Supercritical Extraction Technology and Its Application in Agriculture
Supercritical extraction (SFE) is a new sample pretreatment technology developed rapidly and applied widely in recent years. It overcomes the disadvantages of traditional Soxhlet extraction (liquid solvent extraction), such as time-consuming, laborious, low recovery, poor reproducibility and serious pollution. It makes the sample extraction process more rapid and simple, especially eliminating the harm of organic solvents to human body and environment. In this paper, the modern technology, principle, performance of analytical supercritical extraction and its application in agriculture are described.
The extractant used in supercritical fluid extraction is supercritical fluid. Supercritical fluid is a non-gaseous and non-liquid state between gas and liquid, which can only exist when its temperature and pressure exceed the critical point. Supercritical fluids are dense, similar to liquids, and their viscosities are close to gases. Therefore, supercritical fluid is an ideal extractant.
The solvent strength of supercritical fluid depends on the temperature and pressure of extraction. Using this characteristic, the different components in the sample can be extracted according to the solubility of the fluid by changing the pressure and temperature of the extractant fluid. The weak polar substances can be extracted first at low pressure. With the increase of pressure, the substances with larger polarity and larger molecular weight and their basic properties can be extracted. Therefore, the different components can be extracted by supercritical extraction under programmed pressure. At the same time, it can also play the role of separation.
Temperature changes are reflected in the two factors affecting the density of extractant and the vapor pressure of solute. In the low temperature region (still above the critical temperature), the increase of temperature decreases the fluid density, while the increase of vapor pressure of solute is not much. Therefore, the increase of temperature in the solubility of extractant can make the solute precipitate from the fluid extractant, and when the temperature is further increased to the high temperature region, although the density of extractant enters into the high temperature region. However, with the increase of vapor pressure and volatility, the extraction rate will not decrease but increase.
In addition to pressure and temperature, adding a small amount of other solvents in supercritical fluid can also change its solubility to solutes. The mechanism of its action has not been completely clear up to now. Usually the dosage is not more than 10%, and most of them are polar solvents such as methanol and isopropanol. Adding a small amount of polar solvents can further expand the application scope of supercritical extraction technology to compounds with larger polarity.
II. EXPERIMENTAL EQUIPMENT AND EXTRACTION MODE OF SUPERCRITICAL EXTRACTION
2.1 EXPERIMENTAL DEVICE FOR SUPERCRITICAL EXTRACTION
The supercritical fluid extraction process, as shown in the drawings, includes:
(1) The source of supercritical fluid is composed of extractant storage bottle, high-pressure pump and other accessories. Its function is to convert extractant from normal temperature and pressure to supercritical fluid.
(2) Supercritical fluid extraction (SFE) consists of a sample extraction tube and an accessory device where the extractant in the supercritical state dissolves the extracted solute from the sample matrix. With the flow of the fluid, the fluid containing the extracted solute is separated from the sample matrix.
(3) The part of solute vacuum adsorption separation consists of nozzle and absorption tube. The extracted solute and fluid must be transformed from supercritical state to normal temperature and pressure state through nozzle decompression and cooling. At this time, the fluid evaporates and escapes, while the solute can be eluted and collected on the surface of porous packing in the absorption tube by washing the absorption tube with appropriate solvent.
High Pressure Pump-Extraction Pipe-Absorption Pipe-Collector-Supercritical Fluid Cylinder-Solvent Elution Pump
2.2 Supercritical Extraction
Supercritical fluid extraction (SFE) can be divided into dynamic and static methods. The dynamic method is simple, convenient and fast. It is especially suitable for extracting substances with high solubility in SFE, and the sample matrix is easily permeated by SFE. Static method is suitable for extracting substances which are difficult to separate from the sample matrix or have high solubility in the extractant fluid. It is also suitable for situations where the sample matrix is denser and the supercritical fluid is not easy to penetrate, but the extraction speed is slower.
Ⅲ Selection of Supercritical Fluid and Extraction Conditions
3.1 Selection of Supercritical Fluid
CO2 is the most widely used supercritical fluid to extract low polar and non-polar compounds. Considering the solvent strength, supercritical ammonia is the best choice, but ammonia is easy to react with other substances, which causes serious corrosion to equipment and is too dangerous for daily use. Supercritical methanol is also a good solvent, but because of its high critical temperature, it is a liquid at room temperature. After extraction, it still needs complex concentrating steps and can not be used. Low hydrocarbon substances are not as widely used as CO2 because they are flammable and explosive.
