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有机介质中元素的电感耦合等离子体质谱(ICP-MS)行为特性研究

A Study of Element Behavior in Organic Medium by Inductively Coupled Plasma Mass Spectroscopy

【作者】 胡兆初

【导师】 胡圣虹;

【作者基本信息】 中国地质大学 , 地球化学, 2003, 硕士

【摘要】 电感耦合等离子体质谱(ICP-MS)具有灵敏度高、精密度好、检出限低、多元素同时检测并可提供同位素比值信息的分析特性,被广泛用于不同类型样品中痕量元素的分析研究。随着应用领域的不断扩大,ICP-MS直接用于有机试剂、有机介质中痕量元素的分析研究引起了人们广泛的关注。由于有机介质、有机试剂的引入改变了溶液的物理化学性质及其在ICP中的电离特性,使被分析元素在灵敏度、精密度等方面发生了一系列特异性变化。进行不同有机基体中元素的ICP-MS行为特性的研究,有利于有机基体中痕量元素的直接测定,并能有效地拓展ICP-MS的应用领域。本文详细地研究了EDTA(有机络和物),醋酸(有机酸),异丙醇(醇类),甲醇(醇类),丙酮(酮类)中元素的ICP-MS行为。主要获得了以下认识:一、EDTA介质中元素的ICP-MS行为研究表明:(1)EDTA介质与水溶液介质相比,在相同入射功率条件下,元素分析信号达最佳时,采用稍低的雾化气流速。(2)EDTA的引入对元素分析灵敏度有明显的增强效应,其增强效应有随元素质量数的增加而逐渐降低的趋势。(3)氧化物产率降低明显,从而有效地降低了多原子离子干扰。(4)易电离元素,无论以何种形式(无机的或有机的)引入ICP-MS均会对元素分析信号产生严重的抑制作用。二、醋酸基体浓度变化对元素ICP-MS行为的影响表明:(1)提高有机基体醋酸浓度,所研究元素分析信号达最大时出现在较低雾化气流速。(2)过高的醋酸浓度对等离子体产生的局部冷却作用,使氧化物产率增加。(3)采用较低雾化气流速,能减小醋酸浓度的变化对分析信号、氧化物产率的影响。(4)较高浓度的醋酸往往导致分析信号抑制,然而在一定的条件下也可以增强。一般采用高功率、低雾化气流速可以提高较高浓度醋酸基体中元素分析信号的强度。三、异丙醇介质中元素的电感耦合等离子体质谱行为表明:(1)不同质量数、不同电离能的元素在不同浓度异丙醇基体中的ICP-MS行为不同。在低浓度异丙醇基体中,大多数元素有所增敏,尤其对第一电离能大于9eV的元素,异丙醇的引入对其有强烈的增敏作用。但过高浓度(在选定的仪器操作条件下,大于100mmol/L)的异丙醇基体又导致了元素分析信号的严重抑制,其抑制程度有随元素第一电离能增大而增加的趋势。(2)本文所定义的基体效应判别因子(Distinguish Factor, DF)较好地区分了不同质量数、不同电离能的元素在异丙醇基体中的行为。内标元素In与中间质量数元素的行为趋势具有较好的相似性而与轻、重质量端以及高电离能元素存在着一定的差异。这表明要采用传统的内标<WP=8>法进行异丙醇介质中多元素测定时需要采用多个内标才能较好地校正这种有机基体效应。(3)引入适量的异丙醇试剂可以有效降低氧化物的产率,但过高浓度的异丙醇对等离子体中心通道产生的局部冷却作用,也会促进氧化物产率的增加。四、挥发性有机试剂甲醇、丙酮介质中元素的行为及其影响机制研究表明:(1)元素分析信号的增敏或抑制有赖于仪器的操作条件和甲醇、丙酮基体的浓度。(2)利用甲醇、丙酮的易挥发特性设计了残余效应实验,以探讨引起元素分析信号增强或抑制的主要贡献因素,所设计的残余效应实验有效的避开了由于甲醇、丙酮引入对溶液物理化学性质的变化而引起的元素分析信号的改变。结果表明甲醇、丙酮引入引起的分析信号的增敏不能归功于溶液雾化、传输效率的提高,主要贡献在于改变了元素在ICP中的电离特性。引入7%甲醇后引起的残余效应约需要25分钟的冲洗才能使信号达到稳定状态。(3)甲醇、丙酮对元素的增强效应跟元素的质量数和电离能有关。一般元素质量数越小,电离能越大,其增敏作用越强。其原因被部分归因于等离子体中分析物离子空间分布的变化。对高电离能的元素As、Se,结果显示碳的电荷转移反应是其增敏的主要原因。(4)甲醇和丙酮对分析元素增敏作用的差异主要跟进入等离子体中的含碳量有关,而不是官能团的不同。(5)氧化物产率的降低不足以说明元素分析信号的提高,其降低的程度跟采用的甲醇浓度和最佳化时采用的雾化气流速有关。

