infrared spectrumThe instrument uses substances toinfrared radiationAbsorption characteristicsmolecular structureAnd chemical composition analysis instruments.Infrared spectrometer is usually composed of light source,Monochromator, detector and computer processing information system.according toSpectroscopicThe devices are divided into dispersion type and interference type.Optical Zero Balance for Dispersive Dual Optical PathInfrared spectrophotometerWhen the sample absorbs infrared radiation of a certain frequencyVibrational energy levelThe transition occurs, and the light of the corresponding frequency in the transmitted beam is weakened, resulting in the difference in the intensity of the corresponding radiation between the reference optical path and the sample optical path, thus obtaining the infrared spectrum of the measured sample.
Chinese name
Infrared spectrometer
Foreign name
infrared spectrometer
Scope of application
Industry, agriculture, medicine, science, military
Principle
Absorption of different wavelengths of infrared radiation by substances
Composition
Light source, monochromator, detector and computer processing information system
The infrared part of the electromagnetic spectrum can be divided intoNear infrared light, mid infrared light and far infrared light.Far infrared light (about 400-10 cm-1) is adjacent to microwave, with low energy and can be used for rotationspectroscopy。Mid infrared light (about 4000-400 cm-1) can be used to study foundation vibration and related rotational vibration structures.Higher energy near-infrared light (14000-4000 cm-1) can be excitedOvertoneandharmonicVibration.Infrared spectrometryThe working principle of,Chemical bondThey have different frequencies.resonance frequencyperhapsVibration frequencyDepends on the moleculeEquipotential surfaceShapeAtomic mass, and the final correlationVibration coupling。In order to make the molecular vibration mode active in infrared, there must be a change of permanent dipoles.Specifically, in Bonn Oppenheimer HarmonyVibratorIn approximation, for example, when corresponding to the electronground stateMolecules ofHamiltonianCapable of being characterized by molecular geometryEquilibrium stateNearbyHarmonic oscillatorApproximation, the potential surface of the ground state of the molecular electronic energyNatural oscillationMode determines the resonant frequency.However, the resonance frequency is related to the bond strength and the atomic mass at both ends of the bond after one approximation.In this way, the vibration frequency can be associated with a specific bond type.ordinaryDiatomic moleculeThere is only one key, and that is scaling.More complex molecules may have many bonds, and vibrations may appear conjugated, resulting in somecharacteristic frequencyOfinfrared absorption It can be linked to the chemical group.OftenOrganic compoundFound inCH2Group, can be vibrated in six ways: "symmetric and asymmetric stretching", "scissors swing", "left and right swing", "up and down swing" and "torsional swing".
principle
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Schematic diagram of infrared spectrometer
Fourier transformation infrared lightThe spectrometer is known as the third generation infrared spectrometer, which uses MichelsonInterferometerPut two bundlesOptical path differenceVarying at a certain speedMulticolorThe infrared light interferes with each other to form interference light, and then interacts with the sample.The detector sends the interference signal to the computer for mathematical processing of Fourier transformInterferogramrestoreSpectrogram。
classification
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It is generally divided into two categories:Raster scanningIs rarely used;The other isMichelson interferometerScanned, calledFourier transform infrared spectroscopy, which is the most widely used.Raster scanning usesSpectroscopeThe detection light (infrared light) is divided into two beams, one as the reference light, the other as the detection light to illuminate the sample, and then the grating andmonochromator Separate the wavelength of infrared light, scan and detect the intensity of each wavelength, and finally integrate it into a spectrum.Fourier transformation The infrared spectrum uses 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 fixed mirror. These two beams are broadbandcoherent light, interference will occur.The coherent infrared light shines on the sampledetectorThe infrared interferogram data containing the sample information is collected, and the infrared spectrogram of the sample is obtained after the data is Fourier transformed by the computer.Fourier transform infrared spectroscopy hasScan rateFast, high resolution, stableRepeatabilityIs widely used.
infrared spectrumCan study molecular structure andChemical bond, such asForce constantDetermination andMolecular symmetryEtc., usingInfrared spectral methodDeterminableKey lengthandBond angleAnd from this we can infer the molecular three-dimensional configuration.The strength of chemical bond can be inferred from the force constant obtainedNormal frequencycalculationThermodynamic functionEtc.Corresponding to some groups or chemical bonds in different compoundsSpectral bandwave numberBasically, it is fixed or only changes in a small band, so many organicfunctional groupE.g. methylmethylene、carbonyl, cyano, hydroxyl, amino, etc. have characteristic absorption in the infrared spectrum. Through infrared spectrum measurement, people can determine which organic functional groups exist in the unknown sample, which is the final determination of the unknownChemical structureLaid the foundation.
Due to intramolecular andIntermolecular interaction, organic functional groupcharacteristic frequencyBecause of the functional groupChemical environmentThe micro changes will occur due to the differences, which creates conditions for the research and characterization of intramolecular and intermolecular interactions.
Many molecules in the low wavenumber regionNormal vibrationIt often involves all the atoms in the molecule. Different molecules have different vibration modes, which makes the infrared spectrum as highly characteristic as fingerprints. It is calledFingerprint area。Using this feature, people have collected the infrared spectra of thousands of known compounds, and stored them in the computer to compile the standard spectrum library of infrared spectra.
