Acetonitrile (CH3CN) is a widely used organic chemical raw material. In addition to being used as an extractant for extracting butadiene and isoprene from olefins and paraffins in the petrochemical industry, it is also widely used. It is used as a synthetic raw material for organic synthesis, medicine, pesticides, surfactants, dyes and other fine chemicals, as well as a mobile phase solvent for thin layer chromatography, paper chromatography, spectroscopy, polarography and high performance liquid chromatography (HPLC). It has recently begun to be used. It is used as a DNA synthesis/purification solvent, a solvent for organic EL material synthesis, a cleaning solvent for electronic components, etc. These uses have high requirements for the purity of acetonitrile.
An analysis of the uses of acetonitrile in the laboratory
1. Acetonitrile is a commonly used polar aprotic solvent. In inorganic chemistry, acetonitrile is widely used as a ligand, and its abbreviation is MeCN. For example, the acetonitrile complex PdCl2(MeCN)2 can be prepared by heating a suspension of polymeric palladium chloride in acetonitrile. Acetonitrile is a popular solvent for cyclic voltammetry due to its high dielectric constant.
2. Acetonitrile is also a two-carbon raw material for organic synthesis. It reacts with cyanogen chloride to obtain malononitrile.
3. Acetonitrile is also used as a mobile phase to separate molecules and is often used in column chromatography and more modern high-performance liquid chromatography.
4. In the field of nuclear medicine, acetonitrile is used to synthesize positron-based radioactive drugs such as fluorodeoxyglucose. In the process of synthesizing FDG, the evaporation of acetonitrile can take away the water in the reaction system; the acetonitrile content in the reaction system has a decisive impact on the synthesis efficiency of FDG and the quality of the drug; at the same time, acetonitrile also serves as a solvent and reaction The matrix of the system.
5. In addition, in the routine quality inspection of FDG, acetonitrile: water mixture (for example, 85% v/v) is also used as the mobile phase for thin layer chromatography analysis.
What are the advantages of acetonitrile over methanol?
Reagent selection: acetonitrile and methanol competition
Sometimes using ACN or MEOH alone may not achieve the ideal separation effect. Using ACN and MEOH when the flow is the same can change the selectivity to obtain better separation effect.
In short, when changing the separation selectivity, finely adjust methanol and coarsely adjust acetonitrile. The main purpose is to adjust the viscosity and strength of the mobile phase to improve the separation effect and selectivity.
Based on the different selectivities of solvents, methanol is a protic solvent that can form hydrogen bonds, which improves the selectivity for separating acids, bases or highly electronegative compounds. Acetonitrile is a polar molecule, and its carbon-nitrogen triple bond contains unformed Bonding electrons can combine with compounds containing empty orbitals, and have different compound selectivity from methanol.
In reversed-phase chromatography, you can imagine the chromatographic process as the organic solvent in the mobile phase “extracting” the solute from the stationary phase layer. Then think of the “similar miscibility principle” and you will be able to understand The type of mobile phase in the separated compounds changes the separation results.
When acetonitrile and methanol are mixed with water in the same ratio, acetonitrile has a strong elution ability. Especially when the mixing ratio is low, in order to obtain the same retention time from the elution point of caffeine and phenol, the ratio of acetonitrile only needs to be less than half of the ratio of methanol.
First of all, the price of acetonitrile is high, especially the price of HPLC grade. But why is acetonitrile used more often according to the conditions in the literature?
1. Absorbance. Acetonitrile HPLC grade has low absorbance value. Among the absorption spectra of commercially available HPLC grades and premium grades of acetonitrile and methanol, acetonitrile HPLC has the smallest absorption (especially at short wavelengths). The so-called HPLC grade removes UV-absorbing impurities and limits the absorbance to within the specified value at a specified wavelength. During UV detection, the noise generated is small, so acetonitrile HPLC grade is most suitable for high-sensitivity analysis at short UV wavelengths. In addition, acetonitrile HPLC grade produces fewer ghost peaks on the gradient baseline in UV detection. Although there are various other organic solvents with high miscibility with water, it is difficult to find smaller absorption than acetonitrile HPLC grade. In addition, although the spectra obtained between the HPLC grade and superior grade of methanol are not much different, the superior grade cannot guarantee the absorbance and may cause deviation, and the price is not much different, so try to use HPLC grade.
2. Pressure. The relationship between the ratio of water/acetonitrile and water/methanol mixtures and the infusion pressure varies depending on the type or mixing ratio of the organic solvent. When methanol is mixed with water, the pressure increases, while acetonitrile also mixes with water and the pressure is low. Therefore, acetonitrile does not increase excess pressure in the column at the same flow rate.
3. When degassing the mobile phase, acetonitrile should pay attention to the preparation of mixed solvents. When doing it in the mobile phase bottle (equal concentration system), methanol will generate heat when mixed with water, and the excess dissolved air will easily turn into bubbles. (Degassing is easy); acetonitrile, due to endothermic cooling, slowly returns to room temperature and generates bubbles, so degassing (heating and stirring, membrane filtration, He degassing, etc.) should be considered.
4. Selectivity of separation (elution). Acetonitrile and methanol differ in the selectivity of their separations. This is due to the different chemical properties of organic solvent molecules (methanol and ethanol are protic, acetonitrile and tetrahydrofuran are aprotic). Therefore, if the selectivity of separation cannot be obtained with acetonitrile, try methanol.
5. Peak shape. For example, salicylic acid compounds (phenol compounds having a carboxyl group or a methoxy group in the ortho position) have severe tailing when using acetonitrile, but can be suppressed by using methanol. However, in general, polymersCompared with silica gel columns, � phase columns tend to have wider peak shapes, which is especially common when using polystyrene columns to analyze aromatic compounds. This is very obvious when using methanol as the mobile phase but not when using acetonitrile. For this reason, it is recommended to use the latter (acetonitrile) when using a polymer-based reversed-phase column, because acetonitrile makes the gel swell.
6. Elution ability. When acetonitrile and methanol are mixed with water at the same ratio, generally acetonitrile has a strong elution ability. Especially when the mixing ratio is low, in order to obtain the same retention time from the elution point of caffeine and phenol, the ratio of acetonitrile only needs to be less than half of the ratio of methanol. On the other hand, when the organic solvent is 100% or very close to this, methanol often has a strong elution ability from the perspective of carotene and cholesterol. When the mixing ratio is special, such as 50:1, the modulation error will be large, affecting the retention time, or the equilibrium time will be long. When acetonitrile encounters this situation, use a 10:1 mixture of methanol instead of acetonitrile, which is more convenient to operate. When the solvent is affected by temperature, container quantification is not used, but the weight method (considering specific gravity) can be used to reduce the error in the mixing ratio.
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