选矿测试技术

第二部分 X荧光成分分析x ray Fluorescence Spectroscope (XRF)1 方法特点2 分析原理3 仪器构成4 WDS-XRF5 EDS-XRF6 定量分析1 方法特点XRF是一种最有效的无损成分分析手段之一。

优点：(1)速度快，一般测定一种元素(定量)需10－100秒；(2)可检测固态、粉末、液态样品，以及晶质、非晶质等样品的化学成分；(3)非破坏性测试，不损伤样品(但对于某些高分子物质，如有机物，则会引起颜色变化)；(4)是一种物理分析方法，分析元素范围F9－U92，分析的浓度范围1ppm－100%；(5)制样简单，板状样品把被照射面加工成平面即可，粉末样品经粉碎、压片即可；(6)系表面分析，一般测定深度为0.1mm范围，另外也是表面整体分析，即分析整个照射面范围内的元素种类和含量；缺点：(1)难以作绝对分析，因此定量分析时需参照标准物质进行；(2)原子序数低的元素，其检出限及测定误差都相对较大。

♣ Pros:FastNon-destructive techniquePerforms elemental analysisHigh sensitivity, detects up to 50 ppmCan detect thin single or multi-layer films and measure their thickness.♣ Cons:Does not assist in obtaining chemical informationNot applicable to identifying the first 11 elements.Interfering x-ray lines by matrix effects can possibly complicate analysis.样品要求及处理方法（表1－1）：表1－1 样品要求及处理方法仪器外观：依据光源系统的不同、探测方式的不同，仪器的大小、形状有一定差异（图1－1）。

图1－1 XRF仪器的外观特征光源：X射线光源：白色X射线。

即具有各种波长的X射线，可分析前面所列范围的所有元素。

放射性同位素光源：放射出的射线亦在X射线范围，但能量是固定的。

因此只能分析部分元素，仪器的体积可以很小，最新型的相当于一个计算器的大小。

同步辐射光源：光源的能量更大，对后续的二次射线检测很有利，所以分析精度更高，但仪器造价很昂贵。

2 分析原理When the atoms in a sample material are irradiated with high-energy primary x-ray photons, electrons are ejected in the form of photoelectrons. This creates electron 'holes' in one or more of the orbits, converting the atoms into ions - which are unstable（Figure 1－2(1)）.Figure 1－2 XRF分析原理(1)To restore the atoms to a more stable state, the holes in inner orbits are filled by electrons from outer orbits. Such transitions may be accompanied by an energy emission in the form of a secondary x-ray photon - a phenomenon known as "fluorescence" (Figure 1－1(2)).Figure 1－2 XRF分析原理(2)The various electron orbits are called K, L, M, etc., where K is closest to the nucleus. Each corresponds to a different energy level - and the energy (E) of emitted fluorescent photons is determined by the difference in energies between the initial and final orbit for the individual transitions(Figure 1－3).Figure 1－3 Electron orbits K, L, M,Characteristic x-ray emissions result in an energy spectrum that is a "fingerprint" of the element(Figure 1－4). So we can determine the element kinds in the sample. And also the intensities of the peaks in the spectrum are roughly proportional to the concentrations of the constituent elements.Figure 1－4 Characteristic x-ray emissions result3 仪器构成波长色散型XRF：W A VELENGTH DISPERSIVE XRF (WDS-XRF)通过分析样品在入射X射线作用下产生的二次X射线(荧光射线)的波长，来定性或定量分析样品的元素组成及含量。

能量色散型XRF: ENERGY-DISPERSIVE XRF （EDS－XRF）通过分析样品在入射X射线作用下产生的二次X射线(荧光射线)的能量，来定性或定量分析样品的元素组成及含量。

