X-ray fluorescence spectrometer (XRF) is a new type of instrument that can perform simultaneous simultaneous determination of multiple elements. Under X-ray excitation, the inner electrons of the atom of the element under test undergo an energy level transition to emit secondary X-rays (ie, X-ray fluorescence). Wavelength and energy are two different physical quantities used to describe X-rays from different angles. A wavelength dispersive X-ray fluorescence spectrometer (WD-XRF) is a spectral X-ray signal that is diffracted by a crystal and then received by a detector. If the spectroscopic crystal and the detector move in synchronization, and the diffraction angle is constantly changed, the wavelength of the characteristic X-rays generated by various elements in the sample and the intensity of the X-rays of the respective wavelengths can be obtained. Qualitative analysis and quantitative analysis can be performed accordingly. Wavelength dispersive X-ray fluorescence spectrometer was produced in the 1950s. Because of the multi-component simultaneous measurement of complex systems, it has attracted attention. Especially in the geological department, this instrument has been deployed successively, and the analysis speed has been significantly improved, which plays an important role. With the advancement of science and technology, after the invention of the semiconductor detector in the early 1960s, it was possible to perform energy spectrum analysis on X fluorescence. Energy dispersive X-ray fluorescence spectrometer (ED-XRF), which uses X-ray tube to produce primary X-rays onto the sample, and the resulting characteristic X-rays (fluorescence) directly enter the Si(Li) detector. Conduct qualitative and quantitative analysis. The first ED-XRF was introduced in 1969. In recent years, due to the development of commercial ED-XRF instruments and computer software, the functions have been perfected and the application fields have been broadened. Its characteristics and superiority have been increasingly recognized and developed rapidly. When the sample is irradiated with X-rays, the sample can be excited by fluorescent X-rays of various wavelengths, and the mixed X-rays need to be separated by wavelength (or energy) to measure the intensity of X-rays of different wavelengths (or energies), respectively. For qualitative and quantitative analysis, the instrument used for this purpose is called an X-ray fluorescence spectrometer. Since X-rays have a certain wavelength and a certain amount of energy, there are two basic types of X-ray fluorescence spectrometers: wavelength dispersion type and energy dispersion type. The figure below is a schematic of the two types of instruments. The main components and working principles of the two types of X-ray spectrometers are described below: 1, X-ray tube The main component of the spectroscopic system is a crystal beam splitter, which functions to separate X-rays of different wavelengths by crystal diffraction. According to the Bragg diffraction law 2dsin θ=nλ, when the X-ray of the wavelength λ is incident on the crystal at the angle θ, if the interplanar spacing is d, the first-order diffraction with the wavelength λ=2dsin θ can be observed in the direction of the exit angle θ. And the wavelength is λ/2, λ/3----- and other high-order diffraction. By changing the angle θ, X-rays of other wavelengths can be observed, thus allowing X-rays of different wavelengths to be separated. The split crystal is driven by a crystal rotating mechanism. Since the sample position is fixed, in order to detect a fluorescent X-ray having a wavelength of λ, the spectroscopic crystal is rotated by an angle θ, and the detector must be rotated by a 2θ angle. That is to say, a certain 2θ angle corresponds to a certain wavelength of X-rays, and the spectroscopic crystal and the detector are continuously rotated, and the fluorescent X-rays of different wavelengths can be received (Fig. 10.5). A crystal has a certain interplanar spacing, so it has a certain range of applications. The current X-ray fluorescence spectrometers are equipped with crystals with different interplanar spacings to analyze different ranges of elements. The above spectroscopic system relies on the rotation of the spectroscopic crystal and the detector to enable the characteristic X-rays of different wavelengths to be sequentially detected. This spectrometer is called a sequential spectrometer. There is also a type of spectrometer spectroscopic crystal that is fixed, and the mixed X-rays are diffracted in different directions after passing through the spectroscopic crystal. If the detector is installed in these directions, the X-rays can be detected. This spectrometer that simultaneously detects non-wavelength X-rays is called a simultaneous spectrometer. The simultaneous spectrometer has no rotating mechanism, so the performance is stable, but the detector channel can not be too much, suitable for the determination of fixed elements. In addition, the spectrometer of the spectrometer does not use a planar crystal, but with a