X-ray Fluorescence (XRF) spectrometry is a non-destructive elemental analysis technique that measures the characteristic X-rays emitted when a sample is irradiated with high-energy X-rays or gamma rays. It is applicable to elements from beryllium (Z = 4) to uranium (Z = 92) across a wide concentration range — from trace levels (ppm) to major constituents (percent level). XRF is particularly valued for its minimal sample preparation and ability to analyze solids, powders, and liquids directly.
The principle of XRF is based on the photoelectric effect. When an incident X-ray photon strikes an atom with energy greater than the binding energy of an inner-shell electron (K, L, or M shell), that electron is ejected, creating a vacancy. An electron from a higher-energy shell fills the vacancy, and the energy difference is released as a characteristic X-ray photon. The energy of this photon is unique to the element and the specific electronic transition — Kα, Kβ, Lα, Lβ, Mα lines, among others. By measuring the energies (energy-dispersive) or wavelengths (wavelength-dispersive) and their intensities, both qualitative and quantitative information is obtained.
The X-ray source is typically an X-ray tube with a metal target (Rh, Ag, W, Mo, Cr) that emits bremsstrahlung and characteristic lines when bombarded with accelerated electrons. In portable or handheld instruments, a radioactive source (e.g., ¹⁰⁹Cd, ²⁴¹Am, ⁵⁵Fe) may be used instead. The incident X-rays must have sufficient energy to excite the elements of interest — higher-Z elements require higher excitation energies.
Energy-dispersive XRF (ED-XRF) uses a solid-state detector (silicon drift detector, SDD; or Si(Li)) to measure the energy of each incoming X-ray photon directly, producing a spectrum of counts vs. energy. ED-XRF instruments are compact, fast, and suitable for multi-element screening. Wavelength-dispersive XRF (WD-XRF) uses a crystal to diffract X-rays and a goniometer to select specific wavelengths. WD-XRF offers superior spectral resolution (resolving closely spaced peaks) but requires more instrument time and sample mass. Sequential and simultaneous (multi-channel) configurations exist.
Matrix effects are significant in XRF because the measured X-ray intensities depend not only on analyte concentration but also on absorption and enhancement by other elements in the sample. Semi-quantitative analysis uses fundamental parameters (FP) algorithms to correct for these effects mathematically. Quantitative analysis requires matrix-matched calibration standards or the use of fusion (e.g., lithium tetraborate) and pressed pellet preparation to minimize particle size and mineralogical effects. XRF is widely used in cement, mining, and metals production for process control, in art conservation and archaeology for non-invasive pigment and metal analysis, and in environmental monitoring for soil and sediment screening.
