X-ray fluorescence spectrometry

X-ray fluorescence spectrometry (XRF, X-ray aerosol, X-ray fluorescence spectrometry) is an analytical method for determining the concentrations of elements from Be (No. 4) to U (No. 92) in the range from ppm to 100% in various substances and materials.

X-ray fluorescence spectrometer.

Due to its versatility, accuracy and speed of measurements, as well as ease of use, X-ray fluorescence analysis has found wide application in industry and science.
X-ray fluorescence analysis is based on the dependence of the intensity of X-ray radiation on the concentration of an element in a sample.

When a sample is irradiated with a powerful X-ray tube radiation flux, characteristic fluorescent radiation of atoms arises, which is proportional to their concentration in the sample.
When using wave-dispersive spectrometers, fluorescent radiation is decomposed into a spectrum using crystal monochromators, and then, using detectors and counting electronics, its intensity is quantitatively measured.
In energy-dispersive spectrometers, fluorescent radiation is decomposed into a spectrum thanks to semiconductor detectors (Si or Ge), where all radiation from the sample is recorded and converted into electrical pulses, forming a spectrum in the form of a dependence of the number of pulses on the energy of each element.
Quantitative and qualitative analysis is performed by processing the spectrum using methods of mathematical dependencies and statistics.

X-ray fluorescence .

To perform X-ray analysis, the atom of the analyzed sample must be irradiated with high-energy photons of primary X-ray radiation (from an X-ray tube or radionuclide source). When irradiated, atoms pass into an excited state, which consists of the transition of electrons to higher energy levels. The atom remains in the excited state for a fraction of a second, after which it returns to the ground state. In this case, electrons from the outer shells either fill the resulting vacancies, and the excess energy is emitted as a secondary photon, or the energy is transferred to another electron from the outer shells. The energy of the secondary photon is in the range of X-ray radiation energies, which is located in the spectrum of electromagnetic oscillations between ultraviolet and gamma radiation.

Electron orbitals in an atom are designated K, L, M, etc., K is the orbital closest to the nucleus. Each electron orbital in an atom of each element has its own energy level. The energy of the emitted secondary photon is determined by the difference between the energy of the initial and final orbitals between which the electron transition occurred.

The wavelength of the emitted photon is related to the energy. According to Bragg's law

E = E1-E2 = hc/l ,

where E1 and E2 are the energies of the orbitals between which the electron transition occurred, h is Planck's constant, c is the speed of light, l is the wavelength of the emitted (secondary) photon.

Thus, the fluorescence wavelength is an individual characteristic for each element and is the characteristic fluorescence. The intensity (the number of photons received per unit of time) is proportional to the concentration (the number of atoms) of the corresponding element. This makes it possible to determine the number of atoms of each element included in the sample.

X-ray tube.

The X-ray tube is a source of primary high-energy radiation. It is powered by a highly stable high-voltage generator.
The mechanism of primary radiation is similar to the fluorescence mechanism, but the excitation of the tube anode material occurs when it is bombarded with high-energy electrons, and not by X-rays, as in fluorescence. The spectral composition of the tube radiation depends on the anode material. A rhodium Rh anode is mainly used, but for certain tasks other materials can be used - Mo, Cr, Au, etc.
During X-ray fluorescence analysis, all elements of the sample simultaneously emit photons of characteristic radiation. To determine the concentration of a specific element in the sample, it is necessary to isolate from the total radiation flux coming from the sample the radiation of exactly that wavelength (for VD spectrometers) or that energy (for ED spectrometers) that is characteristic of the element being determined. This is done by decomposing the total radiation flux coming from the sample by wavelengths/energies and obtaining a spectrum. The spectrum is a curve that describes the dependence of radiation intensity on wavelength/energy.

Decomposition of radiation into a spectrum

Wave-dispersive spectrometers. When decomposing radiation into a spectrum (separating out different wavelengths), crystal monochromators with crystal planes parallel to the surface and having an interplanar distance of d are used .
If radiation with a wavelength l falls on the crystal at an angle of q , then diffraction will occur only if the distances traveled by photons upon reflection from adjacent crystal planes differ by an integer ( n ) of wavelengths. With a change in the angle q, when the crystal rotates relative to the radiation flux, diffraction will occur successively for different wavelengths in accordance with Bragg's law: nl = 2d sinq . The angular position ( q ) of the crystal is set depending on the wavelength that must be separated from the spectrum for analyzing the required element. Crystal monochromators. Since the separation of X-ray fluorescence peaks depends on the ratio of wavelength and interplanar distance ( d ), to increase the selectivity and sensitivity of the equipment, the spectrum of the studied sample is measured in a wide energy range using several crystal monochromators made of different materials. Single crystals such as germanium (Ge111), lithium fluoride (LiF200/220/440) are ideal analyzers for the radiation of many elements. Multilayer synthetic coatings are used to increase sensitivity in the analysis of light elements.

