FTIR spectroscopy is a way of measuring the infrared absorption of molecules. It works on the principle that each molecule has unique spectral bands for its vibration modes, determined by the molecular structure. These vibrations are quantized in energy units of formula_1 or cm*formula_2. The measured spectrum will show peaks at specific wavelengths depending on what vibrational modes are present in the sample.
Since each functional group has characteristic frequencies, FTIR can identify certain compounds present in an unknown mixture. This makes it sound like a “fingerprinting” technique for organic compounds since there is no chemical reaction required to analyze them. FTIR can also be used to measure the loss on ignition (LOI). LOI is the total mass of a sample that was lost during a heating process. This percentage is helpful to track quality control for specific products during manufacturing.
A typical FTIR experiment measures reflected infrared radiation from a sample, but transmission measures are also possible. An interferometer creates two beams of light from one beam by splitting it with a partially reflective mirror called a beam splitter to make absorption measurements. Light from both beams is focused onto different parts of the sample and then brought together again by another beam splitter. They interfere constructively or destructively depending on the distance traveled by each path. All constructive interference at a common point produces hot spots that can be detected and used to the intensity of the absorption bands throughout each sample.
The primary components of FTIR spectroscopy basics are the spectrometer, the interferometer, and the detector. A broadband light source can be used to illuminate samples with any range of infrared wavelengths; some systems use white light sources which cover a large portion of the spectrum, while others will use specific lasers or lamps to emit narrowband light at specified spectral peaks. The spectrometer is designed to measure the intensity of infrared radiation over a wide range of frequencies for each point on the sample’s surface; this is achieved by diffraction through several slits to obtain two beams (called “reference” and “sample”) that overlap onto a single detector. More radiation is present at the sensor when these beams interfere constructively, whereas destructive interference results in less radiation.
A Fourier transform converts this time-domain signal into a frequency or wavenumber domain signal used for analysis using fast Fourier transformation. Detailed FTIR data sets are presented as graphs showing each peak’s intensity as either a function of formula_4 or formula_5. Consequently, if one knows the exact wavelengths at which an unknown compound absorbs light, it is possible to identify its presence by comparing its absorption bands to those of known compounds displayed on these types of graphs. If no match exists, then the mixture may be unidentified; however, a lack of a game does not prove that the compound is unknown because other compounds are present in the sample that have yet to be identified.