The limits of detection for XRF (X-ray fluorescence) depend on the concentration of the element in the sample and various other factors. Generally, the detection limits for most elements range from 2-20 ng/cm2 for micro samples, thin samples, aerosols, and liquids. However, it is important to note that the detection limits can vary depending on the specific application and sample type.
Several factors can affect the XRF analysis procedure. Firstly, X-ray emission occurs at characteristic wavelengths that correspond to electron transitions within the atoms of the analyzed sample. These emission peaks are superimposed over a continuous background of X-rays that are scattered by the loosely bound outer electrons. The intensity of the emission peaks and the background scattering are influenced by the particle size, mineral composition, and particle density of the sample.
The depth from which the characteristic X-rays originate also affects the detection limits. Typically, these X-rays are emitted from surface atoms at depths ranging from 1-1000 µm below the sample's surface. The exact depth depends on the atomic weight of the element being detected. Lighter elements are generally more difficult to detect than heavier elements.
Sample preparation is another important aspect of XRF analysis. Samples can be prepared as liquids or solids. One common technique is the use of fused beads, where the sample is ground to a particle size of less than 75 µm and mixed with a flux (usually a lithium tetraborate or tetraborate/metaborate mixture). The mixture is heated in a platinum crucible to high temperatures, potentially up to 1,600 °C. However, the fused bead technique may have limitations in detecting trace elements as the sample needs to be diluted.
XRF spectrometers are typically categorized into two types: Energy Dispersive XRF spectrometers (ED-XRF) and Wavelength Dispersive XRF spectrometers (WD-XRF). ED-XRF spectrometers are simpler and easier to use, allowing for the simultaneous collection of signals from multiple elements. They offer a resolution range of 150 eV to 600 eV. On the other hand, WD-XRF spectrometers collect one signal at a time at different angles using a goniometer. These instruments are more complex and expensive but offer higher resolution ranging from 5 eV to 20 eV.
XRF has various applications in industries such as cement, metal ores, mineral ores, oil & gas, environmental, and geological analysis. However, any laboratory with the necessary expertise can make use of XRF technology.
In terms of sample preparation equipment, it is important to consider avoiding metal contamination. Tungsten carbide lined dies can be used to prevent iron contamination from stainless steel bodies. Different diameters are available, with smaller diameters typically used for Fourier Transform Infrared (FTIR) analysis and larger diameters for XRF analysis.
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