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Supercritical fluid extraction and its application in agriculture

      Supercritical fluid extraction (SFE) is a new sample preparation technique developed rapidly and widely in recent years. It overcomes the problems of traditional Soxhlet extraction (liquid solvent extraction), time-consuming and labor-intensive, low recovery, poor reproducibility, Serious pollution and other shortcomings, the sample extraction process more quickly and easily, especially the basic elimination of organic solvents on the human body and the environment. This article describes the modern techniques, principles, performance and analytical applications of analytical supercritical fluid extraction in agriculture.

      First, the basic principle of supercritical fluid extraction

      Extractant used for supercritical fluid extraction is a supercritical fluid. A supercritical fluid is a non-gaseous or non-liquid state between gas and liquid that can only react when its temperature and pressure exceed the critical point exist. Supercritical fluid density, similar to the liquid, and its viscosity is closer to the gas. Therefore, supercritical fluid is a very ideal extractant.

      The solvent strength of the supercritical fluid depends on the temperature and pressure of the extraction. By using this property, different components of the sample can be extracted according to the solubility in the fluid just by changing the pressure and temperature of the extractant fluid. The weakly polar substances are extracted first at low pressure, Pressure increase, polarity and large molecular weight of the material and the basic nature of the program under the pressure of the supercritical extraction of different extraction components, but also can play a role in separation.

      Changes in temperature are reflected in the extraction solvent density and solute vapor pressure of two factors, in the low temperature zone (still above the critical temperature), the temperature decreases the density of the fluid, while the solute vapor pressure increases little, so extractant Solubility may be precipitated from the fluid extractant when the solvency is increased, and when the temperature is further increased to a higher temperature, the solute vapor pressure is increased and the volatilization degree is increased although the extractant density is further reduced. However, the extraction rate is not reduced but rather Increasing trend.

      In addition to pressure and temperature, the addition of small amounts of other solvents to the supercritical fluid can also alter its ability to dissolve solutes. Its mechanism of action has not yet been fully understood. Usually added in an amount of not more than 10%, and polar solvents methanol, isopropanol and other mostly. Adding a small amount of polar solvent, can make the scope of application of supercritical extraction further expanded to more polar compounds.

      Second, the experimental equipment and extraction methods of supercritical extraction

      2.1 Experimental equipment for supercritical extraction

      The flow of supercritical fluid extraction is shown in the accompanying drawings, which includes:

      (1) The source of supercritical fluid is composed of extractant storage bottle, high-pressure pump and other ancillary devices, whose function is to convert extractant from normal pressure to supercritical fluid;

      (2) supercritical fluid extraction part, the sample extraction tube and ancillary components, in the supercritical state of the extractant here will be extracted solute dissolved from the sample matrix, with the fluid flow, so that the extracted solute Of fluid separate from the sample matrix.

      (3) solute decompression adsorption separation part, composed of the spout and absorption tube, the extracted solute and fluid, must be from the supercritical state through the nozzle decompression temperature and pressure conversion normal temperature and pressure, the fluid volatile escape, and solute Absorption tube in the porous filler surface, with a suitable solvent to wash the absorption tube, the solute can be collected for elution.

      High Pressure Pump - Extraction Tube - Absorption Tube - Collector - Supercritical Fluid Cylinder - Solvent Elution Pump

      2.2 supercritical extraction method

      Supercritical fluid extraction methods can be divided into two kinds of dynamic and static, dynamic method is simple, convenient and fast, especially suitable for the extraction of substances in the supercritical fluid extractant solubility, and the sample matrix can easily be supercritical fluid Penetration of the occasion. The static method is suitable for the extraction of substances that are difficult to separate from the matrix of the sample or have high solubility in the extractant fluid. It is also suitable for the occasions where the sample matrix is ​​more dense and the supercritical fluid is not permeable, but the extraction speed is slower.

