Question

Why are `NaCl` plates used instead of quartz plates in IR spectroscopy to load sample?

Answer 1

Because glass (i.e. quartz) would absorb in the IR spectrum, and mask the absorption of your sample.

Explanation:

`NaCl` absorptions are transparent to most of the `IR` spectrum; there are absorbtions at `400-600cm^(-1)`, which leaves most of the spectrum transparent.