An infrared spectrometer is an instrument that analyzes the molecular structure and chemical composition of substances by utilizing their absorption characteristics of infrared radiation at different wavelengths. An infrared spectrometer typically consists of a light source, a monochromator, a detector, and a computer processing system. Based on the different spectroscopic devices, it is divided into dispersive and interferometric types. For a dispersive dual-path optical null-balanced infrared spectrophotometer, when the sample absorbs infrared radiation of a certain frequency, the vibrational energy levels of the molecules undergo transitions. The light of the corresponding frequency in the transmitted beam is weakened, creating a 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. Below are some brief precautions for the maintenance of an infrared spectrometer.
1. During the measurement, the laboratory temperature should be between 15 and 30°C, and the relative humidity should be below 65%. The power supply should be equipped with a voltage regulator and a grounding wire. Because the relative humidity in the room must be strictly controlled, the area of the infrared laboratory should not be too large, just enough to accommodate the necessary instruments and equipment, but a dehumidification device must be provided.
2. To prevent the instruments from getting damp and affecting their lifespan, the infrared laboratory should be kept dry at all times. Even if the instruments are not in use, they should be turned on at least twice a week for half a day each time, while a dehumidifier is running. Especially during the rainy season, it is best to run the dehumidifier every day.
3. If a single-lens Fourier transform infrared spectrophotometer is used (currently the most widely used), the CO2 content in the laboratory should not be too high. Therefore, the number of people in the laboratory should be minimized, and irrelevant personnel should not enter. Attention should also be paid to proper ventilation.
4. The most common sample preparation method for infrared spectroscopy determination is the potassium bromide (KBr) pellet method (used for over 90% of the varieties listed in the pharmacopoeia). Therefore, to minimize the impact on the determination, the KBr used should ideally be optical reagent grade, or at least analytical grade. Before use, it should be appropriately ground (below 200 mesh) and dried at above 120℃ for at least 4 hours before being placed in a desiccator. If clumping is found, it should be dried again. The prepared empty KBr pellet should be transparent, with a transmittance of over 75% compared to air.
5. If the test sample is a hydrochloride salt, considering the possible ion exchange phenomenon during the tableting process, the standard stipulates that potassium chloride (which is also pretreated like potassium bromide) should be used instead of potassium bromide for tableting. However, the spectra measured after potassium chloride tableting and potassium bromide tableting can also be compared. If there is no difference between the two, potassium bromide can be used for tableting.
6. The sample amount used in the tablet compression method is generally 1-2 mg. Since it's impossible to weigh it using a balance before adding it, and the absorption of infrared light varies for each sample, it's often determined based on experience. Generally, it's required that the vast majority of absorption peaks in the obtained spectrum fall within the 10%-80% transmittance range. If the transmittance of the strongest absorption peak is too high (e.g., greater than 30%), it indicates that the sample amount is too small; conversely, if the strongest absorption peak is close to 0% transmittance and is a flat-topped peak, it indicates that the sample amount is too large. In both cases, the sample amount should be adjusted and the measurement repeated.
7. The amount of KBr used during tableting is generally around 200mg (based on experience). The amount of KBr should be controlled according to the thickness of the tablet after preparation. Generally, the tablet thickness should be less than 0.5mm. When the thickness is greater than 0.5mm, interference fringes can often be observed in the spectrum, which will interfere with the spectrum of the test sample.
8. When compressing tablets, the sample should be finely ground first, then KBr should be added and ground again until homogeneous. This makes mixing easier. An agate mortar should be used for grinding, as the inner surface of a glass mortar is rough and easily attracts sample. Grinding should be done in the same direction (clockwise or counterclockwise) with uniform force. Grinding in different directions may cause crystallization of the sample during grinding, affecting the measurement results. The grinding force should not be too strong; grind until no visible small particles remain in the sample. After grinding, the sample should be poured into the tableting mold through a small funnel (direct pouring is difficult due to the small mold opening), and the sample should be spread as evenly as possible. Otherwise, the transparency of areas with less sample after compression will be lower than that of areas with more sample, thus affecting the measurement. Additionally, if opaque white spots appear on the compressed tablet, it indicates the presence of unground small particles in the sample, and the tablet should be re-compressed.
9. The sample used for testing should be dry; otherwise, it should be dried under an infrared lamp for a few minutes after grinding. After the sample is ground and placed in the mold, it should be connected to a vacuum pump and evacuated for at least 2 minutes to further remove moisture from the sample. Then, the pressure should be increased to 0.8–1 GPa (8–10 T/cm²) and maintained for 2–5 minutes. Failure to evacuate will affect the transparency of the film.
10. After use, tablet compression molds should be wiped clean immediately. If necessary, they should be washed with water and dried, and stored in a desiccator to prevent rust.
