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XRF荧光光谱仪制样方法的原理和依据

更新时间:2021-08-02      点击次数:1730

XRF荧光光谱仪制样方法的原理和依据

样品处理是整个分析检测过程的重要组成部分。其目的是利用各种化学方法将测试元素以离子的形式从固(液)态样品中定量转移到测试溶液中。选择合理的样品分解方法可以大大简化分析程序,大大提高分析方法的适应性和准确性。



设计样品处理的原则是:


①保证样品中所有被测元素都定量转移到试液中,即样品必须*分解;


② 避免在样品处理过程中引入干扰元素,同时有助于去除干扰元素;


③分解方法应尽可能简单、易操作、经济、快速、安全,尽量减少对环境的污染;


④ 便于批量处理样品。



要设计出满足这些条件的样品处理方法,您必须深入了解:



a) 被测元素及其化合物的理化性质;


b) 样品中被测元素的含量范围和出现形式;


c) 样品基体的组成和性质;


d) 使用的最终测试方法和技术。



以国内外测定As、Se、Hg所用样品的各种处理方法为例



对于国内外各种样品中As、Se、Hg的测定,所采用的样品处理方法大致分为三类:


1) 湿酸/碱分解;


2)密闭系统燃烧法(氧弹燃烧法);


3)烧结(半熔)法。


在煤中微量元素的定性和定量检测过程中,样品前处理往往比检测技术本身更为重要。特别是对于各种原子光谱技术,一般需要进行液体进样或液体进样,以达到高精度。为了保证检测的准确性,在充分溶解成分复杂的煤样的同时,要避免被测元素的挥发性元素或化合物形式的损失。目前,国内外有以下几种方法:



(1)直接溶解在密封的坩埚中。



该方法是在密闭容器中用混酸直接分解样品,在微波炉中加热消解样品。具有进样量小、溶解效率高、操作简单、安全、易控制、避免挥发等优点;但是直接分解法不可避免地存在分解不*的缺点。该方法常用于检测燃烧后煤灰或煤提取物样品中的微量元素。



(2)低温灰化法。



样品采用等离子技术在 150°C 左右灰化,然后在聚四氟乙烯容器中用混酸分解。该方法是目前*的较好的预处理技术,但也存在设备运行成本高、耗时长等缺点。



湿酸分解


对于地质样品的测定,常使用HNO3-HF-HClO4系统进行酸溶。用这种方法分解样品时,如果加热强度稍高,蒸发过干,样品中的部分硒会因挥发而流失。原因可能是样品酸分解产生的Se(ClO4)2可以分解为HCl和SeO2,这两种化合物可以反应生成SeCl2,可以在较低温度下升华。据了解,样品在加工过程中会蒸发至干,样品中的硒会损失40%左右。因此,用这种方法分解样品时,必须小心控制加热温度和时间,蒸1~2mL透明溶液时应停止加热。还有利用HNO3-H2SO4体系酸溶解和HNO3-HClO4体系分解。湿法分解操作程序复杂,酸雾挥发到大气中,不利于环保。另外,对于煤,由于煤中含有大量有机物,酸分解不方便。



氧弹燃烧


该方法是将煤样置于充满高压氧的不锈钢筒中,用电点燃煤样。煤中的有机物得到充分燃烧,无机矿物也被氧化分解。煤中的硒元素被转化为氧化物。它溶解在吸收液(水或

稀碱)以气态形式存在于墨盒中。国外一些实验室采用这种方法测定煤中的硒。优点是样品处理不引入除氧气外的其他试剂,减少了引入干扰元素的机会。缺点是高灰分煤可能分解不*(未*燃烧),导致分析结果偏低;操作较麻烦,样品处理效率低;不利于样品的批量分解。




烧结(半熔)法




该方法是将与试样混合均匀,加热燃烧,使煤中的As和Se氧化成氧化物,再与Na2CO3和MgO反应生成盐和硒酸盐。然后用HCl溶解烧成物,将As和Se以离子的形式转移到溶液中。以煤为例,反应方程式为:


这种方法的优点是操作简单,易于批量处理和分解样品。只要正确控制燃烧条件,就可以对被测元素进行定量转化。考虑到目前煤中其他元素的分析标准中,采用半熔体分解煤样的方法很多,分析人员容易接受,工作效率高;国家标准和美国标准也采用半熔融法处理样品。本实验通过对各种处理方法的比较,采用半熔法对样品进行处理。