Energy-dispersive spectrometers. Unlike the wave-dispersive method, the energy-dispersive determination simultaneously records the entire energy range of the secondary (characteristic) radiation from the sample. The spectrum is the dependence of the intensity on the radiation energies of the elements.
The selected radiation enters the X-ray detector to measure the intensity. Intensity is the number of photons received per unit time.

Radiation detection

When detecting fluorescent radiation, the fluorescence energy is converted into voltage pulses of a certain amplitude.

Wavelength-dispersive spectrometers. There are different types of detectors. For relatively long wavelengths, gas-filled proportional detectors (flow-through and sealed) are used to analyze light elements. Their operation is based on the ionization of gas by radiation and the measurement of the number of electrical pulses that have passed through the ionized gas. For the analysis of heavy elements (for short wavelengths), scintillation detectors are used, which measure the current of a photocell sensitive to the luminosity of a special substance - a scintillator (NaI/Tl) when X-rays hit it. The number of pulses recorded is directly proportional to the number of element atoms in the sample.

Energy-dispersive spectrometers. Semiconductor solid-state detectors are used to detect characteristic radiation; their operation is based on ionization inside the semiconductor. A sensitive area is created in the semiconductor detector, in which there are no free charge carriers. Having entered this area, a charged particle causes ionization, accordingly, electrons appear in the conduction zone, and holes in the valence zone. Under the influence of voltage applied to the electrodes deposited on the surface of the sensitive area, the electrons and holes move, and a current pulse is formed. A voltage of up to several kV is applied to the semiconductor crystal, ensuring the collection of all charges formed by the particle in the detector volume. The electron-hole pairs begin to move to the electrodes. As a result of this movement, an electric pulse is generated, which is then amplified and recorded by the counting electronics. The semiconductor detector is made mainly of Si or Ge, it is necessary to cool the detector during analysis using the Pellet effect or liquid nitrogen.

Counting electronics

The counting electronics records the number of pulses coming from the detectors and the energy levels corresponding to the amplitudes.
Modern amplifiers and pulse analyzers allow obtaining a satisfactory statistical measurement error in less than 2 seconds. A longer counting time is required for light elements, as well as for analyzing elements with concentrations close to the detection limit, since in this case, an analysis of a small number of photons with low energies is required.
Analysis and processing of measurement results is performed automatically. For this purpose, analysis methods have been developed in the form of software packages (computer programs). During measurements, the software controls all spectrometer units in accordance with the specified analysis program. All modern spectrometers with an automatic sample feeder allow analysis to be performed continuously and without operator intervention, and upon completion of measurements, concentrations are calculated. Analysis results are transmitted electronically automatically to the specified addresses or accumulated in the measurement database for further processing.

Types of X-ray spectrometers

Wave-dispersive (WD) X-ray spectrometer. X-ray fluorescence spectrometers, in which the characteristic radiation is obtained using crystal monochromators, are called "wave-dispersive". WD are divided into spectrometers of serial and parallel (quantometers) types.

Sequential spectrometers

In spectrometers of this type, each characteristic line of X-ray radiation of any number of elements is sequentially isolated using a moving crystal monochromator and a high-precision goniometer (a device for measuring angles) coupled with a computer-controlled rotation device.

Advantages of sequential VD spectrometers:

• Definition of any number of elements.
• Optimal measurement conditions are programmed for each element.
• High sensitivity, low detection levels.
• Lower cost compared to parallel spectrometers.

Parallel type spectrometers

With parallel spectrometers, measurements are made simultaneously (in parallel). The intensities of the characteristic radiation of the elements are measured simultaneously by using several tuned fixed "channels" located around the sample. It can be said that each of the channels is a separate spectrometer with a crystal monochromator and detector tuned to receive a specific wavelength of one element.

Advantages of parallel spectrometers:

• The highest speed of analysis when used for in-line quality control in industry - express analysis of the technological process.
• Few moving parts, excellent reliability in industrial environments

Energy dispersive spectrometers

Advantages of ED spectrometers:

• Significantly lower cost compared to wavelength-dispersive XRF spectrometers.
• Compactness, convenience, simplicity, possibility of making desktop and portable versions.
• The accuracy and sensitivity when measuring heavy elements is no worse than that of wavelength-dispersive X-ray spectrometers.