Preservation of Unstable Solids

Many samples are unstable in air. Examples of air-sensitive compounds are alkali metal intercalated C₆₀, carbon nanotubes, and graphite, which are 20 sample preparation: an analytical perspective usually prepared in vacuum-sealed tubes. After completion of the intercalation reaction in a furnace, the sealed tubes may be transferred directly to a Raman spectrometer for measurement. Since these compounds are photosensitive, spectra need to be measured using relatively low laser power densities.

For x-ray diffraction, infrared, and x ray photoelectron spectroscopy (XPS), the sealed tubes are transferred to an argon-filled dry box with less than 10 parts per million (ppm) of oxygen. The vacuum tubes are cut open in the dry box and transferred to x-ray sampling capillaries. The open ends of the capillaries are carefully sealed with soft wax to prevent air contamination after removal from the dry box. Samples for infrared spectroscopy are prepared by mixing the solid with hydrocarbon oil and sandwiching a small amount of this suspension between two KBr or NaCl plates. The edges of the plates are then sealed with soft wax. For the XPS measurements, the powder is spread on a tape attached to the sample holder and inserted into a transfer tube of the XPS spectrometer, which had previously been introduced into the dry box. Transfer of unstable compounds into the sampling chamber of transmission and scanning electron microscopes are di‰cult. The best approaches involve preparing the samples in situ for examination.