Principle and Application Prospects of Total Reflection X-ray Fluorescence Spectrometry (TXRF)

Total Reflection X-ray Fluorescence (TXRF) is a multi-element simultaneous analysis technology that has been developed in recent years. By utilizing total reflection, TXRF reduces the stray background of sample fluorescence by about four orders of magnitude compared to Energy Dispersive X-ray Fluorescence (EXRF) spectrometers, significantly enhancing energy resolution and sensitivity. This eliminates the background enhancement or reduction effects commonly encountered in XRF measurements. Additionally, TXRF inherits the advantages of EXRF, making it an irreplaceable new method for elemental analysis. It is regarded as a competitive analytical tool in the field of analysis and holds a leading position in atomic spectrometry.

Within the range of X-ray fluorescence spectrometers, compared to wavelength dispersive spectrometry (WDS) methods, TXRF analysis requires very small sample amounts, does not involve the cumbersome process of sample preparation, and avoids background enhancement or reduction effects. There is no need for different calibration curves for different matrices each time. Moreover, due to the use of internal standard methods, the requirements for environmental temperature are very low. Therefore, in terms of simplicity, economy, and minimal sample requirements, TXRF has significant advantages over WDS methods.

The emergence of total reflection X-ray fluorescence analyzers has greatly advanced the field of elemental analysis. These instruments not only offer high sensitivity, high resolution, and rapid analysis but also can be widely used in the analysis of various materials, providing researchers with a powerful tool.

The principle of Total Reflection X-ray Fluorescence (TXRF)

The Total Reflection X-ray Fluorescence (TXRF) spectrometer primarily consists of the following components: X-ray source, optical system, sample introduction system, detector, data processing system, and other accessories. The following sections will mainly introduce the first four components.

1. X-ray Source: Comprised of a high voltage generator and an X-ray tube. It provides primary X-rays that excite the elements in the sample to produce X-ray fluorescence, with intensity proportional to the primary X-ray's intensity. Typically, XRD or XRF generators can meet the needs of TXRF, with high voltages reaching up to 80 kV, currents up to 80 mA, and overall power up to 3 kW or more. Input stability is generally less than 10%, and output stability is less than 0.01%.

Currently, commercial TXRF spectrometers commonly use X-ray tubes with Mo or W targets, or mixed targets. For example, the TX 2000 Total Reflection X-ray Fluorescence Spectrometer from GNR provides a Mo/W mixed target.

Figure 1: Mo/W Mixed Target

2. Optical System: In order to meet the requirements of TXRF applications (such as incident angle and energy distribution), further adjustments are needed for the geometric shape and spectral distribution of the primary X-rays. This is primarily achieved through the use of apertures, filters, collimating slits, monochromators, etc.

The primary X-rays have a certain divergence angle, and adjustments to the geometric shape can be accomplished using collimating slits.

The emission efficiency of high-energy photons in the continuous spectrum emitted by the X-ray tube is lower than that of low-energy photons, and the critical angle for total reflection of low-energy photons is greater than that of high-energy photons. Therefore, under the condition of satisfying the total reflection of low-energy photons, high-energy photons in the continuous spectrum do not meet the total reflection condition, resulting in a significant increase in background. It is necessary to further filter out high-energy photons, typically achieved using filters and monochromators.

Commonly used filters often employ the principle of total reflection, where low-energy photons undergo total reflection while high-energy photons scatter or are absorbed, thus achieving the filtering effect. Filters can be divided into single total reflection and double total reflection bodies, as shown in the diagram below:

Figure 2: Single Total Reflection Filter

Figure 3: Double Total Reflection Filter

Monochromatic excitation under total reflection is ideal, but achieving monochromaticity solely through filters is not feasible. Therefore, it is common in commercially available instruments to employ monochromators utilizing Bragg reflection and various other techniques. For instance, both GNR's TX 2000 and HORIZON Total Reflection X-ray Fluorescence spectrometers offer dual total reflection optics and multi-layer Si/W monochromators (TX 2000 can also switch between TXRF and conventional XRF), as shown in the figure below:

Figure 4: Optical Path Schematic

3. Sample Introduction System: Providing sample carriers, meeting total reflection conditions, and completing automatic sample introduction operations, mostly made of materials such as quartz glass and organic glass.

4. Detector: Serving as the core component for data readout, the detector requires characteristics such as high energy resolution and minimal thermal effects. Main types include semiconductor detectors, silicon drift detectors, and position-sensitive detectors. Currently, commercially available instruments mostly use silicon drift detectors (SDD), with GNR employing a semiconductor-cooled SDD detector.

Figure 5: Detector

X-ray diffractometers (XRD) are used to measure crystal structure indicators of samples such as powders and thin films, and are commonly applied in molecular structure analysis and research on metal phase transitions. On the other hand, Total Reflection X-ray Fluorescence spectrometers (TXRF) have detection limits reaching picogram levels. Their non-destructive analysis feature is applied in trace element analysis, spanning industries including environmental, pharmaceutical, semiconductor, nuclear, and petrochemical. Specialized residual stress analyzers and residual austenite analyzers, designed and manufactured to meet industrial market demands, have seen widespread use in the field of manufacturing material testing in recent years. Their ease of operation is highly favored by the industry.

The application prospects of TXRF

In the future, with the continuous advancement of science and technology, the performance of total reflection X-ray fluorescence analyzers will continue to improve. For instance, by improving the instrument structure and enhancing the sensitivity and resolution of detectors, the analytical performance of these instruments can be further increased. As the range of applications expands, new application scenarios for total reflection X-ray fluorescence analyzers will continue to emerge.

Additionally, the development of total reflection X-ray fluorescence analyzers should consider environmental and safety aspects. For example, developing low-radiation and low-energy-consumption TXRF analyzers can reduce environmental impact. Furthermore, ensuring the operational safety of the instruments is crucial to guarantee safe usage.

In summary, as an important tool in the field of elemental analysis, total reflection X-ray fluorescence analyzers will play an increasingly significant role in future scientific research. We should continue to focus on their development to provide better support for scientific research.

The current applications and future prospects of TXRF elemental analyzers in the field of elemental analysis are encouraging. They can be widely used for the analysis and determination of major, minor, and trace elements in various fields such as geology, metallurgy, chemical industry, food, biology, medicine, environmental protection, forensic science, archaeology, and high-purity materials. Particularly in the quality control of silicon wafer surfaces in the semiconductor industry, TXRF has an irreplaceable advantage and is currently widely used.