IR Sample Preparation

Solid Samples: Making a KBr Pellet

1. Put the tip of a spatula (about 1 mg) of sample into a pestle. Dilute about 1:20 with KBr from the drying oven (add about 20-30 mg). Grind and mix the two solids together with a mortar. NOTE: The KBr and sample must both be fairly dry to get a good mix.
2. Screw one bolt into the die. Place the die on a counter and transfer a small amount of sample into the die. Distribute the sample completely over the bolt end.
3. Screw the second bolt until it's hand tight and clamp the bolt in the vice with one bolt end sticking up.
4. Use the torque wrench to tighten the bolt to about 30 foot-pounds for about 20 seconds. Remove the bolts.
5. To minimize drift, KBr pellets should be thin and fairly transparent.
6. To clean the bolts and die, remove the pellet with a spatula and wash with deionized water and acetone. Dry in the oven if needed.

Liquid Samples: Using Salt Plates

1. Remove two salt plates from the desiccator. Handle plates by the edges. NOTE: Salt plates are made from compressed sodium chloride. Moisture from your fingers will mark up the plate surface.
2. Place a drop or two of sample in the first salt plate, place the second plate on top.
3. Transfer the plates to a holder. NOTE: Air bubbles will not affect IR spectra.
4. Place the sample holder in the IR chamber and scan.
5. Rinse the plates with acetone or dichloromethane and place the plates in the desiccator.

Why do we use KBr or NaCl?

Typically IR is acquired with the sample either mounted on or suspended in KBr or NaCl. This is because neither of these compounds has an IR-active stretch in the region typically observed for organic and some inorganic molecules. This means that the spectrum is purely that of your compound (and any impurities!).