Fourier Transform Infrared Spectroscopy
FTIR is based on the phenomenon of infrared absorption by molecular vibrations, and is used to identify the chemical bonds present in a sample. In this technique, a compound of interest is mounted within an optically transparent medium, and then the sample's absorption of infrared radiation across a range of frequencies is recorded. Any absorption of infrared radiation should correspond to a mode of vibration for one of the chemical bonds present in the sample. A Fourier transformation of the raw interferogram represents the wavelength dependence of IR absorption, and this provides a unique fingerprint for different chemical compounds.
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 using 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 force was sufficient. The hydraulic press can be calibrated pelletizing pure KBr powder.
- Extract and inspect the pellet. A good pellet is transparent with minimal translucent/powdery regions. A pellet is usable as long as the center is continuous and visually transparent, as 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.
The data from the spectrometer is typically processed to some degree using the software associated with the instrument.
Most instrument software packages are equipped to perform background, atmosphere, baseline and blank corrections. If unavailable, these treatments can be performed by simply subtracting spectra in excel.
This data processing should enhance the clarity of peaks from the sample in the spectrum.
Typical FTIR spectrum consists of intensity of transmittance or absorbance (in percent or arbitrary units) plotted on the Y-axis with the wavenumbers scanned (in inverse centimeter, cm-1) on the X-axis.
FTIR peaks correspond to chemical bonds present in the sample. Different chemical bonds have different characteristic regions. Multiple peaks can exist within a region/band corresponding to the various vibration modes for a bond. The bands for typical bonds is available here. Peak locations for specific compounds can be located in literature. The spectrum of common chemicals can be also found on the website of chemical vendors and NIST.
As an example, in the spectrum for a sample of LiFePO4 shown in the provided image, the peaks from 900-1200 cm−1 correspond to stretching, bending and other oscillation modes of PO43−.The peaks between 500-650 cm−1 belong to the symmetrical stretching vibrations of Fe-O bond.