The sample should be mounted with its smallest surface on the end of the glass fiber to minimize absorption effects and to minimize background scattering from the sample mount. These mountings are usually carried out in an inert atmosphere such as a dish filled with argon gas. Very reactive compounds must be mounted in a glove bag or glove box and sealed in capillary tubes. Crystals of these compounds are usually wedged in capillary tubes or are held in place by a small amount of grease. Capillary tubes containing unstable compounds must be sealed by melting the ends of the glass tube.
Capillaries do introduce two kinds of problems.
The curvature of the capillary distorts the image of the crystal when centering the sample on the diffractometer. Also, the glass itself significantly increases both the background scattering and the absorption of the incident beam of X rays. It is crucial that the capillaries be made out of thin glass similar to that found in commercially available capillaries. Thick glass capillaries absorb X rays so much that very little scattered radiation will leave the capillary.