- Analytical methods
- Gas chromatography
- Liquid chromatography
- Infrared spectroscopy
- Other spectroscopy
- Other methods
- Inductively coupled plasma
- Mass spectrometry
- Nuclear magnetic resonance
X-Ray fluorescence (XRF) is a rapid, relatively non‐destructive chemical or elemental analysis of rocks, minerals, sediments, fluids, and soils. Its purpose is to identify the elemental abundances of the sample. It is used in a wide range of applications, including mining (e.g. measuring the grade of gold ore), metallurgy (e.g. quality control), soil surveys, cement production, ceramic & glass manufacturing, petroleum industry (e.g., sulphur content of crude oils and petroleum products), field analysis in geological and environmental studies, and research in igneous, sedimentary, and metamorphic petrology etc.
The sample is zapped by high-energy, short wavelength X-Rays or Gamma Rays. This radiation excites the sample and dislodges the electrons in inner orbital causing ionization of the sample. With space in the lower orbitals open, electrons in higher orbitals fall into the lower ones. This releases a secondary radiation, the fluorescense, from the sample.
Energy is released during this process because the binding energy of a low orbital is less than that of a higher orbital. The energy released is roughly equal to the difference in the binding energies of the two orbitals involved. Both the energy and the wavelengths of the secondary radiation are much less than the original X‐Ray. Characteristic of the secondary radiation such as energy and wavelength are specific to element whose atom they were released from. These can be detected and converted into computer generated data. The XRF machines output, coupled with a quantitative analysis, will report what percent of each element is within the sample.
A typical XRF spectrometer has X-ray source, sample chamber, analysing crystal, detector and signal processing computer etc. Because the secondary radiation produces such weak photons, the tubes that involve radiation transport must be kept in a vacuum. After the secondary radiation occurs, the fluorescence is channelled into a solid state detector, like a crystal, that is able to produce a steady beam of photons for further detection. Specific wavelengths will come off the crystal at specific angles, which allows for isolated detection. This technique is called wavelength dispersive spectroscopy.
Detectors used for wavelength dispersive spectrometry need to have high pulse processing speeds in order to cope with the very high photon count rates that can be obtained. Furthermore, they need sufficient energy resolution to allow filtering-out of background noise and spurious photons from the primary beam or from crystal fluorescence. Detectors contain their own, specific atoms. The secondary radiation will ionize these atoms contained in the detectors producing a charge. The charge is proportional to the incoming photons energy. Next the charge is collected and the process starts over for the next photon.
Two very common types of detectors are used to measure the intensity of the secondary radiation. They are gas flow proportional detector and scintillation detector. Gas flow proportional detectors are normally used to detect longer wavelength. They are filled with gas, typically a gas mixture of 90% argon & 10% methane (called as “P 10”). The argon is ionised by incoming X-ray photons, and the electric field multiplies this charge into a measurable pulse. The methane is to suppress the formation of fluorescent photons caused by recombination of the argon ions with stray electrons. When very long wavelengths (over 5 nm) are to be detected, the argon may be replaced with neon or helium. Scintillation detectors analyze shorter wavelengths (K spectra from Niob (41) – Iod (53) and L spectra from Thorium (90) – Uran (92)). An embedded crystal produces scintillations that are proportional to the energy of the photon absorbed. This pulse is translated into a voltage proportional to the original photon.
The purity of the P 10 is very important for the accurate analysis results and the lifetime of the instrument. The contaminations in the gas can cause peak tailing of the fluorescence spectrum, thus reduced instrument sensitivity and unstable results. HiQ 10% methane, balance argon promises a good gas purity and BASELINE C 106 regulator is recommended to maintain the gas integrity.