Infrared spectroscopy (IR) relies on the fact that most molecules absorb light in the infrared region of the electromagnetic spectrum, converting it to molecular vibration. This absorption corresponds specifically to the bonds present in the molecule. With a spectrometer, this absorption of infrared radiation by the sample material is measured versus wavelength (as wavenumbers, typically from 4000 – 600 cm-1).
The result is a spectrum that gives a distinctive "molecular fingerprint" which can be used to screen, scan and identify organic and inorganic samples.
Instead of sequentially irradiating the sample with varying single wavelengths (dispersive), FTIR allows to collect all spectral data in one go. For this, you need a continuum source of light to produce light over a broad range of infrared wavelengths. The infrared light then passes through an interferometer and is afterwards directed at the sample.
This yields an interferogram, a somewhat raw signal, that represents the intensity of light as a function of the position of a mirror. This signal first needs to be Fourier-transformed (FT) to produce the more familiar IR plot of intensity vs. wavenumber. Hence the name “FT-IR” or FTIR.
Not only is the acquisition of FTIR spectra much faster than by conventional dispersive Instruments. Additionally, these spectra show a significantly higher signal-to-noise ratio and, since the wavelength scale is calibrated with a very precise laser, spectra obtained have much higher wavelength accuracy.
This depends on the sample that needs to be analyzed. Classically, an otherwise solid sample is either ground with IR transparent potassium bromide (KBr) and pressed into a pellet, or it is thinly sliced and placed onto a KBr window, while liquids are directly measured or diluted with an IR transparent solvent, e.g. CCl4.
By now, this laborious preparation is in many cases succeeded by ATR FTIR spectroscopy, a technique that is mostly non-destructive and suited to analyze solids and liquids in their present states.
ATR stands for attenuated total reflection and has become the standard technique for the measurement of FTIR spectra. The infrared light passes through a crystal of a certain material (diamond, ZnSe or germanium) and interacts with the sample, which is pressed onto the crystal.
From this a spectrum is obtained, that shows all substance specific characteristics, while the intensity ratio of the observed absorption bands might differ from a traditional transmission spectrum due to physical effects.