X射线荧光光谱仪XRF分析误差的来源探讨

X射线荧光光谱仪XRF分析误差的来源探讨

X射线光谱分析仪的好坏常常是以X射线强度测量的理论统计误差来表示的，X射线荧光仪的稳定性和再现性，已足以保证待测样品分析测量的精度，被分析样品的制样技术成为影响分析准确度的至关重要的因素，在样品制备方面所花的工夫将会反映在分析结果的质量上。X射线荧光仪器分析误差的来源主要有以下几个方面：

1． 采样误差：

非均质材料

样品的代表性

2． 样品的制备：

制样技术的稳定性

产生均匀样品的技术

3． 不适当的标样：

待测样品是否在标样的组成范围内

标样元素测定值的准确度

标样与样品的稳定性

4． 仪器误差：

计数的统计误差

样品的位置

灵敏度和漂移

重现性

5． 不适当的定量数学模型：

不正确的算法

元素间的干扰效应未经校正

l 颗粒效应

纯物质的荧光强度随颗粒的减小而增大，在多元素体系中，已经证明一些元素的强度与吸收和增强效应有关，这些效应可以引起某些元素的强度增加和另一些元素的强度减小。图1列举了强度与研磨时间的关系：①粒度的减小，引起铁、硫、钾的强度减小，而使钙、硅的强度增加。②随着粒度减小至某一点，强度趋

于稳定。③较低原子序数的元素的强度随粒度的减小有较大的变化。

l 矿物效应

图2中样品为用不同矿物配成的水泥生料。标为“I"的样品是用石灰石、页岩和铁矿石配成的。标为“F"的样品含有相同的石灰石和铁矿石，但硅的来源是用砂岩代替了页岩。两组原料用同一设备处理，用同一研磨机研磨，每一个样品约有85%通过200目。图2表明这种强度—浓度上的变化首先反映了硅的来源不同，“I"的硅来自页岩，“F" 的硅来自砂岩。然而两组样品的进一步研磨指出这仅仅是一个粒度效应问题。图3表明在全部样品经研磨机粉研到325目（44μm）以后，两组样品的实验点均落在同一曲线上。

l 元素间吸收—增强效应

任何材料的定量X射线荧光分析要求元素的测量强度与其百分含量成正比，在岩石和矿物（由两种或两种以上矿物的组合）这类复杂的基体中，由于试样内其它元素的影响，元素的强度可能不直接与其含量成正比。一般认为，多元素体系中这种非线性是由元素间效应引起的。元素间效应可以是增强效应或吸收效应，也可以是同时包括这两种效应。仍以图2、图3实验为例，图4表明通过简单的研磨可以改进CaO的分析结果。图5表明校正钾对钙的干扰后，两组样品的实验点均落在同一曲线上。

Analysis on the error source of XRF spectrometer

The stability and reproducibility of XRF spectrometer has little influence on the

precision, and the performance of XRF spectrometer is characteristic of the theoretic statistic error of  intensity of XRF spectrometer . the sample preparation technique plays a important role and  will be reflected in the results. the error source of XRF spectrometer is as follows:

1, Sampling error:

Heterogeneous Material

Representative of sample

2. Sample preparation

Stability of the preparation technique

Technique of preparing homogeneous samples

3. Irrelevant standard samples.

Whether the content of unknown samples is in the cover range of the standard samples or not

Accuracy of chemical analysis result of  the standard sample

Stability of the samples and standard samples

4. Instrument error

statistic error of count

location of samples

sensitivity and drift

5. Irrelevant quantitative mathematical models

Irrelevant algorithm

Element interference effect is not calibrated

● granular effect

the fluorescence intensity of pure material increase with the decrease of the granularity. In multi-element system, fluorescence intensity of some element has been proved to be related with matrix effect, which can increase some element

intensity and decrease some element intensity.The relationship between intensity and grinding time is as showed in Fig1:①the decrease of element granularity decrease the intensity of Fe, S, K, and increase the intensity of Ca and Si. ②The intensity tend to be stable when granularity decrease to a certain degree. ③The intensity of low Z element changes a lot with decrease of granularity.

●Material effect

There are some cement raw prepared by different material(Fig 2), the samples marked I are composed of limestone, shale, iron ore. the samples marked F contains the same limestone and iron ore, but contains different silicon, in which, shale is replaced by sandstone. Two kinds of raw material are treated with the same equipment ,and grinded by the same grinding machine. And 85% of them passed 200 mesh. It’s obvious that change of intensity-content relationship show the different resource of silicon. In fact, it is just a matter of granularity, when the sample pass 325 mesh by increasing the grinding time, all points are located in the same curve.

●Matrix effect

All materials measured by XRF are required that the intensity of element is proportional to its content. in the complicated matrix such as rock s and ores(the

composite of at least two kind of materials), for the influence of other elements, the intensity may not be proportional to its content. generally, this nonlinear relationship is caused by elemental matrix effect.

Matrix effect is absorption effect or enhancement effect, or the combination of them. Longer grinding can improve the CaO analysis result(Fig 4). After the calibration of interference of K, the points of two series of samples locate in the same curve.