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Original Title: Seven Common Methods in Pesticide Residue Detection Pretreatment 1. Soxhlet extraction (Automatic Rope Extraction) Soxhlet extraction is a classical extraction method, which is still widely used in sample preparation for pesticide residue analysis. The United States Environmental Protection Agency (EPA) uses it as one of the standard methods to extract organic matter (EPA3540C); Soxhlet extraction is also used as the extraction method in the national standard method 。 Because it is a classical extraction method, other sample preparation methods are generally compared with it to evaluate the extraction efficiency of the method. The main advantage of Soxhlet extraction method is that it does not require the use of special instruments and equipment, and the operation method is simple and easy, many laboratories can be achieved, and the use cost is low. The main disadvantages are high solvent consumption, long time consumption and need of condensed water. The fat extractor made of glass in Soxhlet extraction is one of the glassware which is easy to be damaged, especially the siphon return pipe on the outer wall of the extractor is easy to be damaged, so we should be careful in the experimental operation. In addition to the extraction solvent, the factors that determine the efficiency of Soxhlet extraction are also the number of times of reflux of the extraction solvent (to some extent, it can be said to be the extraction time). Generally, the temperature distribution of the water bath used in the laboratory is not very uniform, and the thickness of the wall of the round-bottomed flask used for extraction is not uniform, which will cause the difference of reflux rate. Generally, the temperature of the water bath should not be too high in the experiment to prevent the loss of the target caused by bumping. ; In Soxhlet extraction, filter paper cartridges are generally used to hold samples, and metal sieve cartridges are not suitable (this will cause the decomposition of some pesticide targets, such as Fe, which may cause the decomposition of some organochlorine pesticides). In addition, attention should be paid to the matching of the filter paper cartridge with the extractor after loading, and in particular, the paper cartridge should not block the siphon return line. The Soxhlet extractor used in the experiment should not be too large, otherwise the solvent vapor will be reduced due to the condensation of ambient air before it reaches the extractor (especially in winter when the ambient temperature is low), which will slow down the extraction efficiency and make the extraction time-consuming too long. As Soxhlet extraction is a relatively open extraction system, attention should be paid to prevent contamination in the extraction operation; it is better to cover the top of the condenser tube in the experimental operation 。 Generally, the Soxhlet extraction tube can be cleaned with chromic acid solution, and deionized water (you can prepare more for extraction with n-hexane before use) can be cleaned, dried or air-dried. There is also an automatic Soxhlet extraction method (Automated Soxhlet Extraction Method) in Soxhlet extraction, and EPA3541 also has a standard method. Compared with the Soxhlet extraction, the automatic Soxhlet extraction method has the advantages of faster extraction time, automatic operation, solvent recovery and the like. Because I have not used this method, I can only simply state it according to the data. 2. Oscillating extraction and tissue trituration (Homogenization method) Shaking extraction and tissue mashing method (homogenization method), these two extraction methods are relatively simpler, generally for plant samples, food, especially fresh samples with high water content, such as vegetables, fruits, etc., are more convenient and simple. The two methods do not need special equipment and can be used by common oscillators, centrifuges, homogenizers and the like; These two methods are used in many standard methods for pesticide residue analysis, such as GB/T5009 series methods and Japan's "JAP Positive List Detection Method for Pesticide Residues in Food, Veterinary Drugs and Feed Additives". In these two methods, the extraction solvent generally used is mostly polar solvent,wiped film evaporator, and acetonitrile is mostly used in the standard method. Because the water content in these samples is generally high, if a single non-polar solvent is used for extraction, the extraction effect will be reduced due to its strong hydrophobicity and limited ability to infiltrate or penetrate the sample. There is also an important pretreatment step, namely solid-liquid separation, in the oscillation method and the tissue mashing method (homogenization method), as well as the ultrasonic extraction and microwave extraction methods mentioned later. This step can be achieved by filtration (suction filtration) and centrifugation. Filtration may be carried out with simple filter paper or by suction filtration with a filter aid such as Celite 545.
