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Laboratory of X-ray Diffractometry and Spectrometry

X-Ray powder diffraction (XRPD)

X-Ray powder diffraction analysis (mineralogical analysis) could be used to measure solid samples in the form of powders or flat compact samples to determine amorphous or crystalline state of samples. The determination of crystalline phases is performed on the base of comparison with database (PDF-4+2015) including d,I values for 365.877 crystalline phases. Database PDF-4+2015 includes even cards (reference 04-) including x,y,z coordinates of atoms, which could be used for quantitative Rietveld`s analysis.

 

Access to measured data

Measured data from all equipments are regularly archived, at least annually. The data are sent to the customer and it is customer‘s responsibility to archive his data.

 

Requirements on samples for standard X-Ray diffraction analysis

XRPD could be used to measure powders or compact samples (certain restriction on size, thickness and weight of the sample – unusual sample better discuss with lab staff). The measured surface of the sample should be flat. Samples like pipes, wires could be measured, but the data quality is lower than in the case of ideal sample (flat, thick enough that no X-Ray will go through the sample). Powders are measured in cylindrical holders, at least 200 mg is necessary depending on the density of the sample, higher density more sample is necessary. Goal is to fill circle cavity with diameter 26 mm and depth of 1 mm. Maximal irradiated size by X-Ray is 20x15 mm. Holders with smaller diameter than 26 mm or lower depth than 1mm can be used to measure smaller amount of sample, but the quality of data will get worse.  Powder pattern can be collected from 1 mg of sample spread on the „low background“ holder made of properly cut Si wafer. Polymeric foils could be measured from the thickness of 1micrometr.  

Requirements on samples for microdiffraction analysis

The quality of powder patterns (intensity of diffraction lines and their width –FWHM) depends on the amount of irradiated sample. The powder pattern obtained from well powdered sample of 1mg collected by standard X-Ray diffraction is better than from microdiffraction!!! Microdiffraction analysis is useful when the amount of sample is lower than 1mg or we need to measure spots with size 0.1-2mm on compact samples. Microdiffraction analysis is useful in the case of compact samples, the maximal height of sample is 42 mm (bigger height – better discuss with lab staff) and the maximal weight 1kg. Microdiffraction analysis could be performed on curved samples!, measured spots could be almost anywhere on the sample.

Requirements for measurement of hygroscopic samples

Hygroscopic samples or liquids with floating crystallites can be measured in capillaries with inner diameter 0.3-2mm. Capillary is horizontally placed in rotation stage. It is necessary to have about 50mg of powder.

X-Ray fluorescent analysis (XRF)

X-Ray fluorescent analysis is used to determine elements in range Be-U in powders or compact samples. It is possible to measure liquids as well with restriction to range Na-U and worse detection limits for Na-Si. Liquids are measured in cups, bottom of the cup is foil, usually 6 micrometer mylar or 4 micrometer polypropylen. Measured liquid must not react with foil, should have pH in range 5-7. XRF is suitable for determination of more elements simultaneously (standard F-U), limit of detection generally  improves with growing atomic (proton) number, advantage is low overlap among elements, light elements Be-Si should be measured on samples with  smooth surface.  This is really necessary for Be-O.

Requirements for sample preparation

The measured surface of the sample should be flat and the size of the sample should cover the circle hole with diameter of 29 mm to get the best data. Smaller samples up to the diameter of 5 mm could be measured but measured element intensities are proportional to irradiated area. Samples like pipes, wires could be measured, but better discuss them with lab staff Powders are pressed into the pellets with diameter of 40 mm, the diameter of pressed sample is 34 mm.

  • powders – amount depends on density of the sample – 1g is appropriate (in the case of  cements is about 0.7 g OK), well powdered (size of grains about 10 mm or less). Goal is to fill circle cavity with diameter 34 mm and depth of 1 mm.
  • smaller amount of sample could be measured, if the sample is not thick enough or the area is smaller than optimal – result is semi-quantitative analysis from amount up to 10mg
  • only major elements (above 1%) can be obtained from 1 mg sample
  • Compact samples should fit in cylindrical cassette with inner diameter of 52 mm and max height of 25 mm.
  • Irregular shapes of compact samples are better discussed with lab staff, some sample preparation will be necessary.
  • liquids – optimal amount 5-15 ml, pH in range 5-7, measured elements Na-U, worse LOD for light elements Na-Si (mylar or polypropylene foil absorbs X-Ray of light elements), less accurate than AAS for concentrations lower than 0.5%, for higher concentrations more accurate than AAS, not very suitable for measurement of smaller number of elements than 3, suitable for screening of elements in liquids. Foil at the bottom of the cup must not react with measured liquid.
  • Think layers on the filtration paper
  • Sample is difficult to press, it means. sample does not stick together or not stick to the support from boric acid - sample could be measured as liquid (worse LOD for light elements Na-Si) or sample must be mixed with some kind of binding agent( e.g.metacrylate)
  • Accurate and precise quantitative analysis can be achieved with the calibration curves using standards. The best results are achieved with preparation samples into the glass disks so called „beads“ by melting the sample with suitable flux (e.g.lithium tetraborate) (fusion changes sample into the amorphous glass, eliminates diffraction - so called mineralogical effect)
Updated: 23.6.2016 07:47, Author: Jan Prchal


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