Due to the strong absorption from pure crystals, polycrystalline aspirin powder was diluted in polyethylene (PE) powder (Induchem, Volketwil, Switzerland) to a 5 % w/w concentration using an agate mortar and a pestle by gentle mixing. The powder mixture was then compressed into a pellet of 13 mm diameter using a hydraulic press (Specac Ltd., Kent, UK) at a load of 2 tons, and a blank PE pellet was prepared in the same way to be used as a reference. For each sample, 1000 waveforms were acquired and averaged with a spectral resolution of 0.94 cm -1 . A spectrum was first acquired at room temperature to check if the sample pellet was at the proper concentration.