What is XRF (X-ray fluorescence) and How Does it Work?

X-ray fluorescence, commonly abbreviated as XRF, is a non-destructive analytical technique used to determine the elemental composition of materials. Each element has a unique characteristic X-ray fluorescence spectrum. An X-ray Fluorescence (XRF) spectrometer can easily identify which elements are present in a sample and determine their respective concentrations.

In this article, we will explore the working principles of XRF, its main components, and the types of materials it can analyze.

How X-ray Fluorescence Works

1. High-energy X-rays are directed at the sample, exciting the atoms within it.

2. An electron in one of the atom’s inner-shell orbitals is ejected, emitting secondary X-rays.

3. As the atom gradually returns to a stable state, electrons from higher-energy orbitals fill the vacancies left in the inner-shell orbitals.

4. When an electron transitions from a higher energy level to a lower one, it emits fluorescent X-rays. The energy of the X-rays depends on the energy difference between the two energy levels before and after the transition. By measuring this energy, X-ray fluorescence analysis can be performed.

Common conventions for naming transitions: When an electron from the L shell fills a vacancy in the K shell, it is called Kα; when an electron from the M shell fills a vacancy in the K shell, it is called Kβ; when an electron from the M shell fills a vacancy in the L shell, it is called Lα; and so on.

Why Can Characteristic X-Rays Identify Elements?

The energy level structure of each element is fixed. Therefore, the energy level differences are also fixed, and the energy of the emitted photons is fixed as well. This energy is uniquely associated with that element.

Moseley’s Law establishes the relationship between energy (or wavelength) and atomic number: the higher the atomic number, the higher the energy of the characteristic X-rays and the shorter their wavelength.

Energy and wavelength can be converted between each other:

E (keV) = 12.4 ÷ λ (Å)

In other words, 12.4 keV corresponds to a wavelength of approximately 1 Å. These two values are essentially different ways of expressing the same information.

This leads to two rules for identification:

1. By measuring the energy of the fluorescent photons, we can determine which element is present → qualitative analysis.

2. By measuring the intensity of the energy line, we can determine the quantity → quantitative analysis.

Why Are Light Elements Difficult to Detect?

1. Low fluorescence yield. When light elements are excited, only a small fraction of the energy is actually released in the form of X-rays; most of the energy is dissipated in other ways.

2. Low energy, making them prone to absorption. The characteristic X-rays emitted by light elements have very low energy. They may be absorbed by the air, the detector window, or the sample itself before reaching the detector. Shortening the optical path, evacuating the chamber, or filling it with helium can improve detection, but the improvement is limited.

Result: The typical measurable range for energy-dispersive X-ray fluorescence (EDXRF) generally extends from Na to U; wavelength-dispersive X-ray fluorescence (WDXRF) can go as low as beryllium. Elements such as hydrogen, helium, lithium, boron, carbon, nitrogen, and oxygen cannot be effectively measured by standard XRF. Methods such as combustion analysis, OES, or GD-OES must be used instead.

Note: XRF measures total elemental content, not chemical speciation. It can detect the presence of iron (Fe) in a sample but cannot distinguish between metallic iron, FeO, or Fe₂O₃. Therefore, methods such as XRD, FTIR, or XPS are required to determine the valence state and compounds of elements.

X-ray Fluorescence (XRF) Spectrometer Operating Procedures

The operating procedure for an X-ray Fluorescence (XRF) spectrometer typically consists of the following six steps:

1. Clean the sample surface. Remove oxide layers, paint, oil stains, and dust. The condition of the sample surface often has a greater impact on the results than the instrument parameters.

2. Select the measurement mode. Common modes include alloys, soil, ores, RoHS, and precious metals. Each mode corresponds to a different combination of voltage, current, and filters.

3. Excitation. The miniature X-ray tube inside the instrument is energized; high-voltage electrons accelerate and strike the target material (typically rhodium, tungsten, or copper), generating a primary X-ray beam.

4. Irradiation of the sample. The primary beam strikes the sample surface, ejecting inner-shell electrons, and the sample begins to emit characteristic X-ray fluorescence.

5. Detection. The detector converts each incoming photon into an electrical signal, and the multichannel analyzer counts the photons by energy channel to produce an energy spectrum.

6. Calculation and Output. The software compares the energy spectrum with a standard spectral library and converts the data into concentrations using calibration curves or the Fundamental Parameters (FP) method. The element names and their percentages or ppm values are displayed on the screen.

Components of an X-ray Fluorescence (XRF) Spectrometer

X-ray tube + high-voltage power supply: Generates primary X-rays, which determine which elements can be excited

Filters: Reduce background noise and highlight target elements

Collimator/optical path: Defines the size of the irradiation spot

Detector: Converts fluorescent photons into electrical signals

Multi-channel analyzer: Counts photons in separate channels based on energy

XRF: Differences Between EDXRF and WDXRF

1. EDXRF (Energy-Dispersive X-ray Fluorescence): Uses a semiconductor detector to directly measure the energy of each photon, obtaining the entire energy spectrum in a single measurement.

