8个校正样品 (64Zn/66Zn：0.6－2.2)；用高纯、高浓缩同位素76Se和82Se配制了16个校正样品（76Se/82Se： 0.05－11.8），用这些样品分别测量并计算了仪器系统误差校正系数K，这些校正样品的K64/66 和 K76/82的相对标准偏差分别为0.034％和0.03％，均在仪器的测量不确定度范围内，说明在校正样品同位素变化范围内，仪器测量同位素丰度比的校正系数没有发生明显变化。
3.在硒同位素丰度比值测量中，氢气碰撞气的使用是SeH产生的重要原因之一，Ar/H在2－7之间都可以满足硒同位素比值测量的要求，即保证较高的硒灵敏度、较小的SeH生成比例、稳定的同位素比值测量结果。本工作建立了SeH的校正计算公式，在对测量结果的质量歧视进行校正时，77Se和78Se的校正更为复杂，因为它们除自身产生的SeH外，还分别受到了来自76SeH和77SeH的影响，故校正质量偏移时应首先对SeH进行校正。对于不同的SeH生成比例，经过校正后，硒的同位素丰度比校正值是一致的，并不受SeH生成比例变化影响。
4.通过对IDMS过程中的关键技术研究，明确了如何正确使用该方法以获得准确测量结果。IDMS方法在测量步骤中引入的不确定度影响因素相对于其它化学分析方法较少，并且可以被明确地表达出来，测量结果可直接溯源到国际单位，因此，该方法对化学计量学研究具有十分重要的意义。
5.建立了适用于ICP-MS测量血清、大豆粉、金枪鱼等多种复杂基体中锌和硒元素的样品前处理方法，建立了锌和硒的ICP-IDMS测量方法。将建立的方法应用于人血清标准物质研制、国家计量院之间的国际比对和合作研究中，取得的优异成绩验证了所建方法的可靠性和可比性。IDMS方法在样品前处理上不怕样品损失和高精度同位素丰度比测量的优点，使其在复杂基体中硒、锌的准确测量方面较其它分析方法具有独特的优势，可在生物、临床、环境、食品等方面的分析研究中广泛应用。 |
| 其他摘要 | Isotope composition of an element is regarded as the unique “finger print” of this element. Isotope composition determination is one of the important research approaches in the fields such as geochemistry, life science, environmental science, geologic science and nuclear science. The advanced research topics by means of isotopic techniques are to explore the life process, physical, chemical and biological process as well as resource of earth system, to research on the effects of environment and disaster, to prospect natural resources and trace to the sources of pollutants. The employing isotopic techniques may be enabling new insights in these scientific fields.
In 1992, the introduction of multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS) provides a powerful technique for isotope analysis. Compared with the traditional thermal ionization isotope mass spectrometry, the main advantages of the MC-ICP-MS are the rapid measurement speed, simple operation and high sensitivity. In addition, due to the high temperature of inductively coupled plasma, all the metal elements and some non-metal elements can be measured using MC-ICP-MS theoretically. In the “real sample world”, difficulties for the isotopic measurement of elements with high ionization potential are well solved using MC-ICP-MS, more and more methods for the accurate measurement of isotopic components in various elements by using MC-ICP-MS having been developed. Currently, the well developed MC-ICP-MS methods are focusing on the measurement of elements such as U, Pb, Sm, Nd, Sr, Hf, B and Li in the nuclear science and geologic science. The systemic methodology studies on the isotopic measurement for Se and Zn (especially for Se) are relatively less. Isotopic measurements for elements with atomic weight less than 80 amu are suffer from severe isobar interferences, the isotope analysis technique, therefore, are need to be further explored and developed. Zinc and Selenium are closely linked to human health. With the development of mass spectrometry techniques there is broad application foreground in fields, such as geochemistry and life science for the two elements. Accurate measurement isotope compositions, total amounts as well as speciations of Zn and Se in various samples are paid more and more attentions. The study of methodology of measurement Zn and Se not only can be used in relative fields, but also establish the technical base for isotope certified reference materials. Consequently, the traceability guarantee can be provided for Zn and Se measurements.
This thesis aimed to solve the problems during the study of Zn and Se isotope composition s as well as the accurate measurement for biological and environmental samples. The methodology for the accurate determination of Zn and Se isotopes using MC-ICP-MS with hexpole collision cell has been developed. From both the methodology and application aspects, main conclusions are achieved as follows:
1.Instrumental parameters including torch axial location, carrier gas flow, collision gas flow, stability of the instrument have significant daily influences of MC-ICP-MS. High performance of MC-ICP-MS can be obtained by instrumental parameters optimization and stable instrument status. It is observed that high purity argon collision gas is preferred for the measurement of Zn isotope using MC-ICP-MS with hexpole collision cell. Measurement precision in the range of 0.002－0.008％ for 64Zn/66Zn、67Zn/66Zn、68Zn/66Zn and 0.01％ for 70Zn/66Zn can be achieved respectively. When high purity of H2 and Ar are mixed at the certain mix proportion and used as the collision gas，precisions for the isotope ratio analysis for 76Se/80Se、77Se/80Se、78Se/80Se、82Se/80Se can be obtained from 0.004 to 0.005%。
2.8 gravimetric synthetic mixtures(64Zn/66Zn：0.6－2.2) prepared from highly enriched isotopes of 64Zn and 66Zn and 16 gravimetric synthetic mixtures(76Se/78Se：0.05－11.8) prepared by highly enriched isotopes of 76Se and 82Se are used to calibrate MC-ICP-MS respectively. The SD of those correction factors K64/66 and K76/82 are 0.034% and 0.03% respectively. The results indicate that SD values of these K values are within the measurement uncertainties of MC-ICP-MS, and no significant correlation is found between those K values and the changes of 76Se/78Se and 64Zn/66Zn respectively.
3.This work indicates that one of the important reasons to form SeH is the use of high purity H2 as the collision gas during the isotopic ratio measurements. It has been showed in the experiments that the ratio of Ar to H2 ranged from 2 to 7 can meet requirements such as higher Se sensitivity, smaller formation ratio of SeH and precision for Se isotope ration analysis. Se correction equation has been established for SeH calibration of obtained results. It should be noted that the corrections of 77Se and 78Se are more complicated due to the self generation of SeH and interference from 76SeH and 77SeH respectively. Consequently, SeH calibration should be performed at the beginning when the mass bias calibration procedure is carried out. For various formation ratios of SeH, corrected values of Se isotope ratios coincide with each other. It indicates that the correction values are not influenced by formation ratio of SeH.
4.The reliable result by using accurate IDMS process has been obtained by studying the key technique issues through the entire experiments procedures. The fewer factors that contributed to the uncertainty have been introduced during the measurement procedure when using IDMS compared with those from other chemical analysis methods. Results obtained from IDMS can be expressed in units for amounts of substance and directly make them traceable to the SI. As a primary method IDMS plays quite important role in the chemical metrology.
5.In this work, Sample preparation methods for Zn and Se in various matrix such as serum, soybean powder, tuna fish and corresponding ICP-IDMS method for these two elements measurement have been developed. The developed methods have been successfully applied to value assignment for human serum certified reference material, and validated by excellent measurement results obtained through related international comparisons and cooperation projects. Moreover, since sample loss is not concerned during the sample preparation and also the inherent the high precision isotopic ratio measurement for this method, it has unique advantage over other analysis technique for the Se and Zn analysis in the complicated matrix. It will be more extensively applied in the fields of biological, clinical, environmental and food science. |
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