If centrifugal separation is used, care should be taken to prevent the container from breaking. Expand the full text In these two extraction methods, in order to avoid the loss caused by liquid transfer, part of the liquid is usually extracted directly from the extraction liquid for subsequent operation. 3. Ultrasonic extraction method Ultrasonic extraction has the advantages of no heating, simple operation, time saving and high extraction efficiency. It is also widely used in sample pretreatment of pesticide residue analysis, such as EPA3550 method. Ultrasonic extraction is generally carried out by using an ultrasonic cleaner (Fig. 2) or a special needle extractor (such as an ultrasonic cell disruptor, Fig. 3). No matter what kind of extraction equipment, it uses the "cavitation" effect of ultrasonic wave. Ultrasonic extraction has obvious characteristics and does not need heating, which is especially suitable for the extraction of thermally unstable targets. At present, the ultrasonic cleaner is mostly used as the extraction instrument in the laboratory. Generally, before ultrasonic extraction,50l rotovap, the sample to be extracted should be soaked in the extraction solvent for a period of time, so that they can fully contact and penetrate each other. In the ultrasonic extraction, it is preferable to use a mixed extraction solvent and extract in steps to improve the extraction efficiency of the target substance. Ultrasonic extraction method also has certain requirements for glass containers, if the quality of glass containers is not good, there are cracks, etc., it is easy to break in the extraction process, so special attention should be paid when choosing glassware. Organic solvents in the use of ultrasonic extraction, the volatility will increase, so it should be noted that the extraction container can not be closed, there should be a certain space. When the ultrasonic cleaner is used for extraction, it should be noted that the distribution of the ultrasonic field in the whole ultrasonic container is not uniform, and there is a dead angle in the distribution of the wave field, which will significantly reduce the extraction efficiency of some samples, resulting in poor reproducibility. Ultrasonic extraction requires a large amount of solvent, which is generally extracted and filtered step by step. Although the operation is simple, the labor intensity of the operation is high, and steps such as filtration are needed to separate the extraction solvent from the sample. 4. Supercritical fluid extraction Supercritical fluid extraction (SFE) has the advantages of short time consumption and less organic solvent consumption, so it is widely used in the sample pretreatment of pesticide residue analysis, especially in the extraction of natural medicine ingredients such as food and effective ingredients of Chinese herbal medicine. (Such as Gilson's, if there is no instructions at the time of operation, it is impossible.) There are many conditions that need to be set for extraction. Uch as the need for carbon dioxide, cryogenic cooling equipment (glycol coolant), the addition of modifiers to increase the solubility of polar compounds, pressure settings, collection of solvents, and the like. Now the supercritical fluid extraction instruments (such as ASI in the United States) have been greatly improved, and the performance, function and volume of the instruments have been greatly improved. Due to the use of special equipment and consumables, the literature on SFE analysis of pesticide residues in China is still relatively small compared with other methods. The biggest advantage of supercritical fluid extraction is the use of organic solvents (basically not used or rarely used), and it is easy to extract some macromolecular compounds, cbd centrifugal extractor ,hemp extraction centrifuge, heat-sensitive and chemically unstable substances. Of course, the cost of equipment is a problem that many laboratories must consider. 5. Accelerated solvent extraction Accelerated solvent extraction was selected as the standard method by the EPA (EPA 3545). Accelerated solvent extraction (ase) is easily automated (sequential extraction) and is currently used as a pretreatment for the analysis of pesticide residues in soil and biological samples (Figure 4). Compared with Soxhlet extraction and ultrasonic extraction, accelerated solvent extraction consumes less solvent, has a high degree of automation and is relatively simple to operate, but the biggest problem of accelerated solvent extraction is the cost of analysis, that is, the instruments and consumables are relatively expensive (especially the filter membrane, disposable), which is difficult for the general laboratory to bear. The efficiency of accelerated solvent extraction is higher, but the general co-extract is relatively more, which will affect the subsequent purification operation. At present, online purification has been reported (for example, there is relevant purification information on the website of Diane Company), that is, the adsorbent required for purification is loaded under the sample to achieve the purpose of extraction-purification. However, for the detection of different samples and pesticide residue targets, the specific method needs to be determined by many experiments.
