What are the types of infrared spectrometers and how they work?

Infrared spectrometers are generally divided into two categories, one is raster scanning, which is rarely used at present; The other is scanned by Michelson interferometers, called Fourier transform infrared spectroscopy, which is currently the most widely used.

Razzle scanning uses a beamsplitter to divide the detection light (infrared light) into two beams, one as a reference light and the other as a detection light to irradiate the sample, and then use the grating and monochromator to separate the wavelengths of infrared light, scan and detect the intensity of each wavelength, and finally integrate it into a spectrum.

Fourier transform infrared spectroscopy uses a Michelson interferometer to divide the detection light (infrared light) into two beams, which are reflected back to the beam splitter on the moving mirror and the fixed mirror. The coherent infrared light is irradiated on the sample, collected by the detector, and the infrared interferogram data containing the sample information is obtained, and the infrared spectrum of the sample is obtained after the Fourier transform of the data by the computer.

FTIR spectroscopy is widely used for its fast scanning rate, high resolution, and stable repeatability.

Fundamentals of infrared spectroscopy.

Infrared spectroscopy is closely related to the structure of molecules, and is an effective means to study and characterize molecular structure. It has a wide range of applications in molecular configuration and conformation research, chemistry and chemical engineering, physics, energy, materials, astronomy, meteorology, remote sensing, environment, geology, biology, medicine, medicine, agriculture, food, forensic identification and industrial process control.

Infrared spectroscopy can study the structure and chemical bonds of molecules, such as the determination of force constants and molecular symmetry, etc., and the bond length and bond angle of molecules can be determined by infrared spectroscopy, and the three-dimensional configuration of molecules can be inferred. According to the obtained force constant, the strength of the chemical bond can be deduced, and the thermodynamic function can be calculated from the simple frequency. Some groups or chemical bonds in the molecule correspond to the band wavenumber in different compounds are basically fixed or only change in the small wavelength range, so many organic functional groups such as methyl, methylene, carbonyl, cyano, hydroxyl, amine and so on have characteristic absorption in the infrared spectrum, through infrared spectroscopy, people can determine which organic functional groups exist in the unknown sample, which lays the foundation for the final determination of the chemical structure of the unknown.

Due to intramolecular and intermolecular interactions, the eigenfrequencies of organic functional groups will change slightly due to the different chemical environments in which functional groups are located, which creates conditions for the study of intramolecular and intermolecular interactions.

Many normal vibrations of molecules in the low wavenumber region often involve all the atoms in the molecule, and different molecules vibrate differently from each other, which makes the infrared spectrum highly characteristic like a fingerprint, called the fingerprint region. Taking advantage of this feature, the infrared spectra of thousands of known compounds have been collected and stored in a computer to compile a library of standard spectra of infrared spectroscopy.

The principle of infrared spectroscopy is that infrared spectroscopy is a kind of molecular absorption spectrum, which uses infrared spectroscopy to qualitatively and quantitatively detect organic matter, emits infrared light through the infrared spectrometer, and then irradiates the light to the surface of the object to be detected. According to the infrared spectrogram, the technician can find a database of chemical groups corresponding to the absorption peak, and perform a qualitative analysis of the composition and state of the substance to be measured.

Classification of infrared spectra

Infrared spectroscopy can be divided into near-infrared spectroscopy, far-infrared spectroscopy and Fourier transform infrared spectroscopy.

There are 4 different forms of energy present in molecules with NIR spectroscopy, which are translational energy, transport energy, vibrational energy, and electron energy. In near-infrared spectroscopy, the absorption of frequency doubling and frequency synthesis generated in the near-infrared region is often weaker than that of the mid-infrared, the background is very complex, and the phenomenon of peak overlap is very serious, and sometimes stoichiometric methods must be used to provide effective information.

Far infrared spectroscopy technology is the use of the absorption spectrum of objects in the far infrared region, the energy of the light source in this region is very weak, and the absorption band is mainly the pure rotational transition in the gas molecule and the expansion and contraction vibration of heavy atoms in the liquid, so it is generally not quantitatively analyzed in the far infrared spectrum region.

FTIR spectroscopy is a fast, non-destructive detection technique for food analysis, mainly by combining with chemometric methods to achieve qualitative and quantitative analysis.

The infrared spectrometer mainly detects: it can determine the existence or change of functional groups or chemical bonds in the sample, which is used to study the qualitative, quantitative, and reaction processes of substances.

In general, inorganics need to be detected with a far-infrared spectrometer. Because most of the vibrational peaks of inorganic matter are in the far infrared band, the detection range of the commonly used infrared spectrometer is in the mid-infrared region.

If an infrared spectrometer is needed to detect the infrared spectrum of inorganic matter, the spectrometer needs to be adjusted, the beam splitter in the Michelson interferometer and the detector of the spectrometer need to be replaced.

Principle:

The Fourier transform infrared spectrometer is known as the third-generation infrared spectrometer, which uses a McKelson interferometer to interfere with two multichromatic infrared lights with a certain speed difference in optical path to form interference light, and then interact with the sample. The detector sends the obtained interference signal to the computer for mathematical processing of the Fourier change, and the interference pattern is restored to a spectral pattern.

The characteristic absorption of functional groups of organic compounds can be referred to the organic spectroscopy book.

4000-1300 is used to identify functional groups, and 1300-400 is a fingerprint area, which is used to identify compounds with similar structures, and can also be used as circumstantial evidence for the presence of a certain group in a compound. The common characteristic absorption frequencies are: 3200-3650 hydroxyl group, free state in 3610-3640 peak type sharp, intermolecular association in 3300, broad peak.

More than 3000 is the unsaturated c-h expansion vibration, 3300 is a typical terminal alkyne, less than 3000 is the saturated c-h expansion vibration, the strong absorption peak of about 1700 indicates the existence of c=o, 1600, 2-4 peaks near 1500, is the skeleton vibration of the aromatic ring, and for more detailed explanation, please refer to Ning Yongcheng's organic compound structure identification and organic popology, a very good book

The proton chemical shift of the n-h peak is at the lower field, and the δ value is.

There are absorption peaks of N-H and C-N bonds. The telescopic vibration of the n-h bond is 3300 3500cm-1. Primary amines are doublets. Secondary amines are unimodal. The telescopic vibration of the C-N bond is generally around 1190 cm-1.

The vibrational forms of molecules can be divided into two main categories: telescopic vibrations and bending vibrations. The former refers to the reciprocating motion of atoms along the direction of the bond axis, and the bond length changes during vibration.

The latter refers to the vibration of the atom perpendicular to the direction of the chemical bond. Different forms of vibration are usually represented by different symbols, for example, telescopic vibration can be divided into symmetrical telescopic vibration and antisymmetrical telescopic vibration, which are denoted by VS and VAS, respectively.

Bending vibrations can be divided into in-plane bending vibrations (δ) and out-of-plane bending vibrations ( ) Theoretically, each basic vibration can absorb infrared light at the same frequency as the simple difference, and an absorption peak appears at the corresponding position of the infrared spectrum.

In fact, there are some vibrating molecules that are infrared-inactive without dipole moment change; In addition, there are some vibrations that have the same frequency and are degenerate; There are also vibrational frequencies that are beyond the range that the instrument can detect, which makes the number of absorption peaks in the actual IR spectrum much lower than theoretical.