IR Spectrum Acquisition

In the organic teaching labs at CU Boulder, almost all samples for IR spectroscopy are prepared as a thin film of the organic compound on salt plates. Two things are important as you are getting ready to run an IR:

• You need to be able tell a good salt plate from a bad salt plate and you must know how to handle these plates properly so that you do not turn a good plate into a bad plate. These topics are covered in the first section below.
• You must choose the proper method to prepare the sample on the salt plate(s): the method is determined by whether the organic compound is a liquid or a solid, as explained in the second section below.

Care and Handling of IR Plates

The IR plates that you use in the organic chem labs are made of polished sodium chloride. Sodium chloride is chosen because it is transparent to infrared radiation. These plates - called "salt plates" - are rather expensive because each plate is cut from a single giant crystal; they are very fragile and sensitive to moisture, including the moisture in your fingers.

What happens when you place salt in water? It dissolves. What happens when you place your sweaty fingers on a salt plate? It dissolves the salt, leaving a fingerprint on the plate. The images below show what a fingerprint on a plate looks like, and how it happens.

The picture below shows the correct way to hold a salt plate. Alternatively, you can wear gloves when you handle salt plates, as long as the gloves are dry.

Moisture in the air causes salt plates to become cloudy; moisture on your fingers causes prints (see above). We store them in desiccators to prevent them from becoming cloudy. Salt plates are very fragile and will chip or shatter if you drop them. They can be scratched by metal spatulas and Pasteur pipets. The photo below shows five plates, one in good condition and four that are slightly damaged. When you are preparing to run a spectrum, try to choose a good plate (and keep it that way!). All of the plates in the photo probably would be useable, especially in organic teaching lab applications.

The damage done to a plate by fingerprints, pits, and cloudiness cause the IR spectrum run on the plate to have broad bands. This is because IR spectra look best when run as a thin, even films of compounds. Pits and fingerprints cause a thick and uneven film to be laid down on the plate, leading to scattering of IR radiation and to bands that are too intense; cloudiness causes scattering of IR radiation and prevent it from going through the sample, leading to broad bands and a spectrum that is less than 100% transmission.

Can plates be used even though they are damaged? In most cases, the answer is "yes". If the last plate available for use looks cloudy or pitted, try running a spectrum and see if it the spectrum is acceptable. A plate that has a big chip off it can still be used, as long as it is big enough to sit on the holder and in the path of the irradiation. IR plates can be resurfaced, so don't throw away a plate if it looks bad.

Thin-film IR Sampling Techniques

Two different methods are used to prepare thin films on an IR plate. If the compound to be studied is a liquid, you need to use thin-liquid film IR sampling technique. If it is a solid, use thin-solid film IR sampling technique. Note: a solution of a solid compound in a solvent does not count as a liquid; if the compound of interest is a solid at room temperature, even if it is dissolved in a solvent like methylene chloride, the compound is a solid and must be run by the procedure for thin-solid films.

Once the sample is prepared, you can collect the spectrum in one of the IR instruments.

Other Sampling Techniques

Solid samples can be prepared for IR examination by several other methods:

• A mull is prepared by grinding the sample with mineral oil and sandwiching the resultant paste between sodium chloride plates.
• The solid can be dissolved in a solvent and placed in a special cell, called a solution cell, which is made of sodium chloride. This gives what is called a solution spectrum.
• A KBr pellet is prepared by grinding the solid sample with solid potassium bromide (KBr) and applying great pressure to the dry mixture. Again, KBr is chosen because it is transparent to infrared radiation. If the pellet is prepared properly, one can actually see through it, as through a pane of glass.

IR Movie

CU Organic Chemistry has produced a movie about IR spectroscopy.