A Comprehensive Understanding Of QuEChERS
The QuEChERS method is essentially the derivation and further development of solid phase extraction technology and matrix solid phase dispersion technology. The core of the method is that it proposes a new mode of extracting pesticides by adding a single solvent (acetonitrile) to the class of protective agents; and replacing the commonly used anhydrous sodium sulfate with anhydrous magnesium sulfate with a stronger water absorption function; using PSA (N -Propylethylenediamine) and GCB (graphitized carbon black) purification to remove fatty acids, organic acids and pigments. Compared with other extraction chromatography fillers such as Florisil, neutral alumina, etc., PSA has less adsorption and higher recovery rate for some pesticides.
The basic steps of the QuEChERS method are as follows:
1. Take 15ml of acetonitrile solution containing 1% acetic acid into a 50 ml Teflon tube containing 6g of anhydrous magnesium sulfate and 1.5g of anhydrous sodium acetate;
2. Add the internal standard, shake and mix;
3. Add 15g crushing sample into the centrifuge tube;
4. Shake for 1 min immediately, mix the anhydrous magnesium sulfate with the sample, and make the sample into a paste;
5. Centrifuge the tube at 3700 r/min for 3 min, the organic solvent is separated from the sample;
6. 1ml of the supernatant is taken in a centrifuge tube containing 0.3 g of anhydrous magnesium sulfate and 50mg of PSA, and vortex mixing;
7. 3700 r/min is centrifuged for 3 min;
8. Take the supernatant to analyze.
The above is the principle of solid phase dispersion of the matrix: the sample is treated by centrifugal purification to remove most of the impurity interference, which is also the most widely used program. In actual operation, it can also be changed according to the specific situation.
For example, when using GC-MS analysis, in order to increase the detection sensitivity, 8 ml or more of the supernatant may be taken at sixth step, and then spin and concentrate before proceeding to the next step; for samples containing more pigments, such as green vegetables, leeks, etc., especially when using a detector that is easily contaminated such as GC-ECD, a small amount of GCB can be added in step 6, but for some pesticides such as dicofol and chlorothalonil have some losses due to their instability in the acetone medium.
F Sehenck et al. (2001) have shown that solid phase extraction can also be used for purification. After step 5, the supernatant can be taken on a prepared PSA SPE cartridge with acetone/toluene (3:1) to elute. If necessary, the supernatant can be concentrated and then passed through the column to improve the sensitivity of the test to meet the requirements of sample analysis.
Advantages of the QuEChERS method. The QuEChERS method has the following advantages:
1) High recovery rate, the recovery rate of a large number of polar and alkaline pesticide varieties is between 85% and 100% (mostly greater than 95%);
2) High accuracy (reproducibility less than 5%), corrected by internal standard method;
3) Short analysis time, and 6 pre-weighed samples can be measured within 30-40 minutes;
4) Small amount of solvent used, little pollution and the no chloride-containing solvent;
5) It is easy to operate and can be completed without high skill or training;
6) Very few glassware used;
7) The method is very strict, and the organic acid is removed during the purification process;
8) It can be completed in a small laboratory;
9) The acetonitrile is sealed immediately after it is added to the container, so that it has less chance of contact with the worker;
10) Low cost of the solvent;
11) The simple device used in the sample preparation process.