Common problems and solutions in infrared spectrum sample preparation
Infrared spectroscopy is an important analytical tool in scientific research and production. Sample preparation is an important part of infrared spectroscopy. In order to obtain a high-quality infrared spectrum, in addition to the performance of the instrument, it depends largely on the selection of a suitable sample preparation method and skilled operation techniques. This paper analyzes the problems that often occur in the preparation of infrared spectroscopy samples for reference by the majority of infrared spectroscopy workers.
In general, sample preparation should pay attention to the following points:
(1) The sample concentration and test thickness should be selected appropriately. Too low concentrations and too thin samples will cause some peaks to disappear, and a complete spectrum will not be obtained. On the contrary, some strong absorption peaks will exceed the scale scale, and the peak will appear, and its true peak position cannot be determined. A good infrared spectrum should have a transmittance of the absorption peaks in the range of 20% to 60%.
(2) The sample should not contain free water. The presence of water not only interferes with the absorption of the sample, but also corrodes the salt window.
(3) Multi-component samples should be separated as much as possible in advance. Otherwise, the spectra of the components overlap each other, rendering the spectrum unresolvable.
Gas sample
Gas samples are typically tested using a gas absorption cell. The gas pool is first evacuated, and the gas sample is sucked into the pool by a negative pressure. The intensity of the absorption peak can be varied by adjusting the sample pressure in the gas cell. The density of gas molecules is much smaller than liquids and solids, so gas samples require a larger sample path length.
The conventional gas absorption cell has a thickness of 10 cm. If the concentration of the gas component being analyzed is small, multiple reflection gas pools can be utilized.
The infrared light is reflected in the gas pool multiple times by the mirror inside the gas pool, and the optical path length can be increased to 10m, 20m or 50m.
Pay attention to the following points when performing gas measurement:
(1) The gas sample should be dried because the water vapor has a large absorption peak in the mid-infrared region. Drying must be done before injection.
(2) The gas pool is cleaned after the measurement, that is, the gas pool and the population pipeline are washed with a dry air stream.
(3) If a multi-reflection gas cell is used, it is preferable to purify the sample. The background absorption is very obvious after multiple reflections, and the interference of the impurity gas on the spectrum is also increased.
(4) Quantitative analysis should make the total pressure in the gas pool equal, because the peak intensity is not only related to the partial pressure, but also related to the total pressure. Therefore, if necessary, the infusion of inactive non-absorbent gas, such as nitrogen or hydrogen, may be supplemented to make the total pressure equal.
Solid sample
Different solid samples have different preparation methods.
Tableting
The tableting method is the most commonly used sample preparation method for infrared spectrum analysis of solid samples. This method can be used for solid samples that are easy to be pulverized.
The amount of sample used varies with the size of the mold. The mixing ratio of the sample to KBr is generally 0.5-2:100. When compressing, first place the solid sample in an agate mortar and grind it, then add KBr powder, grind and mix evenly, then transfer the tablet to the mold, vacuum and press for a few minutes. The mixture is formed under pressure to form a transparent wafer which can be tested.
There are often two abnormal phenomena in the process of tableting. The reasons and solutions are as follows:
(1) The entire film is opaque. Insufficient pressure and poor dispersion. It can be reground or pressed to make it evenly dispersed and increase the pressure, but not overloaded.
(2) When the film is pressed, the film is very transparent, and irregular cloudiness and turbidity appear after one minute or more. Not enough vacuum. Checking the vacuum and extending the vacuum time eliminate this phenomenon.
(3) Clouds appear in the center of the film. The anvil or the tongue is not flat. Should be exchanged or re-polished.
(4) Many white spots appear on the film, and the rest are clear and transparent. Uneven grinding, caused by a small amount of coarse particles. It should be reground.
(5) There are irregular blocks in the film or all of them are cloud-like turbidity. The sample or KBr is caused by moisture. Dry or extend the vacuum time.
(6) Looking at distant objects through the film, the light transmittance is poor and there is light scattering. The KBr is not pure, and at least 5% of the alkali metal halide is mixed in the KBr used. Pure KBr should be used.
In addition, when using the tableting method, you should also pay attention to the following points:
(1) KBr is more hygroscopic. Even if the operating conditions are very strict, such as grinding the sample in an infrared dryer, infrared absorption peaks of free water are inevitably present in the infrared spectrum. To eliminate the interference of free water, a KBr blank (without sample) can be prepared under the same conditions as a compensation sheet.
(2) The alkali metal halide will ion exchange with the sample to produce a responsive impurity absorption peak.
