微波等离子体原子发射光谱分析膨润土中总砷的研究

膨润土是由比例为2∶1的Si—O四面体和Al—O八面体交替组成具有层状结构的硅酸盐天然黏土矿物，膨润土的多孔结构、化学组成、可交换离子类型以及较小的晶体尺寸，赋予了膨润土具有化学活性表面积大、阳离子交换容量大以及孔隙率高的独特性能，广泛应用于石化、冶金、食品、医药和环保等各个领域。在膨润土的开发利用过程中，其所含元素As可能通过迁移对人体健康构成潜在风险。采用硝酸-盐酸-氢氟酸对膨润土进行微波消解，加入高氯酸在电热板上对消解溶液继续进行低温消解，利用微波等离子体原子发射光谱（MP-AES）联用多模式样品引入系统（MSIS）测定膨润土中总As含量。采用25%（w/v）碘化钾为预还原溶液对As的形态进行预还原，将As（Ⅴ）还原为As（Ⅲ），利用硼氢化钠/氢氧化钠在MSIS中将As（Ⅲ）转变为气态氢化物（AsH₃），通过优化MP-AES的最佳观测位置和雾化气流量，获得最佳分析性能，选择188.979 nm为As的分析波长避开了谱线重叠干扰，使用快速线性干扰校正（FLIC）模型校正了背景干扰，选择261.542 nm为内标元素Lu的分析波长校正了基体效应。As的检出限（LOD）为0.41 μg·L-1，通过测定国家标准参考物质对分析方法进行评价，As的测定值与标准参考物质的认定值一致，相对标准偏差（RSD）≤2.80%，验证了方法的准确性好，精密度高。对来自中国不同产地的12个膨润土的分析显示，所有样品中总As的浓度相对较低（As的平均含量在3.51～12.6 mg·kg-1之间），按照中华人民共和国国家标准GB 2760—2014食品安全国家标准食品添加剂膨润土中规定的总As浓度限量标准，所有膨润土样品中的As含量均没有超标。采用大气氮气为等离子体气的MP-AES降低了运行成本，提高了分析效率，为膨润土的质量控制提供了可靠的方法，具有运行安全稳定、操作简单快速、适用性强等优点。

Study on the Analysis Total As in Bentonite With Microwave Plasma Atomic Emission Spectrometry

Bentonite is natural clay of layered phyllosilicates, composed of Si—O tetrahedral and Al—O octahedral sheets in the ratio of 2∶1. Its vesicular structure, chemical composition, exchangeable ion type and small crystal size result in unique properties including large chemically active surface area, high cation exchange capacity and high porosity. It is widely used in petrochemical, metallurgy, food, medicine, environmental protection and other fields. As an element, bentonite may pose a potential risk to human health during the development and utilization of bentonite. This paper determined the total As content in bentonite by microwave plasma atomic emission spectrometry (MP-AES) combined with multimode sample introduction system (MSIS). Nitric acid, hydrochloric acid and hydrofluoric acid were used for microwave digestion of bentonite. After adding perchloric acid, the digestion solution was dissolved at low temperature on an electric heating plate. 25% (w/v) potassium iodide solution was used as prereduction solution to reduce As(Ⅴ) to As(Ⅲ), and sodium borohydride/sodium hydroxide was used to transform As(Ⅲ) into gaseous hydride (AsH₃) in the MSIS. By optimizing the best viewing position and nebulizer gas flow of MP-AES, the best analytical performance of As measurement is achieved. The interference of spectral lines overlapping was avoided by selecting 188.979 nm as the analytical wavelength of As. The background interference was corrected by the fast linear interference correction (FLIC) model. The matrix effect was corrected by selecting 261.542 nm as the wavelength of the internal standard element Lu. The limit of detection (LOD) of As is 0.41 μg·L-1. The analysis method was evaluated by the determination of national standard reference materials. The determination value of As was consistent with the certified value of standard reference material, and the relative standard deviation (RSD) was less than 2.80%, which verified the accuracy and precision of the method. The analysis of 12 bentonite samples from different places in China showed that the total As concentration was relatively low (the average content of As was between 3.51～12.6 mg·kg-1). Based on the limits set by the National Standard of the People’s Republic of China GB 2760—2014 food safety national standard food additive bentonite, the As content in all samples did not exceed the standard. MP-AES using atmospheric nitrogen as the plasma source reduces the operation cost, improves the analysis efficiency, and provides a reliable method for the quality control of bentonite. It has the advantages of safe and stable operation, simple and rapid operation and strong applicability.