Characteristics of the QuEChERS Method
Originally, QuEChERS was used as a test for multiple pesticide residues in vegetables or fruits. Up to now, its application range is no longer limited to this. At present, QuEChERS has a wide range of applications in agricultural and veterinary drug residues in agricultural products and foods, poisons in the blood, and mycotoxins in food.
Compared to other commonly used pre-processing technologies (such as SPE, GPC, etc.), the advantages are:
1) It can detect more than 200 kinds of compounds at the same time, and the recovery rate is better, more than 80%;
2) Correction by the internal standard method, with high precision and accuracy;
3) Simple operation, short detection period, 30-40 pre-weighed samples can be processed in 1h;
4) The solvent is used in a small amount and the price is low. Only acetonitrile is used, the environmental pollution is small, and the risk to the experimenter is small;
5) The use of instruments is simple, mainly using centrifuges and centrifuge tubes.
In addition to the above advantages, QuEChERS has its own shortcomings. For samples with low water content or high-fat content, the purification effect is not ideal, the extraction efficiency is low, and the purification process has a large loss.
Characteristics of the QuEChERS method
1. Selection of extractant
When extracting pesticide residues, the choice of solvent is very important, and acetonitrile has its unique advantages. K Ma tovská et al. (2004) evaluated extraction solvents for pesticide residues. For the stability of pesticide residues, the exchange solvents (cyclohexane, n-hexane, and toluene) are superior to polar solvents (acetonitrile, acetone and ethyl acetate). Acetonitrile degrades N-trihalomethylthio-based fungicides such as captan, folpet, and dichlofluanid; and dicofol and chlorothalonil are unstable in acetone medium; thioether pesticides such as fenthion and disulfoton ethyl acetate are unstable in dissolution. By adding 0.1% acetic acid and using acetonitrile as a solvent, the above unstable pesticide residues have a high recovery rate.
Acetonitrile was identified as the most suitable solvent for extracting a wide range of polar pesticide residues and can be used for GC analysis. If you need to improve the sensitivity of splitless injection, you can use toluene as an exchange solvent, not only the toluene and acetonitrile can be mutually soluble, but also have strong peaks for many polar pesticides, such as acephate, methamidophos. Therefore, the QuEChERS method selected acetonitrile as an organic solvent for extracting pesticide residues.
2. Selection of a dehydrating agent
Anhydrous magnesium sulfate has a water absorption capacity three times higher than that of anhydrous sodium sulfate. It has better dehydration ability when the sample is processed, and the sample can be more pulverized so that the pesticide can be extracted more efficiently. Anhydrous magnesium sulfate absorbs water while also producing heat. The temperature of the extract can reach about 45 °C, which promotes the extraction of pesticides; the centrifugation promotes stratification of the extract and the turbid sample, and the anhydrous magnesium sulfate is added again to absorb excess water.
3. Purifier selection
QuEChERS PSA is used to remove various organic acids, pigments, and some sugars and fatty acids. GCB is used to remove pigments such as carcass and chlorophyll. F Schenck (2005) examined the practical effects of PSA and GCB, and found that PSA can remove more than 90% of fatty acids and about 50% of pigments; GCB can remove more than 90% of pigments, but only about 10% of fatty acids are purified. For some samples with high pigment content, better results will get to combine the two and it can purify most of the pigments and fatty acids. However, the disadvantage of using QuEChERS GCB is that it has strong adsorption characteristics, making some pesticides difficult to elute.