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- Element-to-stoichiometric oxide conversion factors
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- FAQ - XRD/XRF
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FAQ - XRD/XRF
Due to the complexity of XRD data analysis, the chemical composition of your sample must be known in order to accurately pin point the phases present in your sample. This information can be provided with your sample; otherwise a semi-quantitative analysis of a portion of your sample will be carried out with an XRF to determine bulk composition. If this is necessary you will also be charged the costs associated with this test.
What sort of samples can I investigate?
Anything that is crystalline. XRD can differentiate between minerals even when their composition is the same, providing information on crystal structure not evident from other analyses such as XRF. For example, XRD can identify mineral phases or clay minerals as well as products from industrial processes such as boiler scales, corrosion products and asbestos minerals. For accurate identification of crystalline phases they must constitute at least approximately 2% of the sample.
How much sample do I need to provide?
Samples are prepared in two ways. If there is sufficient sample at least 1g of dry, finely crushed powder is packed into a mount. If only very little sample is available then a small amount can be mixed with water and smeared onto a glass slip.
The AAC houses a Siemens D5000 Diffractometer (XRD) theta-2 theta goniometer with a copper anode x-ray tube, fixed slits, monochromator and a forty position sample changer. DiffracPlus with the search/match option is used to collect and analyse the diffraction data. Quantitative XRD Analysis is performed using either DQUANT or SIROQUANT dependent on the material and standards available.
XRD is a non-destructive method. Material that has been prepared as a mount can easily be removed from the holder and returned.
This depends on the angular rage you wish to assess, as well as step size and dwell time for each step. If no special requests are made then samples are usually run using a general scan (1.3° 2θ to 65° 2θ in steps of .02° 2θ for 2.4 seconds per step), which takes approximately two hours.
Equipment required for clay separation is available, as is a heating oven and glycol chamber. Preparation can either be undertaken by the client or a staff member (sample preparation time would then be charged at an hourly rate).
XRD can determine the presence and amounts of minerals species in sample, as well as identify phases. XRF will give details as to the chemical composition of a sample but will not indicate what phases are present in the sample.
It may sometimes be difficult to identify mineral phases due to low concentrations or overlapping peaks. If overlaps occur then reference to compositional data may make interpretation possible regardless. If the sample is poorly crystalline with a high amorphous content identification can also be hampered. Differentiating between high and low temperature variants and between solid solution minerals (e.g. plagioclase feldspars) can also present challenges in some cases.
XRF stands for X-Ray fluorescence. When atoms in a sample are bombarded with X-Rays generated by an X-Ray tube electrons are removed or ejected from inner shells. Electrons from outer shells of the atom drop down to fill the vacant position and to return the atom to a more stable, ground state. When this occurs energy is produced, some of which is in the form of X-Rays. Each element has its own unique X-Ray signature, which means that the X-Rays emitted from a sample can provide qualitative and quantitative compositional information about the sample.
At the AAC we predominately analyse geological samples to determine major and trace element composition. Other types of samples often dealt with include products or residues from industrial processes or refineries.
Both qualitative and quantitative tests are available, depending on your requirements and your sample properties. For quantitative major element analyses the sample must fit within the ranges of the calibrations we currently have to offer. Calibrations are complex constructions based on a large library of standard reference materials which contain different amounts and proportions of each element. A commonly used calibration at the centre assumes a silicate matrix, whilst there are also calibrations available for carbonate and iron-ore matrices. Trace elements and halogens (Cl and F), can also be determined quantitatively with XRF, however major element analyses needs to be completed first.
A robust semi-quantitative analysis (SSQ), which has less stringent requirements in terms of sample composition, is also available. It can detect a wide range of elements and provide information as to approximate amounts of each element present.
How much sample do I need to provide?
Samples can be submitted in powdered form, or samples can be milled by AAC staff at additional cost. For quantitative major elements (including LOI) at least 5g of material is preferred. For quantitative trace analyses an additional 6.5g of sample powder is required. For SSQ analysis 6.5g of sample is ideal, however as little as 1g of material is adequate if necessary.
For quantitative majors powdered sample is usually (except carbonate and iron ore sample types) ignited at 1000°C for 4 hours to oxidise elements such as sulphur, which are potentially damaging in their elemental state to platinum fusion crucibles. From this a loss on ignition or LOI is calculated. The LOI provides information as to the amount of carbon dioxide or any other volatile component present in the sample. The final concentrations of all elements in the original sample are expressed as oxides. A portion of oxidised sample is mixed with a flux, such as lithium tetra or metaborate, which facilitates dissolution of the sample into a homogenous glass bead suitable for XRF analysis. Glass beads are fused in platinum crucibles in a specially designed furnace at temperatures of 1050-1100°C. For trace analyses and SSQ’s, unignited powdered sample is mixed with a binding agent and pressed under high pressure into aluminum cups. The AAC houses a Bruker-AXS S4 Pioneer X-ray Fluorescence Spectrometer, operated by Spectra Plus software.
We strive to maintain a turnaround time of 3 weeks. This is usually dependant on the number of samples, tests requested, amount of other work in progress and on any difficulties encountered with the samples. Often, analyses are completed well within this timeframe.
The range of elements we can assess with XRF can be found here.
Can I get my samples back?
Remaining original and/or oxidised sample powders can be returned, however sample used to make fused beads or pressed pellets cannot then be used for other applications.
Tel: (07) 47814599
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