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

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Mesh

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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