An anhydrous KBr salt can be pressed into a continuous optically transparent pellet. This procedure can be used to mount samples for FTIR.
Creating a KBr Pellet
- Remove the potassium bromide (KBr) from a vacuum oven. It is important that the KBr is fully anhydrous, as residual water in the salt will interfere with the spectra.
- Add potassium bromide (KBr) and the sample to a mortar, such that the mass ratio of sample:KBr is between 1:50 to 1:100. This ratio can be optimized to improve signal to noise ratio and minimize background intensity.
- Grind the powders into a homogenous mixture.
- Cover the pellet press die with a very thin layer of the mixture.
- Pelletize the mixture, either manually or through a hydraulic press. This entails applying enough pressure such that the KBr transforms into a continuous, optically transparent pellet. In our case, with a 1/2 inch pellet press approximately 7 tonnes of pressure was sufficient. This can be calibrated using pure KBr powder.
- Extract and inspect the pellet A good pellet is transparent with minimal translucent/powdery parts. A pellet is usable as long as the center is continuous and visually transparent. Any opacity in this region is detrimental to the quality of the spectrum.
- Carefully place the pellet in to the sample holder.
- Initialize the FTIR machine and collect a background spectra. This background can be subtracted from the measured sample spectra, helping to correct for environmental changes in the instrument. Atmospheric CO2 and H2O signals can be minimized by purging the system with nitrogen.
- Once background collection is complete, insert the sample holder into the FTIR instrument.
- Collect the FTIR spectrum of the sample. The spectrum of a blank KBr pellet to measured be used to identify/remove peaks arising from the impurities present in the KBr.
- The resolution and number of scans for a sample can be tweaked in order to maximize signal-to-noise and capture peak changes.
- If signal intensity is too low or the background is significantly slanted, repeat the procedure using different sample:KBr ratios.
If your sample is soluble in a solvent, it is possible to use pre-made KBr pellets. A dilute sample solution can be drop cast on these pellets and dried to remove the solvent. FTIR spectra can then be collected as described above. The KBr can be cleaned for reuse by using a solvent/water to mildly erode a few layers and expose fresh KBr.
Some samples can not be pressed into pellets. For these, Attenuated total reflectance (ATR)-FTIR can be used.