      Third, the choice of supercritical fluid and extraction conditions

      3.1 The choice of supercritical fluid

      CO2 is currently the most used supercritical fluid for the extraction of compounds of low polarity and nonpolarity. From the viewpoint of solvent strength, supercritical ammonia is the best choice, but ammonia is very easy to react with other substances and corrode the equipment Serious, and daily use is too dangerous. Supercritical methanol is also a good solvent, but due to its high critical temperature, it is a liquid at room temperature and requires complex concentration steps after extraction, which can not be used. Low hydrocarbons are not as flammable and explosive as CO2 Wide range of uses.

      3.2 extraction conditions of choice

      There are several conditions for the selection of extraction conditions: one is to select different pressure with the same fluid to change the extraction conditions to extract different types of compounds; the second is based on the extract in different conditions, the solubility in the supercritical fluid To select the appropriate extraction conditions; third is deposited on the adsorbent analyte, with supercritical fluid elution, in order to achieve the purpose of classification and selection of extraction; Fourth, the more polar components can be directly added to the sample of methanol , With supercritical CO2 extraction, or by another pump by a certain percentage of pumped methanol and supercritical CO2, to achieve the purpose of increasing extractant strength.

      Factors that affect the extraction efficiency In addition to the pressure of the extractant fluid, composition, extraction temperature, the extraction process time and the absorption tube temperature out will affect the efficiency of extraction and collection, extraction time depends on two factors: First, the extract In the fluid solubility, the greater the solubility, the higher the extraction efficiency, the faster the speed; Second, the mass transfer rate of the extracted material in the matrix, the more complete extraction, the efficiency is also higher. Collector or absorber tube temperature will also affect the recovery rate, reduce the temperature is conducive to improving the recovery rate.

      After the supercritical fluid is depressurized, there are mainly two methods for collecting the extract - offline SFE and online SFE or online SFE. The off-line SFE itself is simple to operate, only need to know the extraction step and the sample extract can be analyzed by other suitable methods . The online SFE not only needs to know SFE, but also the chromatographic conditions, and the sample extract is not suitable for other method analyzes. The advantage of this method is that it eliminates the sample processing between extraction and chromatographic analysis and because the extract is transferred directly to It is possible to achieve the maximum sensitivity in the column.

      Fourth, the application of supercritical extraction technology in agriculture

      Supercritical fluid extraction has been widely used in recent years because of its high efficiency, rapidity and simple aftertreatment. It not only has the function of extracting and purifying a small amount of active ingredients from raw materials, but also can remove a small amount of impurities from the crude product to Deep purification effect. Due to the wide range of agricultural samples, complex composition, and the low content of some components, from 10-4 to 10-9 or even 10-12, especially in agricultural products trace pesticide residues and soil chemical pollutants Extraction, often requires the use of a variety of organic solvents and multiple extraction steps, in order to get a large volume containing the target analyte dilute solution, evaporated and concentrated before qualitative and quantitative analysis, this process is not only time-consuming, but also consume a large amount of organic Solvents, and sometimes also often use halogen-containing organic solvents, that have a certain impact on human health, but also cause environmental pollution. More importantly, due to the many extraction steps, the sample recovery rate is reduced, and the reproducibility is poor, which affects the accuracy of the determination result. Since the commercialization of the supercritical extraction apparatus, supercritical extraction has been rapidly promoted in the United States and other western countries. More and more mature supercritical extraction sample preparation methods have been adopted by the National Bureau of Standards as a standard method. The schedule lists examples of agriculturally representative applications of supercritical fluids. Most of these experiments were completed in less than 1 hour, with only a few milliliters of solvent used. To achieve the same extraction effect, solvent extraction took at least 8 to several days with a few hundred milliliters of solvent.

      In China, supercritical fluid extraction technology has just entered a practical stage. The commercial inspection departments and the public security departments have successfully applied the supercritical fluid extraction technology to the extraction of pesticide residues in food and the extraction of active ingredients in the drug. We also extracted the extract of paprika and ginkgo biloba Have tried, achieved better result. Although the current supercritical extraction instrument is more expensive, but in the long-term consideration is worth it