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(1) Choose a purification procedure
When organic solvents are used to extract pesticides in a sample, the oil, wax, protein , chlorophyll and other pigments, amines, phenols, organic acids, sugars, etc.
in the sample will be extracted together with the pesticides
.
There are both pesticides and many interfering substances in the extract.
The amount and type of auxiliary extracts in the sample extract determine the choice of the purification procedure
.
The more auxiliary extracts there are, the more precise the purification procedure is required to ensure that the sample is sufficiently pure during testing
The purification column can separate pesticides from other auxiliary extracts
.
The ideal situation is to completely remove the impurities, but the actual situation is to reduce the concentration of the impurities so that they will not be affected during the analysis process
The choice of purification technology also depends on the nature and type of the sample.
This is because the fat content, water content, sugar content, carotene , organic sulfur compounds or some secondary products of different types of samples vary greatly
.
When these impurities are removed in the purification process, pesticides are often lost
.
Therefore, sample purification is one of the most difficult and important steps in pesticide residue analysis, and it is the key to the success or failure of residue analysis
(2) Liquid-liquid extraction method
Liquid-liquid extraction (LLE) is a method of separation and purification using the difference in partition coefficients of pesticides and interfering substances in samples in two immiscible solvents (solvent pairs)
.
Usually a polar solvent that is compatible with water is paired with another non-polar solvent that is not compatible with water for distribution.
These two solvents are a solvent pair
.
After repeated distribution, the pesticide residue in the sample is separated from the interfering impurities, and the sample is purified
1.
Samples with high water content are first extracted with a polar solvent, and then transferred to a non-polar solvent
.
(1) A pair of solvents that purify organic phosphorus, carbamate and other pesticides with stronger polarity
.
Water, dichloromethane ; acetone , water-dichloromethane; methanol , water-dichloromethane; acetonitrile, water-dichloromethane
(2) Solvent pair to purify non-polar pesticides
.
Water, petroleum ether; acetone, water-petroleum ether; methanol , water-petroleum ether
2.
The main purpose of purification of samples with low water content and high oil content is to remove impurities such as oil and fat in the sample
.
(1) When purifying relatively polar pesticides, first extract the sample with ethyl acetate, acetone, dimethyl sulfoxide, and dimethyl formamide , and then partition with n-hexane or petroleum ether to extract the oil interferences, and discard them.
Remove the n-hexane layer, leave the pesticides in the polar solvent, add salt solution to it, and then reverse extract the pesticides with dichloromethane or n-hexane
.
Commonly used solvent pairs are ethyl-n-hexane, dimethyl sulfide-n-hexane, and dimethyl formamide-n-hexane
(2) When purifying relatively non-polar pesticides, after extracting samples with n-hexane (or petroleum ether), extract with polar solvent acetonitrile (or dimethylformamide) for multiple times, transfer the pesticides to polar solvents and discard Petroleum ether layer, add salt solution in polar solvent, and then use petroleum ether or dichloromethane to extract pesticides
.
(3) Conventional column chromatography
Conventional column chromatography (conventional column chromatogram) mainly refers to conventional adsorption column chromatography, which is a purification method that uses chromatographic principles to separate pesticides from impurities in an open column
.
After the pesticide residue sample extract is processed by liquid-liquid partition extraction, conventional column chromatography is usually used.
Generally, a glass column with a diameter of 0.
2-2 cm and a length of 10-20 cm is used.
The adsorbent is used as the stationary phase and the solvent is the mobile phase.
The extraction concentrate is added to the column to make it adsorbed by the adsorbent, and then the elution solvent is added to the column, and the solvent with a slightly stronger polarity than the extractant is used for elution.
The pesticide with the stronger polarity is eluted first.
The macromolecules and non-polar impurities remain on the adsorbent
.
Only when the activity of the adsorbent and the polarity of the eluent are selected appropriately, and the volume of the eluent is properly controlled, the impurities can be retained on the column, and the pesticides are eluted, which can separate the pesticides from the impurities
.
The most commonly used adsorbents are alumina, silica gel, Florisil, activated carbon and so on
.
The selection of adsorbent and eluent can be summarized as follows based on experience: ①Polar substances are easily adsorbed by polar adsorbents, and non-polar substances are easily adsorbed by non-polar adsorbents
.
②Alumina and Flori silica have strong adsorption capacity for fats and waxes, activated carbon has strong adsorption capacity for pigments, and diatomaceous earth itself has weak adsorption capacity for various substances, but acidic diatomaceous earth has strong adsorption capacity for various substances in the sample.
Pigment, fat and wax have good purification effect
.
③Change the composition of the elution solvent to obtain specific selectivity
.
If elution is performed on a column with different polar solvent ratios, various pesticides can be eluted in different order
.