Analytical Services:
XRD and XRF: A brief early history of the use of x-rays for analysis
XRD
LSM Analytical Services large range of analytical techniques includes instrumentation for XRD analysis; one of two of the most widely applied analytical techniques based on X-rays:
XRF (X-Ray Fluorescence) for the elemental or oxide content of a material
XRD (X-Ray powder Diffraction) for identification of crystalline compounds or phases
The techniques are often considered to be complimentary. For example a sample may be submitted for XRF and found to contain 50% Al 2 O 3 and 30% SiO 2 and this may be sufficient information. However, XRD could then distinguish between samples containing different phases; for example Quartz (SiO 2 ), Corundum (Al 2 O 3 ), Mullite (Al 6 Si 2 O 13 ) or Kaolinite (Al 2 Si 2 O 5 (OH) 4 ) or any combination of these.
The X-rays used for XRD have one wavelength (they are all one “colour”). Normally, a sample is a few grams of powder and is packed into a holder with a flat surface. The beam of x-rays enters the sample surface and is reflected by the very small crystals (crystallites) in the sample. When it is reflected the single beam of x-rays is split so the x-rays come out of the sample in several beams at different angles to the sample. The instrument has a detector that swings round the sample as the sample itself is rotated and registers the position and strength of these beams. This data is plotted by software as strength (intensity) vs. position (angle) to give a series of “peaks” or “lines”, which is called the diffraction pattern.
Each chemical compound or phase reflects x-rays slightly differently and so has a different diffraction pattern. A mixture of compounds gives a pattern that is made up of the patterns of all the individual compounds. So to identify the compounds present in a mixture the pattern obtained is compared to a large database of patterns. Often there are overlapping lines so experience and judgement are important. To give a guide when phase identification is complete the peaks are classified as major, minor or trace.
LSM has developed a suite of programmes internally to allow a range of determinations to be made including:
- phase identification of crystalline materials
- phase quantification, including crystalline silica
- glass content of GGBS (ground granulated blast furnace slag)
- customer specific quality control methods
LSM`s analysis development team also develops new methods to meet customer requirements.
XRF
LSM Analytical Services large range of analytical techniques includes instrumentation for XRF analysis; one of two of the most widely applied analytical techniques based on X-rays:
X-Ray Fluorescent (XRF) Spectrometers use a spectroscopic technique, which is commonly used for solids, in which x-rays are used to excite a sample and generate secondary x-rays.
XRF provides determination of major and trace elements in solids. There are 2 types of XRF spectrometers:
- Wavelength dispersive (WDX or WDS); superior resolution and detection limits
- Energy dispersive (EDX or EDS); smaller, often portable
As with any analytical method, sample preparation is crucial and LSM was one of the early pioneers of the Borate glass bead technique, developing methodology for analysis of refractory and rare earth oxide mixtures as well as a range of Ferroalloys, Steels and hard metal Carbide powders. This is a valuable way to eliminate errors with materials, which exhibit varying composition, or are difficult to present in other forms.
The x-rays used for X-Ray Fluorescence (XRF) have as wide a range of wavelengths as possible (i.e. they are as near as possible to white light). The beam of x-rays enters the atoms of the sample and for each different element one wavelength (“colour”) of x-rays is given out, the characteristic radiation.
The detector is moved to the position at which the characteristic radiation for each element being analysed leaves the sample and registers the strength of the beam. The intensity (strength) of the characteristic x-rays given out measures the amount of that element in the sample. In reality elements are measured simultaneously using a number of pre-assembled fixed channels placed around the sample. Each is effectively a self-contained spectrometer, with a crystal and detector tuned to receive a specific wavelength.
LSM also provides an excellent semi-quantitative package, which covers all the elements from Fluorine to Uranium measurable by XRF in one analysis. Useful results can be obtained from even small samples. By combining a semi-quantitative analysis with XRD, a cost effective investigative tool is available for deposits, corrosion, contamination or other unknowns.
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