Transcript of "Sample preparation and Presentation XRD Webinar June 19 2012 AJK Analytical Services"
Sample preparation andPresentation XRD Webinar June 19 2012
Contents The goal of sample preparation Different analytical questions may require different sample preparation techniques Sample related issues in X-ray diffraction An overview of various sample preparation and presentation techniques Some Special cases
An ideal ‘powder’ Consists of very many, randomly oriented crystallites Depending on the analytical needs the optimal crystallite size may vary from 40µm down to <1µm Crystallites too big Unreliable intensities Crystallite size of 40µm only 12 crystals contributing to the intensity profile at any time To obtain a 1% you need about 53000 contributing crystals
An ideal powder Particles too small, or wrongly ground Peaks get broader at the cost of the maximum intensity Crystallites not randomly oriented (preferred orientation, or texture) Deviating relative intensities One very dominant orientation Sometimes just one reflection is visible (Mica and Muscovite, for example)
Issues to pay attention to Take care that you grinding doesn’t affect the crystal structure, or induces strain into the crystallites (pharmaceuticals, but also cryolite; the solvent of alumina for the elctrolysis must be dealt with carefully) The material of the grinding vessel is of paramount importance (Cross) Contamination
Effect of Back Loading Equant Elongate Tabular Bladed
Effect of Back loading For Equant particle shape the effect of Back Loading versus Front Loading is small and BL is usually not needed For Elongate and Tabular shapes the effect of BL is not always sufficient Most effect of BL is expected for Bladed particle shape
Zero Background holder Needed when: Sample is transparent to X-rays Very small amount of sample is available and the specimen must be presented as a flat plate Sample cannot be packed properly
Zero Background Holder Obliquely cut single crystal Most commonly either Si, or Quartz Direction of the cut is chosen to avoid any reflections occurring in the reflection sphere (i.e. no reflecting lattice planes parallel to the crystal surface)
Alternative Presentations Protect the specimen during the measurement Specimen presentation for transmission measurements Capillaries
Protecting your specimen Mount your specimen in a reaction chamber (the Anton Paar XRK900, for example) and flush with an inert gas Cover your specimen with a transparent foil, which does not give a diffraction pattern Mount the specimen inside a glass capillary tube (Mark tube)
Transmission Het monster wordt in een holle ring tussen twee folieën geklemd De folieën worden m.b.v. drie in elkaar passende ringen (vaak teflon) gespannen Of het monster wordt in een capillair gemonteerd
Capillair Gebruikt erg weinig monster Voorkomt meestal voorkeursoriëntatie Beschermt tegen invloeden van buitenaf Is lastig te vullen
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