ASE has a high degree of automation, simple operation, easy parameter setting, and automatic filtering in the subsequent operation, which greatly reduces the labor intensity of the experimenter and avoids the loss of the target. When extracting samples with high moisture content, anhydrous sodium sulfate shall not be used for dehydration (mainly to prevent caking and pipeline blockage). For the requirement of sample quantity, the sample should be filled in combination with the extraction pool of each volume, and it should not be filled too much or too tightly, otherwise the extraction efficiency will be affected. Also, because of the high temperature extraction, it is not suitable for some target substances which are easy to pyrolyze. In addition, the accelerated solvent extraction device is also a good instrument for the study of subcritical water extraction. Microwave extraction Microwave extraction is to heat the substance directly and uniformly through the two effects of molecular polarization and ionic conduction (the current theory includes microwave thermal effect and non-thermal effect, which will not be discussed here). According to the principle of microwave action, microwave extraction needs polar solvents, but it is generally mixed solvent extraction. There are relatively more literatures on microwave extraction in China, which can also explain the applicability of this method to some extent. There are two main ways of microwave extraction: open type (Fig. 5) and closed type. (For more microwave principles, please refer to the page of microwave extraction/digestion in this forum.) The general household microwave oven, its microwave generation is in the form of pulse, if we look at the experimental phenomenon, that is, when the microwave is disconnected, the solvent will have a strong bumping phenomenon. The distribution of the microwave field in the cavity of a domestic microwave oven is not uniform (this problem is overcome by rotating the disk). There are also many discussions about whether a modified household microwave oven can be used for microwave extraction experiments. In my opinion, the feasibility can not be discussed for the time being, but can be seen from the open microwave extraction experiment itself: first, when using microwave extraction, there should be a stirring device, preferably a magnetic stirrer, so that the extraction solvent can reach temperature equilibrium in the fastest time to avoid the drawbacks caused by uneven distribution of microwave field; second, stirring can also improve the efficiency of extraction. 。 Secondly, for the experiment, a more accurate temperature control system is needed, which is a difficulty, because the current temperature measurement methods (thermocouple, thermal resistance, optical fiber, infrared, etc.) Have their limitations in the application of this microwave extraction method. ? The actual temperature of the solvent in the extractor is difficult to visualize and control in a timely manner. From these two points of view, if you want to modify it, it is still very troublesome. When non-pulsed microwave extraction is used, bumping can be avoided, mainly because of the continuous supply of microwave, which will not form a very large pulse. Another problem with microwave extraction is microwave decomposition, because microwaves act not only on the solvent but also on the target itself; however, in practice, As long as the power of the microwave is set reasonably, the influence of the microwave on the decomposition of the target substance is within an acceptable range. The efficiency of microwave extraction needs to be optimized by microwave power, solvent ratio, extraction time, extraction temperature and so on. Due to the high pressure and high temperature conditions, the airtight microwave extraction device is superior to the normal-pressure microwave extraction device in the aspects of extraction efficiency, extraction time, solvent consumption and the like, and the extraction environment is high pressure and high temperature, which is somewhat similar to the effect of accelerating solvent extraction; therefore, The efficiency of extraction, the time of extraction and the solvent consumption are all due to atmospheric extraction. However, the vexing problem of closed microwave extraction is the problem of temperature control, which also leads to safety issues. Since not every extraction tank has temperature or pressure control (it is not known whether there are products with relevant functions at present), when the sample components in each extraction tank are different, there may be some changes in temperature or pressure. Because the temperature of microwave extraction is relatively high, it is not suitable for the target compounds. The operation of open microwave extraction experiment is somewhat similar to ultrasonic extraction. The extraction can be carried out step by step, and the liquid-solid separation can also be achieved by means of filtration; the extraction efficiency of microwave extraction is higher Moreover, the co-extraction of pigments in samples, such as plants, is smaller, which makes the purification slightly easier.
Relatively speaking, open microwave extraction is slightly inferior to closed extraction in terms of the amount of samples processed. Generally, only one sample can be extracted at a time by normal pressure microwave extraction, and the extraction solvent needs to be cooled by condensed water, while closed microwave extraction can extract multiple samples at the same time. At present, domestic microwave instruments have microwave digestion and extraction products. 7 Matrix Solid Phase Dispersion Technology MATRIX SOLID-PHASE DISPERSION (MSPD) is a technique in which a sample (solid or liquid) is directly mix and ground with a silica gel suitable for reverse phase bonding (such as C18, C8, etc.) (now extend to other materials, such as diatomaceous earth, etc.). A uniform fraction of the sample is placed on the surface of the solid phase particles to form a semi-solid column, which is then washed and eluted using an operation similar to SPE. Its advantages are as follows: Depending on the mechanical shearing force of filler particles and the impurity removal effect of C18 and other fillers, the sample homogenization and extraction are completed in the same process without solvent and impurity removal steps; C18 and the like can destroy the cell membrane of the lipid, so that the cell components are released and redistributed in the filler; and the sample matrix and the components to be tested are uniformly distributed in the filler. The various components of the sample are dissolved and distributed in the bonding phase on the surface of the filler according to the polarity according to the law of similar dissolution; The retention of components is related to the filler, sample matrix and solvent; The MSPD sample processing speed is fast, the amount of solvent is small, and the amount of sample is also small, so the detection method (instrument) is required to have high sensitivity. The commonly used detection methods include the determination of chloropropanol in soy sauce,winterization filtration, the determination of pesticide components in apple juice, the extraction and determination of persistent compounds (polychlorinated biphenyls), etc. Return to Sohu to see more Responsible Editor:. toptiontech.com