(3) Due to the hygroscopic interference of the alkali metal halide, care must be taken when analyzing the stretching vibration absorption peaks of the ON and NH bonds, and the C = C and C = N stretching vibration absorption peaks. In order to avoid such interference, it is sometimes possible to measure the sample by mixing it with polyethylene powder or paraffin and pressing it into a sheet.
(4) The sample may undergo physical changes (such as polycrystalline conversion phenomenon) or chemical changes (partial decomposition) during the tableting process, which may cause differences in the appearance of the spectrum. Therefore, for certain inorganic compounds, sugars, solid organic acids, solid phenols, amines, imines, amine salts, amides and the like, it is not necessarily suitable to prepare a sample by KBr tableting.
Powder method
The powder method usually involves depositing a solid sample in an agate mortar to a thickness of about 2 microns. The powder is then suspended in a volatile liquid. The suspension is moved to the salt window and the solvent is removed to form a uniform thin layer which is then scanned.
A common problem with powder methods is particle scattering, that is, infrared light is irradiated onto the sample particles, and the incident light is scattered. This cluttered scattering reduces the energy of the sample beam reaching the detector and increases the baseline of the spectrum. The scattering phenomenon is particularly severe in the short-wave region, and even no absorption peak appears. In order to reduce the scattering phenomenon, the sample particle diameter should generally be made smaller than the wavelength of the incident light. Since the mid-infrared region starts from 2 microns, it is necessary to grind the sample to a size of 2 microns.
Thin film method
The sample is dissolved in a suitable solvent, and the sample solution is poured on a glass plate or a KBr window. After the solvent is evaporated, a uniform film is formed to be tested. The film thickness is generally controlled to be 0.001-0.01 mm. The thin film method requires that the solvent has good solubility to the sample and the volatility is appropriate. If the solvent is difficult to volatilize, it is not easy to remove from the sample film. If the volatility is too large, the sample will become opaque during the film formation process.
Paste method
This method can be used for solid samples that do not form a film without a suitable solvent. 2-5 mg of the sample was ground into a powder (particles < 20 μm), a drop of liquid dispersant was added, and a paste was formed, similar to a toothpaste, and then uniformly applied to a KBr salt sheet. Common liquid dispersion media are liquid paraffin, fluoro oil and hexachlorobutadiene. Since the liquid dispersion medium absorbs in the spectral range of 4000-400 cm-1, the interference of the dispersion medium should be noted by this method. Secondly, although this method is simple and rapid, it can be applied to most solid samples. However, due to the interference of the dispersion medium, especially when the refractive index of the sample and the dispersion medium differ greatly or the sample particles are not fine enough, the spectral quality will be seriously affected. This is not suitable for quantitative analysis.
Liquid sample
Liquid samples are divided into pure liquid and solution. Generally, the solution should not be used as much as possible to avoid interference with the absorption of human solvent. Only the absorption of the sample is strong, the liquid film method cannot produce a very thin absorption layer, or the solution method is used in order to avoid the influence of the association between the molecules of the sample. When the solution is selected for testing, the commonly used solvents are carbon tetrachloride, carbon disulfide, dichloromethane, acetone, and the like. The choice of solvent must pay attention to the following two points:
(1) There is sufficient solubility for the sample at normal temperature, and the sample should be chemically inert. Otherwise, the position and strength of the absorption band of the sample will be affected.
(2) The solvent has no absorption, or only weak absorption, in the region of the main absorption zone of the sample, or the absorption energy is compensated.
The various solvents themselves absorb more or less in the infrared region, so to obtain a spectrum of the sample solution with a wide spectrum, two or more solvent segments must be used in combination.
The concentration of the formulation solution is generally between 3% and 5%. Depending on the application and the amount of sample, different types of liquid sample cells are used. The thickness of the liquid sample cell must be corrected during quantitative analysis. There are two commonly used correction methods: interference fringe method and optical density comparison method. Pay attention to the following points during the operation of the solid pool:
(1) Prevent bubbles during filling.
(2) The sample should be fully dissolved, and no insoluble matter should enter the pool.
(3) During the cleaning process of the pool or when the cleaning is completed, do not allow the window to get wet due to solvent evaporation.
(4) Do not spill the sample solution onto the window when loading the tank. For pure liquid samples, it is usually made into an extremely thin film of about 0.001-0.05 mm. Only such a small path length can obtain a satisfactory spectrum. A drop of pure liquid is typically pressed between two pieces of salt window and placed in the light path for testing. This method is simple, fast and solvent-free, but not suitable for volatile liquid samples, and this method does not provide very reproducible spectral data, so it is not suitable for quantitative analysis.
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