People can quickly determine the composition of unknown compounds by comparing the infrared spectra of unknown substances measured with the spectra in the standard library.
The development of modern infrared spectroscopy technology has made the significance of infrared spectroscopy far beyond the stage of simple routine testing of samples and inferring the composition of compounds.Many new infrared spectrometers are derived by combining infrared spectrometers with other testing methodsMolecular spectrumFor example, the combination of chromatography technology and infrared spectrometer has created opportunities to deepen the understanding of the chemical structure of various components in complex mixture systems;Combining infrared spectrometer with microscope method, infrared imaging technology is formed for researchHeterogeneous systemOfmorphological structure Because infrared spectrum can effectively distinguish different compounds by using its characteristic spectral bands, this method has chemical contrast that other methods cannot match.
Carry out quantitative analysis of compounds to study the transient relationship between chemical reaction kinetics, crystal change, phase change, material tension and structure
Industrial process and continuous detection of air pollution
Carry out quantitative analysis of compounds to study the transient relationship between chemical reaction kinetics, crystal change, phase change, material tension and structure
Industrial process and continuous detection of air pollution
crystalquartzComponent analysis of hydroxyl measurement polymer Drug analysis
matters needing attention
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1. The temperature of the laboratory shall be 15~30 ℃ during the measurement,relative humidityIt shall be less than 65%, and the power supply used shall be equipped with voltage stabilizing device andground wire。The relative humidity in the room should be strictly controlled, so the area of the infrared laboratory should not be too large, and it can accommodate the necessary instruments and equipment, but there must beDehumidifier。
2. For example, single beam FourierInfrared spectrophotometer(Most used), the CO2 content in the laboratory should not be too high, so the number of people in the laboratory should be as small as possible, unrelated personnel should not enter, and pay attention to proper ventilation.
3. Such asTest articlebyhydrochloric acidSalt, due to the possible ion exchange phenomenon during tablet pressingpotassium chloride(Same asPotassium bromideThe same is used after pretreatment) instead of potassium bromide, but potassium chloride tablet andPotassium bromide tabletPosttestIf there is no difference between the two spectra, potassium bromide can be used for tablet pressing.
4. In order to prevent the instrument from being affected by moisture and affecting its service life, the infrared laboratory should always be kept dry. Even if the instrument is not used, it should be turned on at least twice a week, half a day each timedehumidifierDehumidification.especiallyMildew rainSeason, it is better to turn on the dehumidifier every day.
5. The most commonly used infrared spectroscopySample preparationThe method isPotassium bromide(KBr)Tablet pressing method(pharmacopoeiaThis method is used for more than 90% of the collected varieties), so in order to reduce the impact on the determination, the KBr used should preferably be optical reagent grade, at leastAnalytically pureLevel.Before use, it should be properly ground (below 200 meshes) and dried at 120 ℃ for more than 4 hours before usedryerMedium standby.If caking is found, it shall be dried again.The prepared empty KBr sheet should be transparent, compared with air,TransmittanceIt should be more than 75%.
6. The amount of the test sample taken in the tablet pressing method is generally 1~2mg, because it is impossible to add it after weighing with a balance, and the infrared radiation of each sampleAbsorption of lightThe degree is inconsistent, so it is often used according to experience.Most of the spectrograms required in generalabsorption peakIt is within the range of 10%~80% light transmittance.If the transmittance of the strongest absorption peak is too large (if more than 30%), the sampling amount is too small;On the contrary, if the strongest absorption peak is close to the transmittance of 0%, and it is a flat peak, it means that there is too much samplingShould beRe measure after adjusting the sampling amount.
7. The sample for determination should be dry, otherwise it should be placed after being groundInfrared lampLet dry for a few minutes.After the sample is grinded and installed in the moldVacuum pumpAfter connectionVacuumizeAt least 2 minutes, so that the moisture in the sample is further removed, and then pressurized to 0.8~1GPa (8~10T/cm2) for 2~5min.No vacuum pumping will affect the transparency of the film.
8. The amount of KBr taken during compression is generally about 200mg (also based on experience). The amount of KBr should be controlled according to the film thickness after production. Generally, the film thickness should be less than 0.5mm. When the thickness is greater than 0.5mm, it can often be observed on the spectruminterference fringe, causing interference to the spectrum of the test article.
9. When pressing the tablet, the test article should be taken for grinding first, then KBr should be added for grinding again, which is easierEvenly mix。The grinding shall beAgate mortar, due to glassMortarInternal surfaceIt is rough and easy to adhere to the sample.Grind in the same direction（Clockwiseoranti-clockwise）Even force, if not grinding in the same direction, it may cause crystal transformation of the test article during the grinding process, thus affecting the determination results.The grinding force should not be too large, and it is enough to grind until there are no small particles visible to the naked eye in the sample.After the sample is grinded, it should be poured into the tablet mold through a small funnel (because the mold mouth is small, it is difficult to pour directly), and the sample should be spread evenly as far as possible, otherwise the transparency of the place where there are fewer samples after the tablet is pressed will be lower than that of the place where there are more samples, which will affect the measurement.In addition, if it appears on the pressed filmOpaqueIf there are small ungrounded particles in the grinded sample, it should be pressed again.
10. Wipe all parts of the die immediately after use, and clean and dry them with water if necessary, and store them in a dryer to avoid corrosion.