【Abstract】 During the last decades inductively coupled plasma-mass spectrometry (ICP-MS) has been developed into an accurate and sensitive technique for multi-element determinations in a range of sample matrices. Compared with AFS, AAS and ICP-AES, ICP-MS offers better sensitivity and excellent accuracy with multi-element and isotope ratio measurement capabilities. Organic solvents are widely used for the purposes of preconcentration and separation of trace elements in analytical chemistry. In this study, the effects of EDTA, acetic acid, 2-propanol, methanol and methane on the behaviors of elements in ICP-MS were investigated. The effects of ICP-MS operation parameters, concentration of organic reagent on analyte signals were studied. The possible mechanism of the effect of organic medium on analyte signals were discussed in detail using carry-over effect caused by volatile organic solvent.The main results were summarized as follows:1. Experimental results show that the signal intensities of elements are enhanced in the EDTA medium. The enhancing factor decrease with increasing mass. The yields of oxides are reduced significantly in EDTA medium. Compared with 2% nitric acid solution, a lower nebulizer gas flow rate is needed for EDTA solutions to obtain the maximum signal. The easily ionizable element in EDTA matrix is also contributed to the decrease of analyte signal intensities.2. The analyte signal intensities were suppressed in a higher concentration of acetic acid. However, signal enhancements occur under certain conditions. The matrix induced-changes highly depend on the nebulizer gas flow rate and power. Higher concentration of acetic acid is thought to result in local cooling in the central channel of the plasma. As the concentration of acetic acid increases from 2% to 5%, analyte signal intensities decrease and oxide ratio increase, especially in the high nebulizer gas flow rate. Higher incident power and low nebulizer gas flow rate allow the plasma to accept a change in the concentration (2% to 5%) of acetic acid without significant variation of analyte signal. 3. Experimental results show that the most of analyte signals were enhanced when the concentration of 2-propanol medium was less than 100mmol/L, especially for elements with ionization energy greater than 9eV. However, the analyte signal intensities were suppressed when the concentration of 2-propanol medium was more than 100mmol/L. The behaviors of elements were identified with distinguish factor of matrix effect (DF) and these elements were assorted into three clusters. The mechanism of analyte suppression and enhancement effects in 2-propanol medium was discussed according. 4. Enhancement or suppression of analyte signals in the presence of methanol and acetone depends on volatility of the compound, its concentration, mass and ionization potential of analyte and operating conditions of ICP-MS. Presence of a low concentration of methanol or acetone enhances the intensities of elements in order of decreasing mass. This may be related to the spatial shift of the zone of maximum ion density in plasma, which in turn affects the extraction of ion from<WP=10>the plasma to the sampling cone. In contrast, the more volatile acetone more easily depresses signals of elements from low mass(7Li) to high mass(238U). A high concentration of methanol also depresses intensities of all elements due to its cooling effect on the central channel of the plasma. The enhancement effect of methanol and acetone appears to be more related to the amount of carbon present in the plasma than the difference in the functional group of the organic solvents. Carry-over experiments of methanol and acetone show signal enhancement of analyte is not due to the improvement in the nebulization-transport process of the sample. The oxide productivity decreases in the presence of methanol, the level of which depends on the nebulizer gas flow rate used. However the reduced oxide productivity is insufficient to account for the signal enhancement. The C+-analyte atom charge transfer

  • 【分类号】O539
  • 【被引频次】6
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