curved crystal, the crystal lattice plane used is bent into a circular arc with a radius of curvature of 2R, and the incident surface of the crystal is ground into an arc of radius R. The first slit, the second slit and the spectroscopic crystal are placed on a circumference of radius R such that the surface of the crystal is tangent to the circumference, and the distance between the two slits to the crystal is equal (see Fig. 10.6), which can be proved by geometric method. When X-rays are directed from the first slit to the points of the curved crystal, they are all at the same angle to the plane of the lattice, and the reflected beam is again concentrated at the second slit. Because of the convergence of reflected light, such a beam splitter is called a focusing spectroscope, and a circle with a radius of R is called a focusing circle or a Roland circle. When the spectroscopic crystal is rotated to different positions around the center of the focus circle, different glancing angles θ are obtained, and the detector detects X-rays of different wavelengths. Of course, the second slit and the detector must also rotate accordingly, and the rotational speed is twice the crystal speed. The biggest advantage of focusing method is that the fluorescent X-ray loss is small and the detection sensitivity is high. 3. Inspection and recording system Another type of detection device is the scintillation counter as shown above. The scintillation counter consists of a scintillation crystal and a photomultiplier tube. After X-rays are incident on the crystal, light can be generated and amplified by a photomultiplier tube to obtain a pulse signal. The scintillation counter is suitable for the detection of heavy elements. In addition to the above two detectors, there are also semiconductor detectors for the detection of energy dispersive X-rays (see next section). In this way, the fluorescent X-rays generated by the X-ray excitation are detected by the detector after being split by the crystal, and the relationship curve of the 2θ-fluorescence X-ray intensity, that is, the fluorescent X-ray spectrum is obtained, and the figure below is the fluorescence X of an alloy steel. Ray spectrum. 4 , energy dispersive spectrometer  Roundness Measuring Instruments Roundness Measuring Instruments ,Roundness Tester,Roundness Measuring Machine,Instrument Used To Measure Roundness Zhejiang dexun instrument technology co., ltd , https://www.dexunmeasuring.com
The target and tube operating voltage of the X-ray tube determines the intensity of the portion of the primary X-ray that is effective to excite the excited element. The working voltage of the tube is increased, and the proportion of the X-rays at a short wavelength is increased, so that the intensity of the generated fluorescent X-rays is also enhanced. However, it does not mean that the higher the tube operating voltage, the better, because the fluorescence excitation efficiency of the incident X-ray is related to its wavelength, and the closer to the absorption limit wavelength of the measured element, the higher the excitation efficiency.
The X-rays generated by the X-ray tube are incident on the sample through the é“ window, and the characteristic X-rays of the sample elements are excited. When working normally, about 0.2% of the power consumed by the X-ray tube is converted into X-ray radiation, and the rest become heat energy. The X-ray tube is heated, so it is necessary to continuously cool the target electrode with cooling water.
2, the light distribution system
The above describes the use of a spectroscopic crystal to separate and detect fluorescent X-rays of different wavelengths to obtain a fluorescent X-ray spectrum. The energy dispersive spectrometer is characterized by the fact that fluorescent X-rays have different energies, which are separated and detected without using a spectroscopic crystal, but by means of a semiconductor detector. Such semiconductor detectors include lithium-drift silicon detectors, lithium drift detectors, and high-energy germanium detectors. After the X-ray photo is incident on the detector, a certain number of electron-hole pairs are formed, and the electron-hole pair forms an electric pulse under the action of the electric field, and the pulse amplitude is proportional to the energy of the X-photon. Over a period of time, the fluorescent X-rays from the sample are sequentially detected by the semiconductor detector to obtain a series of pulses whose amplitude is proportional to the photon energy, which is amplified by the amplifier and sent to a multi-channel pulse analyzer (usually more than 1000 channels). The number of pulses is separately counted according to the magnitude of the pulse amplitude. The pulse amplitude can be scaled by the X-photon energy, so that the distribution curve of the count rate with the photon energy is obtained, that is, the X-ray energy spectrum. The energy spectrum is corrected by a computer and then displayed, the shape of which is similar to the spectrum, except that the abscissa is the energy of the photon.
