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Supercritical fluid extraction (SFE) is developing rapidly in recent years, the application is very wide of a new sample pretreatment technique, it overcomes the traditional soxhlet extraction solvent extraction (liquid) time-consuming, low recovery rate, poor reproducibility, pollution is serious, such as the extraction process of sample is more quick and easy, especially basic elimination of the organic solvent to the harm of human body and environment. This paper describes the modern technology, principle, performance and application of analytical supercritical extraction in agriculture.
The basic principle of supercritical extraction
Supercritical fluid is a non-gaseous and non-liquid state between gaseous and liquid. This substance can only exist when the temperature and pressure exceed the critical point. Supercritical fluid has a higher density, similar to the liquid, and its viscosity is closer to the gas. Therefore, supercritical fluid is an ideal extractant.
The solvent strength of supercritical fluid depends on the temperature and pressure of the extraction. Using this feature, simply change the pressure and temperature of fluid in the extraction, can the different components of the samples according to the size of the solubility in the fluid, successively extracted, weak polar material extraction, first under low pressure with the increase of pressure, larger and larger molecular weight of the material and the basic properties, so the program booster supercritical extraction under different extraction components, can also have the effect of separation. Temperature changes in the density of extraction solvent and solute the vapor pressure of the two factors, the low temperature zone (still above the critical temperature), temperature to reduce the fluid density, and solute the vapor pressure increases, therefore, the solution ability of extracting agent temperature can make the solute precipitation from fluid extraction agent, and further raise the temperature to high temperature, although further reduce the density of the extractant, but increased solute steam pressure, increased volatility, instead, not only will not reduce the extraction rate has a tendency to increase.
In addition to pressure and temperature, the solute solubility of supercritical fluid can be changed by adding a small amount of other solvents. The mechanism of action is not yet fully understood. Generally, the amount of addition is no more than 10%, and the polar solvent methanol, isopropanol, etc. are the majority. The application of supercritical extraction technology can be extended to more polar compounds by adding a small amount of polar solvent.
Experimental device and extraction method of supercritical extraction
2. Experimental facility for supercritical extraction
The process of supercritical fluid extraction is shown in the attached figure, 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 transform the extractant from normal temperature pressure state to supercritical fluid;
(2) the supercritical fluid extraction part is composed of sample extraction tube and accessory device, where the extractant in the supercritical state dissolves the extracted solute from the sample matrix. As the fluid flows, the fluid containing the extracted solute is separated from the sample matrix.
(3) the solute vacuum adsorption separation part, absorbed by the nozzle and tubes, extracted solutes and fluid, supercritical state by the nozzle has to be reduced pressure cooling transformation under normal condition, the fluid volatile escape, and solute transport in porous filler surface absorption tube, wash with a suitable solvent absorption tube, can put the solute elution collect spare. High pressure pump -- extraction tube -- absorption tube -- collector -- supercritical fluid cylinder -- solvent eluting pump
2. 2. Supercritical extraction method
Supercritical fluid extraction can be divided into dynamic and static methods, the dynamic method is simple, convenient and fast, especially suitable for extraction of substances with high solubility in supercritical fluid extraction agent, and the sample matrix is easy to be infiltrated by supercritical fluid. The static method is suitable for those substances which are difficult to be separated from the sample matrix or are highly soluble in the fluid of the extractant. It is also suitable for the situation where the sample matrix is relatively dense and the supercritical fluid is difficult to permeate, but the extraction speed is slow.
Selection of supercritical fluid and extraction conditions
3. Supercritical fluid selection
CO2 is a much used supercritical fluid, which is used to extract low polar and non-polar compounds. Considering the solvent strength, supercritical ammonia is a suitable choice, but ammonia is easy to react with other substances, severely corrodes the equipment and is too dangerous for daily use. Supercritical methanol is also a good solvent, but because it has a high critical temperature and is a liquid at room temperature, complex enrichment steps are required after extraction, and low hydrocarbons are less widely used than CO2 because they are flammable and explosive.
