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Preparing a Nujol Mull IR Sample

A Nujol mull is generally only used for solid samples and is quite different from the thin film and cast film methods. The solid is finely ground and mixed well with Nujol (a hydrocarbon oil). The resulting IR spectrum therefore consists of both the solid sample and Nujol oil.

Click here to see the corresponding sample recorded as a cast film

The Nujol mull method has the advantage that all solids (not just those soluble in organic solvents, as in the cast film method) can be treated in this manner, but has the disadvantage that the IR active C-H stretches (approx. 2990 - 2820 cm^-1) and bends (approx. 1490 - 1420 and 1390 - 1360 cm^-1) in Nujol oil might obscure IR active bands in the solid product. For this reason, it is an excellent practice to obtain a reference spectrum of Nujol oil and tape it into your lab notebook.

Grind a few mg of the solid compound (enough to cover a circle 2 mm in diameter) into a fine powder (like flour) with a small mortar and pestle.

Add 1 drop of Nujol oil to the powder.
Grind the mixture into a paste.

2. Add 2 drops of Nujol oil to the powder

3. Grind the mixture into a paste

Take a drop of the paste (using the tip of the pestle) and transfer it to the center of the IR plate.
- do not let the pestle tip scratch the plate
Place the second plate over the paste and gently turn the top plate to spread the paste.

4. Transfer the mull to a salt plate

5. Add a second plate and gently twist

Place the sandwich of IR plates in the sample cell mount.
Gently press and twist on the cover that holds the plates together.
- air bubbles should not be visible when the plates are held together

6. Place the plates in the cell mount

7. Put the cover on top

A Few Practical Problems to Watch For

the most common problem encountered with this method is that the film is too thick which results in the Nujol 1470 - 1370 cm^-1 peaks going off scale
conversely, if the film is too thin or if the plates are not held together properly, air bubbles will cause signals for carbon dioxide and water vapour to appear in the spectrum
check the intensity of the bands from the compound against those of Nujol; ideally, the compound peaks should be of similar intensity to the Nujol peaks that are found in the 1470 - 1370 cm^-1 region. If not, try adding more solid to the mull.
if there is too much solid in the mull, this may be manifested by a baseline that does not fall near 100% and/or peaks going off scale
if there is too little solid in the mull, the spectrum will not appear to be much different than that of Nujol oil

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Last modified: Monday, February 28, 2011
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