IR thin liquid films

Preparation of Samples for IR Spectroscopy: Liquids

Liquid organic compounds can be examined directly as a thin film, "neat”, between two NaCl plates. By "liquid organic compound" we mean pure organic compounds that are liquids at room temperature, not solutions of a solid organic compound in a solvent. Although it is possible to run a solution as a thin film between two plates, the resultant spectrum would show bands of both the compound of interest and the solvent. (Other methods of liquid IR sampling are discussed on a separate page: other methods.)

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

How to prepare a liquid sample:

(click on the small photos below to see a larger version of the same)

Obtain two salt plates. Salt plates are stored in desiccators, both in each lab room on the TA's desk and in the hood in the IR instrument room.

Ideally, the plates were put away clean, although not all students are considerate enough to clean plates after use. If necessary, clean the plates with a small amount of acetone (see below). Ideally, the plates should be transparent, but quite foggy plates usually give acceptable spectra (see good, acceptable, and bad IR plates).

Use a Pasteur pipet to place a drop of your liquid unknown on one salt plate . . .

. . . then put a second salt plate on top so that the liquid spreads into a thin film. When running a liquid sample, you need to have two plates, both to prevent the liquid from running off the plate and to prevent it from evaporating.

Pick up the plates and take them to the IR. Carry them by the edges and/or wear gloves. (Gloves are advised because sometimes the liquid compound leaks out from between the plates.)

Take the plates to one of the FT-IR instruments . . .

. . . and place them in the V-shaped sample holder inside the instrument.

Obtain the IR spectrum.

Once you have obtained your spectrum, clean the salt plates with acetone: separate the salt plates and place them dirty side up on a paper towel, then squirt a small amount of acetone on the plates. Do not hold the plates in your hand when you rinse them because acetone is an irritant.

Wipe the plates clean with a kimwipe. Then, put the salt plates in a desiccator.
Never use water to clean salt plates since water will dissolve the sodium chloride of which the plates are made.

Movie of this procedure:

The following movie was taped in c1993. Most of the movie is still valid, although we now use acetone to rinse plates instead of methylene chloride (methylene chloride is a lot more toxic and we found that acetone does not significantly harm the plates). Also, the FT-IR in the movie has been replaced with a different model.

how to prepare liquid samples, or thin liquid films, at the beginning of the IR sample preparation movie

Obtain two salt plates. Salt plates are stored in desiccators, both in each lab room on the TA's desk and in the hood in the IR instrument room.
Ideally, the plates were put away clean, although not all students are considerate enough to clean plates after use. If necessary, clean the plates with a small amount of acetone (see below). Ideally, the plates should be transparent, but quite foggy plates usually give acceptable spectra (see good, acceptable, and bad IR plates).
Use a Pasteur pipet to place a drop of your liquid unknown on one salt plate . . .
. . . then put a second salt plate on top so that the liquid spreads into a thin film. When running a liquid sample, you need to have two plates, both to prevent the liquid from running off the plate and to prevent it from evaporating.
Pick up the plates and take them to the IR. Carry them by the edges and/or wear gloves. (Gloves are advised because sometimes the liquid compound leaks out from between the plates.)
Take the plates to one of the FT-IR instruments . . .
. . . and place them in the V-shaped sample holder inside the instrument.
Obtain the IR spectrum.
Once you have obtained your spectrum, clean the salt plates with acetone: separate the salt plates and place them dirty side up on a paper towel, then squirt a small amount of acetone on the plates. Do not hold the plates in your hand when you rinse them because acetone is an irritant.
Wipe the plates clean with a kimwipe. Then, put the salt plates in a desiccator. Never use water to clean salt plates since water will dissolve the sodium chloride of which the plates are made.