3. 2. Selection of extraction conditions
The extraction conditions can be selected in several cases. First, the extraction conditions can be changed by selecting different pressures for the same fluid to extract different types of compounds. The other is to select suitable extraction conditions according to the solubility of the extract in supercritical fluid under different conditions. Thirdly, the analyte was deposited on the adsorbent and eluted with supercritical fluid to achieve the purpose of classification and extraction. Fourthly, for the components with greater polarity, methanol can be directly added to the sample and extracted with supercritical CO2, or another pump can be used to pump methanol and supercritical CO2 in a certain proportion to achieve the purpose of increasing the strength of the extractant.
Factors affecting the extraction efficiency as well as the extraction agent fluid pressure, composition, extraction temperature, extraction time and the temperature of the absorption tube out of the process of affecting extraction and collecting efficiency, extraction time depends on two factors: one is by extraction solubility in the fluid, the greater the solubility, the higher extraction efficiency, the faster the speed; Second, the higher the mass transfer rate in the matrix, the more complete the extraction and the higher the efficiency. The temperature of the collector or absorber will also affect the recovery rate, and lowering the temperature will improve the recovery rate.
After supercritical fluid decompression, there are mainly two kinds of methods to collect extracts -- offline SFE and online SFE or online SFE. Offline SFE itself is easy to operate, only the extraction steps need to be understood, and the sample extract can be analyzed by other appropriate methods. Online SFE requires not only the understanding of SFE but also the chromatographic conditions, and the sample extract does not apply to other methods of analysis. Its advantages are mainly that it eliminates the sample processing between extraction and chromatographic analysis, and it is possible to achieve greater sensitivity due to the direct transfer of the extract to the chromatographic column.
Application of supercritical extraction in agriculture
With its high efficiency, fast speed and simple post-treatment, supercritical extraction device has been widely used in recent years. It not only extracts and purifies a few effective components from raw materials, but also removes a few impurities from rough products to achieve the effect of deep purification. Due to agricultural sample involves a wide range of complex, content is very low, and some components from 10-4 to 10 to 9, and 10-12 level, especially the trace pesticide residues in agricultural products and the extraction of harmful chemical pollutants in soil, often need to use a variety of organic solvents and multiple extraction steps to get bulk dilute jersey solution containing the target analyte, behind the evaporation concentration and quantitative analysis can be carried out, the process is not only time-consuming, also consumes large amounts of organic solvent, sometimes often need to use organic solvent containing halogen, namely to produce certain effect to human body health, still can cause environmental pollution. More importantly, because of the multiple extraction steps, the recovery rate of the sample is reduced and the reproducibility is poor, which affects the accuracy of the determination results. Since the commercialization of supercritical extraction, supercritical extraction has been rapidly popularized in the United States and some other western countries. More and more mature pretreatment methods of supercritical extraction samples have been adopted as standard methods by the national bureau of standards. A representative application of supercritical fluid in agriculture is listed in the attached table. Most of these experiments were done within an hour, with only a few milliliters of solvent. To achieve the same extraction effect, solvent extraction will take at least 8 hours to several days, with the amount of solvent up to several hundred milliliters.
In China, the supercritical extraction technology has just entered the practical stage. The commercial inspection department and the public security department have successfully applied the supercritical extraction technology to the extraction of pesticide residues in grain and the extraction of active ingredients in drugs. Although supercritical extraction is expensive at present, it is worthwhile in the long run.
Supercritical fluid because of the large density, low viscosity and high diffusion rate of the inherent characteristics of supercritical fluid extraction as a sample preparation and pretreatment technology has the advantages of other classical methods can't compare with, it can shorten the processing time is 1 to 2 orders of magnitude, avoid the use of a large number of poisonous solvent, reduce the likelihood of contamination. Supercritical extraction is used in combination with other methods to avoid the loss of sample transfer, which is of great significance for reducing human error and improving the sensitivity and accuracy of the method.
At present, the application of supercritical fluid extraction technology is mostly non-polar or low-polar substances, and there are still many difficulties for substances with high polarity. Although modifier can be added in the extraction agent to improve the strength of the extraction agent, the application range is still limited. How to extend it from weakly polar material to polar and even ionic material will be one of the future development directions of supercritical extraction technology.