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Sample preparation and Presentation
XRD Webinar June 19 2012
ContentsThe goal of sample preparationDifferent analytical questions may require different sample preparation techniquesSample related issues in X-ray diffractionAn overview of various sample preparation and presentation techniquesSome Special cases
An ideal ‘powder’Consists of very many, randomly oriented crystallitesDepending on the analytical needs the optimal crystallite size may vary from 40µm down to <1µmCrystallites too bigUnreliable intensitiesCrystallite size of 40µm only 12 crystals contributing to the intensity profile at any timeTo obtain a 1% you need about 53000 contributing crystals
An ideal powderParticles too small, or wrongly groundPeaks get broader at the cost of the maximum intensityCrystallites not randomly oriented (preferred orientation, or texture)Deviating relative intensitiesOne very dominant orientation Sometimes just one reflection is visible (Mica and Muscovite, for example)
Need a fine powder? Grind!
Coarse powder
Effect of spinning
Effect of spinning
Grinding devices
Swing Mill
Planetary Ball Mill
Effect of grinding
Issues to pay attention toTake 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
Sieving
0
10
20
30
40
50
60
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0 200 400 600 800 1000 1200
Mesh
Effe
ctiv
e P
artic
le S
ize
(µm
)
Preparing a flat specimenCavity front loadingCavity Back loadingSpray (atomizing) (on a zero background holder)Suspending (on a zero background holder)Side drifting
The specimen holder
Reference surface must be flat within 13µm
Cavity Front Loading
Cavity Back loading
Cavity Back Loading
Effect of Back Loading
Equant
Elongate
Tabular
Bladed
Effect of Back loadingFor Equant particle shape the effect of Back Loading versus Front Loading is small and BL is usuallynot neededFor Elongate and Tabular shapesthe effect of BL is not alwayssufficientMost effect of BL is expected forBladed particle shape
Effect of Back Loading
Zero Background holderNeeded when:Sample is transparent to X-raysVery small amount of sample is available and the specimen must bepresented as a flat plateSample cannot be packed properly
Zero Background HolderObliquely cut single crystalMost commonly either Si, or QuartzDirection of the cut is chosen to avoid any reflections occurring in the reflection sphere (i.e. noreflecting lattice planes parallel to the crystal surface)
Zero Background Holder
Dusting on a 0-BG holder
Compare with Back Loading
Compare with Back Loading
Alternative PresentationsProtect the specimen during the measurementSpecimen presentation for transmission measurementsCapillaries
Protecting your specimenMount your specimen in a reaction chamber (the Anton Paar XRK900, for example) and flush with an inert gasCover your specimen with a transparent foil, which does not give a diffraction patternMount the specimen inside a glass capillary tube (Mark tube)
TransmissionHet monster wordt in een holle ring tussen twee folieën geklemdDe folieën worden m.b.v. drie in elkaar passende ringen (vaak teflon) gespannenOf het monster wordt in een capillairgemonteerd
CapillairGebruikt erg weinig monsterVoorkomt meestalvoorkeursoriëntatieBeschermt tegen invloedenvan buitenafIs lastig te vullen