Infrared spectroscopy, also known as "infrared spectrophotometric analysis"A type of molecular absorption spectrum. The force method of utilizing the selective absorption of electromagnetic radiation in the infrared region by substances for structural analysis and qualitative and quantitative analysis. The molecules of the tested substance only absorb infrared spectra that are consistent with their molecular vibration and rotation frequencies under infrared irradiation. Analyzing infrared spectra can enable qualitative analysis of substances. There are many atomic groups in compound molecules^[1]After each atomic group is excited, it will produce characteristic vibrations, and its vibration frequency will inevitably be reflected in the infrared absorption spectrum. Based on this, various atomic groups in the compound can be identified and quantitative analysis can also be performed.
　　1.infrared spectroscopyGeneral characteristics
Strong characteristics, fast measurement, no damage to the sample, small sample usage, easy operation, ability to analyze samples in various states, low analysis sensitivity, and large quantitative analysis errors.
2. Requirements for samples
① The purity of the sample should be greater than 98% or meet commercial specifications
This is the only way to compare with the standard spectrum or commercial spectrum of pure compounds
Multi component samples should be separated and purified in advance using fractionation, extraction, recrystallization, or chromatography methods, otherwise the spectra of each component overlap and are difficult to resolve
② The sample should not contain water (crystalline water or free water)
Water has infrared absorption, interferes with hydroxyl peaks, and can erode the salt window of the absorption pool. The samples used should undergo drying treatment
③ The concentration and thickness of the sample should be appropriate
Make the maximum absorption transmittance between 5-20%
3. Qualitative analysis and structural analysis
Infrared spectroscopy has distinct characteristics, and the number, position, shape, and intensity of its spectral bands vary from compound to compound. Therefore, infrared spectroscopy is a powerful tool for qualitative identification and structural analysis
① Identification of known objects
By comparing the spectrum of the sample with the spectrum measured by the standard sample, or with the standard spectrum in literature (such as the "Atlas of Infrared Spectroscopy for Drugs", Sadtler Standard Spectrum, Sadtler Commercial Spectrum, etc.), qualitative analysis can be achieved
When using spectra from literature, it should be noted that the physical state, crystal shape, solvent, measurement conditions, and instrument type of the sample should be the same as the standard spectrum
② Identification of unknown objects
If the unknown substance is not a new compound and the standard spectrum has already been recorded, there are two methods to check the standard spectrum:
A. Using the band index of the standard spectrum, search for spectra in the standard spectrum that have the same absorption band as the sample spectrum
B. Perform spectral analysis to determine the possible structure of the sample. Then use the chemical classification index to search for standard spectra for comparison and verification
Preparation before Spectral Analysis:
Understand the source of the sample to estimate its possible range
Measure the physical constants of the sample, such as melting point, solubility, refractive index, optical rotation, etc., as qualitative evidence
Calculate the molecular formula based on elemental analysis and determination of molecular weight
The program for calculating the unsaturation Ω of a compound to estimate its structure and verify the validity of spectral analysis results is generally:
A. Starting from the strongest spectral band in the characteristic region, infer the possible functional groups that the unknown substance may contain, and determine the functional groups that cannot be contained
B. Verify the spectral bands in the fingerprint area, identify relevant peaks that may contain functional groups, and use a set of related peaks to confirm the presence of a functional group
C. For simple compounds, after confirming a few functional groups, the molecular structure can be preliminarily determined
D. Verify the standard spectrum
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 needs to be combined with analytical methods such as ultraviolet spectroscopy, mass spectrometry, and nuclear magnetic resonance for comprehensive spectral analysis to determine the molecular structure.
④ Identify bacteria, study the structure of cells and other living tissues
4. Quantitative analysis
Infrared spectroscopy has many spectral bands to choose from, which is more conducive to eliminating interference.? Infrared light sources have low emission energy, and the sensitivity of infrared detectors is also very low, ε＜103
The absorption tank has a small thickness, the slit width of the monochromator is large, and the measurement error is also large
Infrared spectroscopy is a good analytical method for determining pesticide components, soil surface moisture, field carbon dioxide content, as well as protein, fat, and moisture content in grains, oil crops, and meat products