2. WDXRF (Wavelength-Dispersive X-ray Fluorescence): Uses a diffraction crystal to disperse fluorescence of different wavelengths to different angles according to Bragg’s law, then counts them individually.

A simple rule of thumb: If you need to take the instrument to the field, require results in a matter of seconds, and primarily want to measure major and trace elements, choose EDXRF. If you need lower detection limits, need to distinguish between adjacent elements, and require data that is comparable across laboratories, choose WDXRF.

Precautions for X-ray Fluorescence (XRF) Spectrometers

The penetration depth of primary X-rays is typically only a few micrometers to a few millimeters, depending on the X-ray energy and sample density. Heavy-element matrices absorb X-rays strongly, resulting in a shallower effective analysis layer; for light matrices such as plastics and soil, the penetration depth can reach several hundred micrometers or even millimeters.

XRF measures the surface layer. Therefore, if necessary, the sample should be polished or machined to expose a fresh surface before measurement.

Matrix Effects and Peak Overlap

1. Absorption: Other elements in the matrix absorb the fluorescence of the target element, resulting in underestimated values. A typical example is the absorption of the nickel signal by an iron matrix.

2. Enhancement: The fluorescence of a certain element in the matrix further excites the target element, resulting in overestimated values.

3. Z+1 Problem: Elements adjacent to each other on the periodic table naturally have spectral lines that are close together. For example, the Lα peak of lead (Z=82) overlaps with the Kα peak of arsenic (Z=33); the Kβ peak of vanadium overlaps with the Kα peak of chromium.

There are three solutions: calibrate using a standard sample that matches the sample matrix; enable the software’s matrix correction and overlapping peak resolution functions; and, if necessary, modify the chemical sample preparation method.

XRF Spectrometer: Measurement Capabilities and Limitations

Can Measure

Conventional elements from Na (11) to U (92); WDXRF can detect elements as low as Be (4)

Sample Types: Metal alloys, ores and rock cores, soil and sediment, cement and clinker, glass and ceramics, plastics and coatings, electronic and electrical products, petroleum products, precious metals

Special Features: When equipped with dedicated software and standard samples, it can measure coating thickness and multilayer film structures

Cannot Measure

Light elements such as carbon, nitrogen, oxygen, and fluorine: due to low fluorescence yield; low-energy X-rays are easily absorbed

Chemical forms and valence states: XRF measures only total elemental content

Isotopic composition: characteristic X-rays do not vary with isotopes

Ultra-trace levels (ppb to ppt): detection limits are insufficient

Molecular structure of organic compounds: the principle does not involve molecular bonds

Typical Industries for XRF Applications

1. Metal Manufacturing and Recycling: Alloy grade identification, scrap metal sorting, and quality inspection of raw materials and finished products.

2. Oil, Gas, and Energy Industries: Positive material identification (PMI) of pipes, pressure vessels, and welded components to ensure materials meet specification requirements.

3. Ore: Ore grade analysis, core sampling, mineral exploration, and soil geochemical analysis.

4. Environmental Monitoring: Heavy metal screening in soil, sediments, and atmospheric particulate matter to support pollution investigations and environmental assessments.

5. Precious Metals and Jewelry: Gold purity and karat testing, precious metal composition analysis, and evaluation of recycled materials.

6. Petrochemicals and Lubricants: Analysis of sulfur and additive-related elements in oils, catalyst composition testing, and process quality control.

7. Glass and Ceramics: Elemental composition analysis of raw materials, formulations, and finished products to support production process optimization and quality control.

If you’re struggling with the detection of a specific element—such as magnesium in an aluminum matrix, cadmium in soil, or lead in plastic—please provide us with your sample information and the target element. We’ll offer a comparative recommendation based on the element’s detection range, limit of detection, and compatibility with the matrix.

How to Choose a Handheld X-ray Fluorescence (XRF) Spectrometer

1. Elements to Be Analyzed: Confirm which elements need to be detected, especially whether light elements such as Mg, Al, and Si are required. This helps determine the appropriate detector and instrument configuration.

2. Detection Limits: Identify the target concentration range and minimum detection limit required. Trace-element analysis may require longer measurement times and optimized testing conditions.

3. Sample Types and Applications: Consider whether you will analyze metals, alloys, plastics, soil, or other materials. Different sample matrices may require specific calibration methods and application modes.

4. Detector Performance: Evaluate the detector type, energy resolution, and X-ray tube configuration. These factors influence elemental identification and analytical performance, particularly for light elements and low-concentration samples.

5. Compliance Requirements: If results must meet specific industry standards or regulatory requirements, verify the instrument's applicable testing methods, calibration options, and reporting capabilities.

6. Total Cost of Ownership: Beyond the initial purchase price, consider calibration standards, software, maintenance, operator training, battery replacement, and after-sales support.

Conclusion

If you have any further questions about X-ray Fluorescence (XRF) spectrometers, please contact us. Learn how our professional XRF testing, elemental analysis, and non-destructive testing solutions can meet your material composition analysis needs. Whether you work in manufacturing, environmental science, or research, our advanced technology delivers accurate, reliable, and customized industry-specific analysis results.