The biggest advantage of energy dispersion is the ability to simultaneously measure almost all of the elements in a sample. Therefore, the analysis speed is fast. On the other hand, since the total detection efficiency of the X-ray by the spectrometer is higher than that of the spectrum, the fluorescent X-ray can be excited using a low-power X-ray tube. In addition, the spectrometer has no mechanical mechanism as complicated as the spectrometer, so the operation is stable and the instrument volume is small. The disadvantage is that the energy resolution is poor and the detector must be stored at low temperatures. It is difficult to detect light elements.
5 , sample preparation
The sample subjected to X-ray fluorescence spectrometry may be either a solid or an aqueous solution. Regardless of the sample, the condition of the sample preparation has a great influence on the measurement error. For metal samples, attention should be paid to the error caused by component segregation; for samples with the same chemical composition and different heat treatment processes, the count rate is different; metal samples with uneven composition should be remelted, rapidly cooled and turned into wafers; The sample is polished and polished; for powder samples, it is ground to 300 mesh to 400 mesh and then pressed into a wafer, which can also be placed in a sample cell. If a uniform sample is not obtained for a solid sample, the sample can be dissolved with an acid and precipitated into a salt for measurement. For the liquid sample, it can be dripped on the filter paper, and it can be measured by evaporating the water with an infrared lamp, or it can be sealed in the sample tank. In summary, the sample tested should not contain water, oil and volatile components, and should not contain corrosive solvents.
6 , qualitative analysis
The fluorescent X-rays of the different elements have respective specific wavelengths, so the composition of the elements can be determined according to the wavelength of the fluorescent X-rays. In the case of a wavelength dispersion type spectrometer, for a crystal having a certain interplanar spacing, the wavelength λ of the X-ray can be obtained from the 2θ angle of the rotation of the detector, thereby determining the elemental composition. In fact, in qualitative analysis, the line can be automatically identified by a computer to give qualitative results. However, if the element content is too low or there is spectral line interference between the elements, manual identification is still required. First, the characteristic X-rays of the X-ray tube target and the accompanying lines of the strong peaks are identified, and then the remaining oblique lines are marked according to the 2θ angle. When analyzing the unknown spectral line, it is necessary to take into account the source and nature of the sample in order to make a comprehensive judgment.
7 , quantitative analysis
The quantitative analysis by X-ray fluorescence spectrometry is based on the fact that the fluorescent X-ray intensity I1 of the element is proportional to the content Wi of the element in the sample:
Ii=IsWi
In the formula, when Is is Wi=100%, the intensity of the fluorescent X-ray of the element. According to the above formula, quantitative analysis can be performed by using a standard curve method, an incremental method, an internal standard method, or the like. However, these methods must make the composition of the standard sample as identical or similar as possible to the composition of the sample. Otherwise, the matrix effect of the sample or the influence of the coexisting element may cause a large deviation in the measurement result. The so-called matrix effect refers to the effect of changes in the basic chemical composition and physicochemical state of the sample on the intensity of X-ray fluorescence. Changes in chemical composition can affect the absorption of the sample by X-ray and X-ray fluorescence, as well as the fluorescence enhancement effect. For example, when measuring elements such as Fe and Ni in stainless steel, NiKα fluorescent X-rays are generated by the excitation of one X-ray, NiKα may be absorbed by Fe in the sample, and Fe is excited by Fe, and the absorption effect of Fe is measured when Ni is measured. When the result is lowered, when Fe is measured, the result is high due to the fluorescence enhancement effect. However, it is almost impossible to configure the same substrate.
In order to overcome this problem, the current X-ray fluorescence spectrometry method generally adopts the basic parameter method. This method is based on the absorption and enhancement effects between the elements, the theoretical intensity of the elemental fluorescent X-ray is calculated from the standard or pure substance, and the intensity of the fluorescent X-ray is measured. Comparing the measured intensity with the theoretical intensity, determining the sensitivity coefficient of the element. When measuring the unknown sample, first measuring the fluorescent X-ray intensity of the sample, setting the initial concentration value according to the measured intensity and the sensitivity coefficient, and then calculating the theoretical value of the concentration value. strength. The measured intensity is compared with the theoretical strength so that the two reach a certain predetermined precision, otherwise it is corrected again. The method is to measure and calculate all the elements in the sample, and the mutual interference effect between the elements is considered, and the calculation is very complicated. Therefore, you must rely on a computer for calculations. This method can be considered as a standard-free quantitative analysis. When the content of the sample to be tested is greater than 1%, the relative standard deviation may be less than 1%.
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