WDS-XRF and EDS-XRF(图1－5)图1－5 XRF工作流程框图4 WDS-XRFWavelength dispersive XRF uses a crystal to separate the various wavelengths: for every angle of incident radiation(入射辐射), the only wavelength reflected to the detector is the one that conforms to Bragg’s formula: nλ = 2d sin θwhere λ is the wavelength of the x-ray radiation produced by the sample; d is a constant characteristic of every crystalline substance (i.e. the x-ray crystal); and θ is the angle on incidence of the x-radiation on the sample.The crystals and their planes often used are as follow（表1－2）.表1－2 常用的分光晶体How to determine the wavelength（图1－6）:Detector is rotating when doing the wavelength determination, also the crystal is rotating by half speed. So, by changing the angle of the crystal, you can select any wavelength for specific elements of interest.Different crystal can be used determine different elements. When doing measurement, we often need to change crystals for the various elements, finally we can yield results in any form desired: qualitative, ratio, quantitative, graphic, etc.The relationship between the range of analyzing element and the crystals, and the 2 theta scanning range（表1－3）.图1－6 波长测定示意图表1－3 The relationship between the range of analyzing element and the crystalsAnalyzing Procedure:Every element has a strongest X ray wavelength. In order to determine it, first we should measure the intensity of that wavelength.For example, strongest Line and their 2θ position for Ni, Fe, & Ru, when detected by different crystal（表1－4）.表1－4 Strongest Line and their 2θ position for Ni, Fe, & RuIn order to make the determination more accurate, we should also measure their accompanying peaks（表1-5）.表1－5 Ni, Fe的K系特征Analyzing Procedure（图1－7）:图1－7 分析程序框图小结♣WDS was introduced in the early 1950s.♣WDS spectrometer systems employ diffraction by a single crystal to separate characteristic wavelengthsemitted by the sample.♣ IN WDS, polychromatic radiations emanating from the X-ray tube impinge on the specimen and reject K, L, … level electrons from atoms of the elements present in the specimen. This results in the emission of fluorescence radiation (K, L, …. series lines) which are dispersed by an appropriate crystal according to the well-known Bragg equation.♣In order to measure x-rays of different energies (wavelengths), the position of the diffracting crystal and detector must be mechanically changed relative to the fixed specimen to alter the angles.♣ To cover the range of X-ray wavelength to be measured, several different diffraction crystals must be utilized. ♣ The detectors used will often be a flow-proportional detector a gas-filled scintillator and photo multiplier tube5 EDS-XRFThe second and more common method for analyzing obsidian is EDS-XRF（图1－8）.EDSXRF systems detect elements on the periodic table between atomic numbers 11 (Na) and 92 (U).Samples can be analyzed non-destructively with little or no sample preparation in minutes and in some cases seconds.Elements in concentrations from as low as a few parts per million to 100% may be analyzed in the same sample simultaneously.Accuracy of less than one percent relative error are attainable with comparable reproducibility.Analysis by EDSXRF, like WDSXRF involves use of ionizing radiation to excite the sample, followed by detection and measurement of X-rays leaving the sample that are characteristic of the elements in the sample.However, unlike the crystal (wavelength) XRF, EDS－XRF spectrometer electronics digitize the signal produced by X-rays entering the lithium drifted detector, and send this information to the PC for display and analysis. The Na to U spectral data are in the 1-37 KeV range.图1－8 EDS-XRF的工作流程框图The high resolution lithium drifted silicon X-ray detectors provide low detection limits, few spectral interferences, and accuracy over a wide range of concentrations. These detectors are able to separate the electron energy coming from the sample into elemental components,which in turn are translated into ratio, qualitative, or quantitative elemental data in spreadsheet or word processor formats.Additionally, the tube voltages on EDS－XRF are quite low and the entire instrument is plugged into a standard wall socket. WD－SXRF requires filtered voltage at 220 V.Advantages:(1) NON-DESTRUCTIVEIn the vast majority of cases, analyzed samples are not destroyed or changed by exposure to x-rays. They can thus be saved for future reference or used for other types of testing that may be destructive.(2) MINIMAL PREPARATIONMany samples can be examined with little or no pre-treatment. Many of the alternative techniques requiredissolution procedures that are both time-consuming and costly in terms of the acids or other reagents required.(3) FASTX-ray spectrometry enables chemical compositions to be determined in seconds.(4) EASY TO USEModern instruments run under computer control, with effective software to handle measurement set-up and results calculation. Tasks that once required the constant attention of a trained analyst can now be handled by skilled operators — or even carried out by fully automated systems.Comparing with WDS-XRFWDS- slow, but accuracy is high.EDS- fast, but accuracy is low.WDS – Quantitative is easier than qualitative.EDS – both are same.Generally the differentiating ability of Wavelength is higher then that of Energy.Advantages of Energy Dispersion:* simplicity of instrumentation - no moving parts* simultaneous accumulation of the entire X-ray spectrum* qualitative analysis can be performed in 30 s,* a range of alternative excitation sources can be used in place of high-power x-ray tubes with their large, heavy, expensive and power-consuming suppliesAdvantages of Wavelength Dispersion:* resolution is better at wavelengths longer than 0.08 nm* higher individual intensities can be measured because only a small portion of the spectrum is admitted to the detector* with multichannel analysers sensitivity for weak lines in the presence of strong lines is limited because the strongest line determines the counting time* lower detection limits are possible* alternative sources include, low power x-ray tubes, secondary monochromatic radiators, radioisotopes and ion beams.6 定量分析定量分析的基本原理是测定出样品所产生的2次X射线的强度，然后与标准样品所产生的2次X射线的强度比较，从而确定出样品中某种元素的含量。