3.2 Selection of Extraction Conditions
There are several conditions for the selection of extraction conditions: firstly, the same fluid is used to select different pressure to change the extraction conditions, so as to extract different types of compounds; secondly, according to the solubility of extracts in supercritical fluid under different conditions, the appropriate extraction conditions are selected; thirdly, the analyte is deposited on the adsorbent and eluted by Supercritical fluid to achieve the classification and selection of extraction. Fourthly, for the components with larger polarity, methanol can be directly added to the sample, extracted by supercritical CO2, or pumped into methanol and supercritical CO2 by another pump according to a certain proportion to achieve the purpose of increasing the strength of extractant.
In addition to the pressure, composition and extraction temperature of the extractant fluid, the extraction time and the temperature of the absorption tube will affect the extraction and collection efficiency. The extraction time depends on two factors: one is the solubility of the extracted substance in the fluid, the higher the solubility, the higher the extraction efficiency and the faster the extraction speed; the other is the mass transfer of the extracted substance in the matrix. The higher the extraction rate, the more complete the extraction and the higher the efficiency. The temperature of the collector or or absorption tube also affects the recovery rate, and lowering the temperature is conducive to improving the recovery rate.
After supercritical fluid decompression, there are two main methods to collect extracts: off-line SFE and on-line SFE or on-line SFE. Off-line SFE itself is simple to operate and only needs to know the extraction steps. Sample extracts can be analyzed by other suitable methods. Online SFE not only needs to understand SFE, but also the chromatographic conditions. Sample extracts are not suitable for other analytical methods. The main advantage of online SFE is that it eliminates the sample processing process between extraction and chromatographic analysis, and it is possible to achieve maximum sensitivity by directly transferring extracts to chromatographic columns.
Supercritical extraction (SCE) has been widely used in recent years because of its high efficiency, rapidity and simple post-processing. It not only extracts and purifies a small amount of active ingredients from raw materials, but also removes a small amount of impurities from crude products to achieve the effect of deep purification. Due to the wide range, complex composition and low content of some components in agricultural samples, from 10-4 to 10-9, even 10-12, especially the extraction of Trace Pesticide Residues in agricultural products and harmful chemical pollutants in soil, it is often necessary to use a variety of organic solvents and extraction steps in order to obtain a large volume of dilute liquid containing target analytes, which can be concentrated by evaporation. Qualitative and quantitative analysis, this process is not only time-consuming, but also consumes a large number of organic solvents. Sometimes halogen-containing organic solvents are often used, that is, they have certain effects on human health, but also cause environmental pollution. More importantly, because of the various extraction steps, the recovery of samples is reduced and the reproducibility is poor, which affects the accuracy of the determination results. Since the commercialization of supercritical extraction instrument, supercritical extraction technology has been rapidly promoted in the United States and some other Western countries. More and more mature sample pretreatment methods of supercritical extraction have been adopted as standard methods by the National Bureau of Standards. Schedule lists representative examples of supercritical fluid applications in agriculture. Most of these experiments were completed in one hour with only a few milliliters of solvent. To achieve the same extraction effect, solvent extraction takes at least 8 hours to several days, and the amount of solvent reaches several hundred milliliters.
Supercritical extraction technology has just entered the practical stage in our country. Commodity inspection departments and public security departments have successfully applied supercritical extraction technology to the extraction of pesticide residues in grain and effective components in drugs. We have also tried to extract pepper powder and Ginkgo leaves, and achieved good results. Although the price of supercritical fluid extractor is expensive at present, it is worth considering in the long run.
V. Concluding remarks
Supercritical fluid extraction as a sample preparation and pretreatment technology has the advantages that other classical methods can not match because of its inherent characteristics of high density, low viscosity and high diffusivity. It can shorten the processing time by 1-2 orders of magnitude, avoid the use of a large number of toxic solvents, and reduce the possibility of pollution. Supercritical extraction combined with other methods avoids the loss of sample transfer, which is of great significance to reduce human error and improve the sensitivity and accuracy of the method.
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