Infrared spectroscopy is also called "infrared spectrophotometry". Abbreviation "IR", a kind of molecular absorption spectrum. A method for structural analysis and qualitative and quantitative analysis of various compounds that absorb infrared light by utilizing the selective absorption of electromagnetic radiation in the infrared region by substances. Under the irradiation of infrared rays, the molecules of the tested substance only absorb the infrared spectrum that is consistent with the vibration and rotation frequency of its molecules. Analysis of infrared spectrum can be used for qualitative analysis of substances. There are many atomic groups in compound molecules. After each atomic group is excited, it will produce characteristic vibration, and its vibration frequency must be reflected in the infrared absorption spectrum. According to this, various atomic groups in the compound can be identified, and quantitative analysis can also be carried out.
1. General characteristics of infrared spectroscopy
It has strong characteristics, fast measurement, no damage to the sample, less sample consumption, easy operation, and can analyze samples in various states, with low analytical sensitivity and large quantitative analysis error.
2. Requirements for samples
① The purity of the sample should be greater than 98%, or meet commercial specifications
This facilitates comparison with standard or commercial spectra of pure compounds
Multi-component samples should be separated and purified by fractional distillation, extraction, recrystallization or chromatography in advance, otherwise the spectra of each component overlap each other and it is difficult to analyze
②The sample should not contain water (crystal water or free water)
Water has infrared absorption, interferes with hydroxyl peaks, and can erode the salt window of the absorption cell. The samples used should be dried
③ The concentration and thickness of the sample should be appropriate
Make the strongest absorption and transmittance between 5 and 20%
3. Qualitative and structural analysis
Infrared spectroscopy is characteristically distinct in that the number, position, shape and intensity of bands vary from compound to compound. Therefore, infrared spectroscopy is a powerful tool for qualitative identification and structural analysis
①Identification of known objects
Comparing the spectrum of the sample with the spectrum measured by the standard, or with the standard spectrum in the literature (such as "Drug Infrared Spectrum Atlas", Sadtler standard spectrum, Sadtler commercial spectrum, etc.), it can be qualitative
When using the spectrum in the literature, it should be noted that the physical state, crystal shape, solvent, measurement conditions and the type of instrument used of the sample should be the same as those of the standard spectrum.
② Identification of unknown objects
If the unknown substance is not a new compound, the standard spectrum has been included. There are two ways to check the standard spectrum:
A. Use the band index of the standard spectrum to find the spectrum in the standard spectrum that has the same absorption band as the sample spectrum
B. Perform spectral analysis to determine the possible structure of the sample. It is then checked against the standard spectrum by looking up the chemical classification index
Preparation before resolving the spectrum:
Know the source of the sample to estimate its possible extent
Determination of the physical constants of the sample such as melting point, solubility, refractive index, optical rotation, etc. as qualitative circumstantial evidence
Obtain the molecular formula from elemental analysis and molecular weight measurement
To calculate the degree of unsaturation Ω of a compound, the procedure for estimating the structure and verifying the rationality of the spectral analysis results is generally as follows:
A. Starting from the strongest spectral band in the characteristic area, infer the groups that the unknown may contain, and determine the groups that are impossible to contain
B. Use the band verification in the fingerprint area to find out the relevant peaks that may contain groups, and use a set of relevant peaks to confirm the existence of a group
C. For simple compounds, after confirming a few groups, the molecular structure can be initially determined
D. Check the standard spectrum for verification
③ Structural analysis of new compounds
Infrared spectroscopy mainly provides structural information of functional groups. For complex compounds, especially new compounds, infrared spectroscopy alone cannot solve the problem. It is necessary to cooperate with analytical methods such as ultraviolet spectroscopy, mass spectrometry and nuclear magnetic resonance to conduct comprehensive spectral analysis to determine the molecular structure. .
④Identify bacteria and study the structure of cells and other living tissues
4. Quantitative Analysis
Infrared spectroscopy has many bands to choose from, which are more conducive to eliminating interference. The luminous energy of the infrared light source is low, and the sensitivity of the infrared detector is also very low, ε<103
The thickness of the absorption cell is small, the width of the monochromator slit is large, and the measurement error is also large
☆Infrared spectroscopy is a better analytical method for the determination of pesticide components, soil surface moisture, carbon dioxide content in the field, and the determination of protein, fat and moisture content in cereals, oil crops and meat foods.