Final answer:
Preparing liquid samples for IR involves creating a thin film between salt plates, while solid samples are usually made into a pellet with KBr or analyzed directly using the ATR method.
Step-by-step explanation:
Differences Between Preparing Liquid and Solid Samples for IR Spectroscopy
The preparation of samples for infrared (IR) spectroscopy differs significantly between liquid and solid states due to their physical properties. For liquids, a common method is to place a drop between two salt plates (commonly made of sodium chloride or potassium bromide) because these materials don't absorb IR light and the liquid forms a thin film that allows IR light to pass through for the analysis. On the other hand, solid samples are usually milled with potassium bromide (KBr) powder and pressed into a pellet or disc, or they can be directly analyzed using attenuated total reflectance (ATR) technique where the solid sample is pressed against a crystal with a high refractive index to measure the absorbed IR radiation.
In the context of solid sample preparation with KBr, the pellet method involves finely grinding the sample with KBr in a mortar to get a homogeneous mixture, which is then placed into a die and subjected to high pressure to form a translucent pellet. This pellet can then be placed in the light path of the IR spectrometer. The ATR method for solids doesn't require extensive sample preparation as it involves simply placing the sample on the ATR crystal, making it suitable for samples that are hard to prepare or which might suffer damage from the pellet-making process.