What is the principle of infrared spectroscopy testing?

The atoms of any molecule are constantly vibrating and spinning. The frequency of these molecular motions is of the same order of magnitude as the frequency of infrared radiation (1013 to 1014 cycles per second). When a molecule is moving at the same frequency as the infrared radiation hitting the molecule, and when the dipole moment changes during that motion, the molecule can absorb the incident radiation. These curves or graphs of absorption as a function of wavelength or frequency include IR spectra.

Figure 1 Infrared spectra of alkyd resins and oils in exterior primers. The spectrum can serve as a fingerprint of the material. Many absorption bands are characteristic of specific functional groups and can be used to obtain structural information about a sample.

The mid-infrared region is from about 2 to 25 um (5000 to 400 cm-'), which is a useful range for chemical analysis. The most convenient unit for infrared wavelengths is the micron or micron (um, 10m). It is now more common to express infrared spectra in terms of wavenumbers, which are expressed in reciprocal centimeters (cm-'). The wave number is proportional to the frequency and is calculated by dividing the wavelength (unit: um) by 10,000 divided by the wavelength (unit: cm-'). The near-infrared region of the spectrum, which occurs at higher frequencies (wavenumbers) and shorter wavelengths than the mid-infrared, has found considerable use in recent years, especially in process control and monitoring relatively well-defined materials. It is not useful for identifying complete unknowns Very useful.

An infrared spectrum, as shown in Figure 1, is a plot of the y-axis of infrared radiation transmitted or absorbed by a sample versus the wavenumber or wavelength on the x-axis. Since no two substances that absorb infrared radiation absorb it at the same frequency, the infrared spectrum is a substance's "fingerprint" that helps distinguish one molecule from another. Also, apart from certain hydrogen bonding situations or chemical reactions taking place, the spectrum of a mixture is simply the sum of the spectra of the individual components that make up the mixture. Figure 8.1 shows the infrared spectrum of an alkyd resin and oil on an exterior primer. In addition to being a fingerprint, infrared spectroscopy can explain molecular structure. Functional groups have characteristic absorption properties. For example, carbonyl compounds, such as polyester in Fig. 8.1, have strong absorption near the 1700 wavenumber.