What is the main purpose of mixing the solid samples with KBr or any other salt?
The main purpose is sample dilution. Anyway, it is possible to analyse solid samples directly using reflectance techniques (like ATR or DRIFTS), with the limitation that signal intensity depends on factors like wavelength (depth of light intrusion into the sample depends on refraction index, which differs for different wavelengths), area of contact (ATR), surface roughness (DRIFTS). Several algorithms exist for correction (like ATR-correction).
Most molecules absorb in the IR, thus the term molecular spectroscopy. Salts like KBr, NaCl, CSCl... have a very stable and low absorbance across the wavelengths, thus they are used both as binding or dilution agents, and windows.
Water has a huge and broad peak at 3300 cm-1 and would interfere with any other peak in this region. Thus, drying the sample before analysis.
Yes, you can directly measure dry powder samples of chemicals, drugs including raw herbal drugs, medicinal plants or extracts by using ATR FT IR as described by Thomas above, kind regards
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