Theory of FTIR Spectroscopy

Early Infrared Spectrometers

Infrared spectrometers were first developed in the United States in the mid-1940s. Initially, their applications were confined to research and development work on organic compounds. In 1954, the first Japanese instruments were manufactured by the Applied Optics Research Institute, the predecessor of JASCO.

Modern Infrared Spectrometers

In the 1970s, the first commercial Fourier Transform infrared (FTIR) spectrometer appeared. In FTIR spectrometers, spectra are generated in the time domain by following the position of a moving mirror and the occurrence of constructive and destructive interference. A Fast Fourier Transform (FFT) then converts the signal from the time into the frequency domain. Since the FFT calculation takes time to compute, the development of commercial FTIR spectroscopy closely followed the trend of the increased power and miniaturization of computers. By the 1980s, dispersive infrared spectrometers were phased out for the more desirable interferometer type spectrometers as computers became more widespread. In 1982, JASCO developed its first FTIR spectrometer.

FTIR spectrometers have three main advantages over dispersive spectrometers:

1. Fellgett’s Advantage – the full spectrum reaches the detector simultaneously  (instead of a small band of radiation as in grating-based spectrometers), the spectrum takes much less time to collect.
2. Jaquinot’s Advantage –  since there are fewer optics and no slits, the total power at each data point is much greater.
3. Conne’s Advantage – the resolution is much greater than with a grating, enabling spectral subtraction, concentration curves, and library searches among other functions.

By using an interferometer, it should be noted that a couple of features in a dispersive monochromator, such as fixed wavelength measurements and double beam optical configurations are not allowed in a FTIR spectrometer.

The electromagnetic spectrum consists of different regions corresponding to different energy (E), frequency (ѵ), and wavelength (λ) ranges as seen in Figure 2. The unit for near-, mid-, and far-infrared, the wavenumber (cm^-1), is derived from the inverse relationship between wavelength and frequency.

FTIR spectroscopy takes advantage of how IR light changes the dipole moments in molecules (Fig. 3) that correspond to a specific vibrational energy. Vibrational energy corresponds to two variables: reduced mass (μ) and bond spring constant (k) (Equation 1). For k constant, we can look at C-C, C=C, and C≡C showing an increase of 800 cm^-1 across the series (Table 1). Substituting atoms in a C-C bond with nitrogen and oxygen causes a shift of 100 cm^-1 (Table 2). By looking at the two series, it can be seen that bond strength alters the wavenumbers more than mass.

The Near-IR portion of the electromagnetic spectrum falls between 4,000 to 12,800 cm^-1. This region consists of overtones (two of the same vibrational modes occurring simultaneously) and combinations (two different vibrational modes occurring simultaneously). Since these modes are not strictly quantum mechanically allowed, the intensity of the modes is often quite low. These spectra are often complex, and chemometric techniques, such as multivariate analysis, are used. In spite of the drawbacks, there are clear advantages to Near-IR spectroscopy. Firstly, the path length of the light is such that bulk samples can be analyzed with little to no sample preparation. Secondly, water does not affect signal as it does in mid-IR. These two benefits have been of great value to process chemistry and bulk analysis of incoming/outgoing goods.

The Far-IR region lies between 10 cm^-1 and 700 cm^-1. The bonds that show in this region are 3+ atom functional groups, such as -C-C-C- bending, and lattice vibrations in crystalline materials. Since these are highly dependent on conformation or crystal structure, materials with the same chemical structure, but different crystal structures may be distinguished using Far-IR. There are two disadvantages to Far IR. Firstly, water absorbs strongly in this region making a purged or evacuated system absolutely necessary. Secondly, the intensity of these modes is weak, so sensitive detectors and high-powered sources are needed.

FTIR is also used for microscopy for microanalysis of the same types of materials that are studied using macro IR measurement. An FTIR Microscope uses reflection optics to observe and focus infrared light onto samples.  Transmission, Reflection, Attenuated Total Reflectance, and Grazing Angle objectives are used to transmit and collect infrared energy to a small spot (5-50 μm) on the sample of interest.  Multiple measurements in selected, spots, lines or 3D grids can be used to generate a chemical map.  This mapping may be enhanced using imaging analysis, chemometrics and library search to automatically identify components and their distribution in a sample. More about FTIR microscopy can be found here.