Principles of infrared spectroscopy (5) Various measurement methods in FTIR spectroscopy

October 1, 2019

There are various measurement methods in FTIR spectroscopy, and these are summarized below.

・Transmission methods

KBr tablet method
This method is used for powder samples, and precise measurements can be performed even with very small samples. However, due to the fact that KBr absorbs moisture, it is not suitable when measuring samples whose spectra contain absorption bands in the same regions as those due to water. Samples that contain water, are highly hygroscopic, have a high concentration of carbon or pigments, have an unstable crystal structure, or are relatively opaque to infrared light, are also unsuitable. For chlorides and hydrochlorides, since a halogen exchange reaction occurs between Br and Cl, sample pellets are generally produced using KCl.

Fig. 10 KBr tablet method

Nujol method
A powder sample is mixed with liquid paraffin to form a paste, which is then applied to the window plate. The Nujol method is used for samples that are not suitable for the KBr method, such as those that absorb moisture during sample preparation (e.g., sugars), and samples that do not dissolve in solvents (e.g., inorganic substances). It is also used for reagents specified by official standards such as pharmacopoeia, or for far-infrared measurements under a N2 gas purge. However, this method is not suitable for evaluating bands associated with ​​CH groups (3,000 to 2,800, 1,500 to 1,300 cm-1) because of absorption by the liquid paraffin in these wavenumber regions.

Fig. 11 Nujol method

KBr plate method
This method uses crystalline KBr. A powder sample is sandwiched between KBr plates and measured. Since there is no need for crushing, sample preparation is easy. It is an effective method when simple sample preparation is required, or for measuring highly hygroscopic samples. However, samples that contain large particles are susceptible to scattering of light, so the measured absorbance may be lower than the true value. This method cannot be used for samples containing water.

Fig. 12 KBr plate method

Thin-film method
This method is for samples that can be formed into thin films with a thickness of 10 μm or less. Various thinning methods can be used, such as stretching, hot pressing, and cutting. If the sample is soluble in a solvent, the solution can be applied to an infrared transmitting material (KBr plate), and evaporating the  solvent to form a thin film becomes simple.

Fig. 13 Thin-film method

Liquid-film method
This method is suitable for samples of non-volatile liquids that have a high viscosity, and also for qualitative analysis. The liquid is held between two window plates in the assembled cell. Depending on the peak intensity, a spacer may be needed to adjust the optical path length. For samples containing water, a KBr window cannot be used.

Solution method
This method can be used to measure volatile solutions or to analyze the concentration of trace additives dissolved in a solution. Before performing the measurements, it is important to ensure that the solvent itself does not give rise to absorption bands that interfere with those of the target components.

Gas method
This method is used for gaseous samples. The cell path length should be adjusted based on the gas concentration. Since gaseous samples usually produce sharp absorption peaks, high-resolution measurements are required. To determine the amount of water vapor and carbon dioxide in a sample, use an optical system that is under a full vacuum, or perform a N2 gas purge to remove these components completely.

Fig. 14 Gas method

・Reflection methods

ATR method
This is a reflection measurement method based on penetration of infrared light into a sample, where the light is totally internally reflected by a prism. Measurements can be performed simply by placing the sample in close contact with the prism. When measuring acidic and alkaline solutions, use a diamond prism that will not corrode.

Fig. 15 ATR method

Diffuse reflection method
This method is used for powder samples, and the light that is measured has been repeatedly transmitted, scattered and reflected inside the sample. This light is called diffuse reflection. If the sample is strongly absorbing, it should be diluted using an infrared transmitting material such as KBr. If a quantitative analysis is needed, the reflectance can be calculated using the Kubelka-Munk transformation, and the resulting value is proportional to the concentration.

Fig. 16 Diffuse reflection method

Specular reflection method (surface)
This method detects specularly reflected light from the smooth surface of a flat sample irradiated at an incidence angle of about 10°. The measured reflectance is compared to that for a reference mirror. The reflection spectrum may contain distortions due to variations in the refractive index, but these can eliminated using a Kramers-Kronig (KK) transformation.

Fig. 17 Specular reflection method  (surface)

Reflection absorption method
If the sample to be measured is a thin film on a metal plate, infrared light is reflected at the interface with the metal plate, so that the absorption spectrum for the thin film can be measured. There is no need to perform a KK transformation.

Fig. 18 Reflection absorption method

High-sensitivity reflection (RAS) method
When a very thin thin film on a metal substrate is irradiated with light that is polarized in a specific direction, only molecules with dipole moments that respond to this polarization vibrate. This leads to an increase in absorbance and a sensitivity that is one to two orders of magnitude higher than that for the transmission method. This is a highly sensitive method for thin films.