Determination of infrared transmission spectra of samples in different physical states experiment

Summary

This experiment is from the official website of Wuhan University School of Pharmacy

Operation method

Determination of Infrared Transmission Spectra of Different Physical Samples Experiment

Principle

Infrared spectroscopy is the study of molecular spectroscopy of molecular vibration and rotation information. When a compound is irradiated by infrared light, and the vibration or rotation frequency of a chemical bond in the compound is comparable to the frequency of infrared light, it will absorb the light energy and cause changes in the permanent dipole moment of the molecule, resulting in the molecular vibration and rotation energy level from the ground state to the excited state of the jump, so that the intensity of the transmitted light of the corresponding frequency is weakened. Different chemical bonds in the molecule vibration frequency is different, will absorb different frequencies of infrared light, detect and record the intensity of transmitted light and wave number (1/cm) or wavelength of the relationship between the curve, you can get the infrared spectrum. Infrared spectra reflect the characteristic absorption frequencies of molecular bonds and can be used for structural analysis and quantitative determination of compounds. According to different experimental techniques and applications, we divide the infrared light into three regions: the near-infrared region (0.75~2.5 μm;: 13158~4000), the mid-infrared region (2.5~25 μm;: 4000~400) and the far-infrared region (25~1000 μm;: 400~10). Most of the molecular vibrations accompanied by rotation are in the mid-infrared region, and general infrared spectroscopy is detected in this wave number interval. Infrared light source Fourier transform infrared spectrometer is mainly composed of infrared light source, Michaelson interferometer, detector, computer and recording system of five parts. Infrared light through the Michaelson interferometer irradiation samples, and then by the detector will be detected in the form of interferogram signal sent to the computer, the mathematical processing of Fourier transform, and finally get the infrared spectrum.

Move

1. Sample preparation

1.1 Preparation of solid samples. (L-Tyrosine pressed tablets)

1.1.1 L-Tyrosine pressed tablets.

Take about 1 mg of solid sample in a clean agate mortar and grind it to a fine powder under infrared light, then add about 200 mg of dry potassium bromide powder and grind it together until the two are completely mixed and homogeneous (particles are about 2 μm or less). Take out the press mold, put one press sheath glossy side up into the mold core, put on the collar, use the sample spoon to add the sample into the mold carefully and stack it evenly, take another press tongue glossy side down into the mold core, put a little pressure to make the sample spread out, and put on the cover. Put the loaded mold on the hydraulic press, close the air pressure valve, manually pressurize until the pressure gauge indicates about 400kgf, stop pressurizing, keep 1~3min after deflating and relieving pressure. Remove the mold, demold the sample to get a transparent disc, use small tweezers to put it on the specimen holder, and insert it into the detection cell to determine the infrared spectrogram.

1.1.2 Liquid paraffin paste.

Take 2~3mg of solid sample in a clean agate mortar research fine, add 1~2 drops of liquid paraffin, fully ground and mixed to paste, dry under the infrared lamp, take out the sample holder and potassium bromide (or sodium chloride) salt tablets, will be ground with a stainless steel spoon scraped to the salt tablets, coated with another salt tablets pressed on the lower panel of the sample holder, the position of the appropriate adjustments, inserted into the upper panel, the diagonal of the sample holder with a After that, insert it into the upper panel and fix it by screwing the opposite corners of the sample holder, and then insert it into the detection cell to measure the infrared spectra.

1.1.3 Thin film method (mostly used for the determination of polymer compounds).

Usually the specimen is hot-pressed into a film, the film is sandwiched between two salt tablets, and then put into the sample holder for fixing, and then the infrared spectrum is measured (film samples can be measured directly by this method). The polymer can also be dissolved in a suitable solvent (concentration of 1% to 20%), and then drop the solution in the salt sheet spread evenly in the infrared lamp so that the solvent gradually evaporated into a film, covered with another salt sheet, loaded into the sample frame fixed, inserted into the detection cell to determine the infrared spectral pattern.

1.2 Preparation of liquid samples. (Methyl salicylate)

Liquid film method: for high-boiling point, low-viscosity samples, the sample can be dropped directly on the salt sheet, covered with another salt sheet; for larger viscosity samples, a stainless steel spoon will be a small amount of sample coated on the surface of the salt sheet, baked under the infrared lamp, scrape the sample, covered with another salt sheet, so that between the two salt sheets to form a certain thickness of the liquid film, loaded into the sample holder is fixed, and then inserted into the detection cell to determine the infrared spectral map. For low-boiling point volatile samples, closed liquid cell should be used for detection.

1.3 Preparation of gas samples.

Take out the gas feed tank, open the feed tank any one of the two pistons, connect it with the vacuum pump; open the vacuum pump, pump out the original air in the air tank, close the pumping piston and oil pump switch. The gas sample into the sample tank any one of the population, open the piston injection, the size of the absorption peak intensity of the gas sample is realized by adjusting the pressure of the sample in the gas tank, so in the injection of the sample, the other inlet of the gas tank can be connected to the manometer, so that the pressure of the gas tank to control the detection of the required range.

2.Sample Detection

Insert the pre-prepared sample into the sample holder and record the infrared spectrum.

Caveat

1. The concentration of the potassium bromide sample and the thickness of the tablets should be appropriate, and special attention should be paid to drying during sample grinding and placement.

2. Never touch the surface of sodium chloride and potassium bromide salt tablets with hands; clean the salt tablets with acetone, wipe them with lens paper or degreased cotton wool, and then put them into a desiccator for storage.

3. Liquid samples should be dried and de-watered before preparation, and CaF2 or BaF2 window tablets should be used for aqueous solutions; corrosive samples should not be prepared with conventional salt tablets.

Common Problems

1. Adopting the conventional spectrum processing function, carry out baseline correction and appropriate smoothing processing on the measured spectrum, label the wave values of the main absorption peaks, and store the data.

2. Discriminate the attribution of functional groups.

3. Summarize the frequency range of the same group in different compounds.

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