为了将被测元素从燃烧后的样品中转移到溶液中,燃烧后的物质必须用合适的酸溶解。为此,需要对酸的种类和加酸方法进行实验。



酸的种类


使用不同的酸处理样品对结果有影响,因此本实验考察了 HCl、H2SO4 和 HNO3 在烧伤治疗中的溶解情况及其对测定的影响。




H2SO4:测试发现用H2SO4溶解烧焦会引起CaSO4沉淀。此外,H2SO4 中含有高 As 和 Se,导致空白过高。如果提前去除,会增加实验工作量。因此,不宜选用 H2SO4 作为溶解酸。 .



HNO3:测试发现,当HNO3溶解样品时,其原子吸收信号低于HCl介质。原因可能是HNO3起氧化剂作用,与样品中的还原性物质反应生成亚硝酸,而NO2-对氢化硒的生成有抑制作用。国外一些实验室采用HNO3作为测定氢化硒发生原子吸收的介质,以降低灵敏度为代价,换取更好的稳定性。



HCl:用HCl溶解烧焦是国内外广泛使用的方法。大多数氯化物是易溶盐。 HCl 具有还原性。它是一种还原剂,可将 Se6+ 还原为 Se4+。 HCl中As、Se含量少,试剂空白低,有利于测定。该方法使用 HCl 来溶解燃烧的材料。

ilute alkali) in the cartridge in a gaseous form. Some foreign laboratories use this method to measure Se in coal. The advantage is that the sample processing does not introduce other reagents except oxygen, which reduces the chance of introducing interfering elements. The disadvantage is that the high-ash coal may be incompletely decomposed (not completely burned), which can lead to low analysis results; the operation is more troublesome, and the sample processing efficiency is low; it is not conducive to batch decomposition of samples.




Sintering (semi-melting) method




This method uses Aldrin and the sample to mix uniformly, heat and burn, so that the As and Se in the coal are oxidized to oxides, and then react with Na2CO3 and MgO to form arsenate and selenate. Then dissolve the burned material with HCl, and transfer As and Se into the solution in the form of ions. Taking coal as an example, the reaction equation is:








The advantage of this method is that it is easy to operate and easy to process and decompose samples in batches. As long as the burning conditions are correctly controlled, the measured elements can be quantitatively transformed. Considering that in the current analysis standards for other elements in coal, there are many methods that use aldrin semi-melt to decompose coal samples, which are easy for analysts to accept and have high work efficiency; national standards and American standards also use aldrin semi-melt method for processing sample. Through the comparison of various treatment methods, this experiment uses the semi-melting method to treat the samples.




In order to transfer the measured element from the burned sample into the solution, the burned substance must be dissolved with a suitable acid. To this end, experiments should be conducted on the type of acid and the method of adding acid.




Types of acid




The use of different acids to process the samples has an impact on the results, so the experiment investigated the dissolution of HCl, H2SO4 and HNO3 in the treatment of burns and their influence on the determination.




H2SO4: The test found that the use of H2SO4 to dissolve the burnt will cause CaSO4 precipitation. In addition, H2SO4 contains high As and Se, which causes the blank to be too high. If it is removed in advance, it will increase the experimental workload. Therefore, it is not suitable to choose H2SO4 as the dissolving acid. .




HNO3: The test found that when HNO3 dissolves the sample, the atomic absorption signal is lower than that of the HCl medium. The reason may be that HNO3 acts as an oxidant and reacts with the reducing substances in the sample to form nitrous acid, and NO2- has an inhibitory effect on the formation of Se hydride. Some foreign laboratories use HNO3 as the medium for the determination of Se hydride generation atomic absorption, at the cost of reduced sensitivity, in exchange for better stability.




HCl: The use of HCl to dissolve the burnt is a widely used method at home and abroad. Most of the chlorides are easily soluble salts. HCl has reductive properties. It is a reducing agent that reduces Se6+ to Se4+. The content of As and Se in HCl is small, and the reagent blank is low, which is beneficial for the determination. This method uses HCl to dissolve the burnt material.


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