FT-IR spectroscopy

Fourier-transform infrared spectroscopy (FT-IR) is a method used to analyse compounds that absorb electromagnetic radiation in the infrared region.
The absorbance is based on the dipole movement of a diatomic molecule. There are two main events in molecules that indicate infrared absorbance, vibrations and rotations. These movements describe the various quantum states molecules will have when absorbing infrared radiation. The energy difference of these can be measured and plotted either based on absorbance or transmittance.
The principle of FT-IR is based on interferometry. Analysis using FT-IR produces an interferogram which is a complex convolution of signals. Within the IR spectrometer a radiation source emits infrared light which passes a beam splitter, at 45° angle, creating deflected and unaffected beams of infrared light. The beams are directed towards a stationary and movable mirror before being reflected to the detector, passing the sample. The movable mirror allows for constructive or destructive interference by changing the difference in phase between the two beams.
The detector passes information via a computer that applies the Fourier Transformation, producing an FT-IR spectra where transmittance of emitted infrared is plotted against wavenumber in cm-1 (frequency).
\begin{equation} \label{eq:fourier} F(v) = \int_{-\infty}^{\infty}f(t)e^{(-2\pi i v t)} dt \end{equation} The Fourier Transformation allows conversion of time or space, function \( f(t) \), into frequency or spatial frequency, \( F(v) \), respectively. As infrared is part of the radiation spectrum with notated wavelength as unit, which is a unit of space, the output is spatial frequency or wave number.

Samples were analysed using a PerkinElmer Spectrum™ 100, FT-IR attenuated total reflectance (ATR) spectrometer. The background used for powdered samples was air, ultrapure water for liquids. Backgrounds were read before scanning of the sample. The samples were tightly clamped before scanning. Scans were carried out between wave number 4000 and 500. Each measurement was scanned for a minimum of 8 scans. This was to improve the signal-to-noise ratio (SNR). Data were outputted in % transmission versus wave number. Peak analysis was done using a custom MatLab™ script. Peak values were identified using literature databases.