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IR Spectroscopy of Solids

In the organic teaching labs at CU, the preferred method of solid sample preparation is the "Thin Solid Film" method. Briefly, the solid sample is dissolved in a suitable solvent (usually methylene chloride) and the solution is dropped onto a salt plate. After the solvent evaporates, a thin-solid film of the compound remains on the plate. The IR is run directly in the FT-IR.

Obtain a single salt plate from the desiccator in the main fume hood in your lab. Ideally, the plate will already be clean. If necessary, clean the plate with a small amount of acetone. The plate should also be transparent, but quite foggy plates usually give acceptable spectra.
Put about 50 mg of solid into a beaker, vial, or test tube - no need to weigh it, just judge by the photo below.
Add couple of drops of solvent (usually acetone or methylene chloride) to dissolve the solid.
Place a drop of this solution on a salt plate. Allow the solvent to dry; you should observe a thin solid film on the plate. If you do not, add another drop of the solution and allow it to dry (see below for a visaul guide of how this film should look).
Place the plate in the V-shaped sample holder inside the instrument. Note that you use only one salt plate in this procedure, not two plates as when running a thin liquid film. Obtain the IR spectrum. If the peaks are of too low an intensity, remove the salt plate, add more of the solution, allow the solvent to dry, and re-run the FTIR. If the peaks are of too high an intensity, remove the plate and clean it. Dilute the solution of solid compound with acetone or methylene chloride and repeat the above process. Once you have obtained your spectrum, clean the salt plates again with acetone and return them to the desiccator.

Visual Guide for Film Thickness

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