Principles of XRF analyzer
X-ray fluorescence spectroscopy is one of the widely used elemental analysis techniques in industrial and laboratory projects, which works based on the excitation of studied atoms by an external X-ray source. The excited atom emits X-ray photons with characteristic energy and wavelength when returning to the ground state, which are detected and read by a detector.
Principles of XRF device
Most XRF devices are divided into two categories based on the method of X-ray fluorescence detection: Energy Dispersive and Wavelength Dispersive.
source The first part of both ED-XRF and WD-XRF models is the X-ray source. The source consists of a vacuum chamber in which there is a cathode (usually in the form of a tungsten wire) and an anode with a potential difference of several thousand kilovolts compared to the cathode. Electrons are released from the cathode due to heat and are accelerated towards the anode. When the accelerated electrons hit the anode, by the bremsstrahlung mechanism, white radiation occurs, which contains the characteristic X-ray photon of the anode. Finally, these photons exit through the beryllium window embedded in the tube.
Schematic of an X-ray tube
Needless to say, there is no one-size-fits-all XRF method resource that meets all needs. The power of X-ray tubes can be adjusted from several watts for EDXRF to several kilowatts for WDXRF. In the last case, the tube must be a cool liquid because a large part of the power of the tube appears as heat. Also, the anode material must be carefully selected because its characteristic beam energy will be used for proper excitation of the sample. Some single element anodes are aluminum, chromium, tungsten, palladium and gold. It is enough to identify light elements with a high intensity and low energy beam in the range of 1 to 10 electron volts. While for the excitation of heavier elements, more energy is needed up to the range of 50 keV. It is also important to know that the main reason for the background in the detector is the source of the X-ray radiation.
In devices designed to cover a wide range of atomic numbers from light to heavy, a different approach to X-ray generation is employed. An anode is used for white radiation. Then this continuous radiation hits a secondary target material, which is the characteristic beam of the target and does not cover the rest of the spectrum. Then this radiation is used to excite the sample.
Detector
The two main types of XRF devices, WD and ED, differ in their overall detector system. EDXRF relies on a semiconductor detector that captures the entire spectrum of light emitted from the sample and then converts it into a histogram of photon numbers and energies. On the other hand, WDXRF uses a crystal that disperses and separates the waves emitted from the sample based on their wavelength and places the detector in the right place to receive photons with a specific and desired energy.
WDXRF spectrometer
The following figure shows the schematic of a wavelength resolution spectrometer. The X-rays that come from the sample and contain the secondary fluorescence beam of the tested element are first passed through the collimator, which is made of very close metal layers, and then reach the analyzing crystal.
Schematic of WD-XRF wavelength separation method
Analyzing crystal scatters incoming rays according to Leaf’s law (nλ=2dsinθ) depending on their wavelength in different angles. In order to cover different wavelengths, different crystals with different plate spacing are used. Below is a list of common crystals and their range of use.
The waves scattered by the crystal finally reach the detector. Two series of detectors are used in WDXRF. First, there is the gas-flow proportional detector, which is used for long wavelengths. The high-energy beams pass through this detector and are read by the NaI detector.
In the WDXRF system, it is possible to use several detectors placed at certain angles. In this way, several elements can be analyzed simultaneously over and over again. This system has high flexibility along with extremely good sensitivity in research work. Also, the disadvantages of this method in comparison with the EDXRF method include the lower speed for processing the entire spectrum for all elements, higher cost, and larger size devices.
EDXRF spectrometer
Next, let’s examine the energy dissociation spectrometer. The following figure shows the schematic of this spectrometer. As you can see, unlike the wavelength separation spectrometer, there is no crystal to separate the different wavelengths, and the photons are detected by a detector based on their energy. While the placement of the detector in front of the sample is simple, it requires detectors with sophisticated and advanced electronics, which are not difficult to prepare with the advancement of technology.
Schematic of ED-XRF energy resolution method
Energy separation in EDXRF method is done in semiconductor detectors. For example, we can refer to germanium detectors and also silicon detectors, the latter being more widely used. Semiconductor detectors work on the basis that X-ray photons hitting the semiconductor diode produce electron-hole pairs, the higher the number, the higher the energy of the incident photon. Then, by applying a high voltage, the electric charge created is stored in a capacitor and after being amplified by an amplifier, it is converted into digital signals and finally processed with a multi-channel analyzer. The resulting data is sent to a computer and displayed by software and computational algorithms in the form of numbers and understandable information for the operator. In some newer devices, in order to achieve greater sensitivity, two or more detectors for different wavelengths have been used. However, usually the detection limit of EDXRF method is not as good as WDXRF.
Spectron manufactures XRF instruments in both energy dispersive (EDXRF) and wavelength dispersive (WDXRF) models. For example, the Spectron MAKC-GVM device, which works by the wavelength separation method, has the ability to measure elements from atomic number 11 (Na) to atomic number 92 (U) due to various crystals. Also, other models have been designed and supplied to measure specific elements such as sulfur and allow the operator to perform daily and routine tests well and in the shortest possible time without making any initial settings. Among these, we can mention the SW-D3 models for measuring sulfur using the wavelength separation method, CLSW for measuring chlorine and sulfur simultaneously using the wavelength separation method, and SL for measuring sulfur using the energy separation method.
which are widely used in laboratories of oil and gas refineries as well as petrochemical industries.
Other applications of XRF devices include steel and cement industries, which routinely use elemental analysis and XRF methods both for research and development of new materials and for quality control. Also, in the plastic and polymer industries, there are cases where online XRF analyzers are used to control the production line and monitor the metals in the extrusion line.
Mining, geology and archeology are other fields where the XRF method is used to study the composition of the soil and identify the elements in the sample down to very small amounts.
Relying on advanced and strong devices as well as the technical knowledge of its experts in the field of X-ray fluorescence spectroscopy (XRF), Spectron has been able to provide practical solutions to problems in the fields of oil and gas, petrochemicals, environmental monitoring, mining, metallurgy, Provide geology.
In short, compared to other elemental analysis methods, the XRF method is a flexible and fast method that has the ability to test different samples from solid powder to liquid. Also, the XRF method is easily available and has a much lower cost than other methods. The major part of the costs of this method is related to the purchase of the device, and preparing the sample, performing the test, as well as maintaining the device itself, include very little time and cost.
In general, the most important weakness of the XRF method is its high detection limit compared to methods such as Graphite Furnace Atomic Absorption Spectroscopy and Inductively Coupled Plasma Mass Spectrometry (ICP-MS), both of which have much lower detection limits than They have XRF. On the other hand, it is worth mentioning that although the atomic absorption technique has a better detection limit, it is a destructive method that can only measure one element at a time, so the analysis speed is lower. It also requires more consumables such as argon gas and maintenance, which increase the overall cost of the test. In addition to having a far lower and better detection limit than the other mentioned methods, the ICP-MS method also has a higher operating speed and is also able to identify isotopes of an element. However, only liquid samples can be injected into it, and solid samples require more preparation steps such as dissolving in acid. The consumption of pure argon gas in this method is very high and greatly increases the cost of the test. It goes without saying that the initial cost of preparing an ICP-MS device is much higher than that of an atomic absorption and XRF device.
Artin Azma Company, as the representative of Spectron in Iran, provides XRF devices for domestic applicants and provides customers with a valid warranty and ten-year after-sales service.