Preparation of Solid Samples for IR Spectroscopy: Solids

In the organic teaching labs at CU, the preferred method of solid sample preparation is the “Thin Solid Film” method. (Other methods of liquid IR sampling are discussed on a separate page: other methods.) 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.

Be sure to read the section on Care and Handling of Salt Plates.

Thin-Film Sampling Technique for Organic Solids

Put a small amount of your solid sample (about 0.05 g) in a small amount of acetone or methylene chloride (about 0.5 mL) and let it dissolve.

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 a mL or less of solvent to dissolve the solid
please see: how to handle Pasteur pipets!

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.

place a drop or two on a single IR plate and allow it to dry - it should then look like the "just right" plate below
too much . . . too little . . . just right!

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.

plate being placed in the Nicolet on the left plate being placed in the Nicolet on the right
this is how the plate sits on the holder inside the instrument

Movie of this procedure:

The following movie was taped in c1993. Most of the movie is still valid, although we now use acetone both to rinse plates and to dissolve the solid samples instead of methylene chloride (methylene chloride is a lot more toxic and we found that acetone does not significantly harm the plates). Another change is that we now advise students to dissolve a few crystals of the solid sample in a few drops of acetone instead of 1 mL. Also, the FT-IR in the movie has been replaced with a different model.