series: [Analysis of Pesticides in Food and Environmental Samples, edited by Jose L. Tadeo]

Overview #

The most commonly used extraction solvents are acetonitrile, acetone, ethyl acetate, and methanol.

While water-miscible solvents, such as acetonitrile and acetone, will effectively extract pesticide residues from high moisture fresh fruits and vegetables, they will not adequately extract pesticide residues from dry samples, such as grains or feeds.

Acetonitrile–water (65/35, v/v) will effectively extract pesticides from dry products.

Pesticides could be extracted from well-comminuted produce samples by shaking with acetonitrile.

An advantage of using acetonitrile as an extraction solvent is that while it is completely miscible with water, it can be readily separated from water not only by liquid–liquid partition with nonpolar solvents (Mills method) but also by the addition of salts (salting out). While the resulting extracts were not as clean as those obtained by the Mills method, both polar and nonpolar pesticides could be recovered.

Using the Mills method, water and NaCl are added to the sample extract, and pesticide residues are partitioned from the acetonitrile–water mixture into a very nonpolar solvent, petroleum ether.

The Mills method worked very well with the nonpolar OC pesticides. Some of the more polar OPs that were developed in the 1970s were not easily recovered.

A number of methods have been presented that employ organic solvent extraction by shaking instead of using mechanical blenders or homogenizers.

Shaking may not work as well as homogenizing for some of the more nonpolar OCs.

Polar solvents are needed for the extraction of nonpolar OC pesticides from the plant matrix.

In 2003, a new approach to the extraction of pesticides from fresh fruits and vegetables with acetonitrile, called quick, easy, cheap, effective, rugged, and safe (QuEChERS) was reported. ¹ This method entailed shaking the sample with acetonitrile, followed by shaking with sodium chloride (NaCl) and MgSO4 to remove the water. The salts create an exothermic reaction with water, induce phase separation between water and acetonitrile, and bind water to drive the pesticide analytes into the acetonitrile phase, resulting in high recoveries, including the polar and water-soluble pesticide, methamidophos. A modified QuEChERS extraction, using 1% acetic acid–acetonitrile extraction solvent and sodium acetate rather than NaCl, was developed to facilitate the recovery of base-sensitive fungicides like chlorothalonil and captan.

“Luke method". Produce samples were extracted with acetone, and water was removed from the extract by a series of liquid–liquid partition steps, first with petroleum ether–dichloromethane, followed by dichloromethane–NaCl. Both polar and nonpolar pesticides could be recovered.

Ethyl acetate is only slightly miscible with water, which simplifies the problem of separating water from the sample extract. Since ethyl acetate is more nonpolar than the other solvents discussed, the polar pesticides do not readily partition into ethyl acetate. Large amounts of sodium sulfate are usually added to bind the coextracted water and force the polar pesticides into the organic phase. Most methods using ethyl acetate extraction entail two extractions of the sample matrix rather than the single extraction commonly used with acetonitrile and acetone. Polar solvents like ethanol may be added to the extraction solvent to increase the recovery of polar compounds. Ethyl acetate extraction will result in a cleaner extract as it will extract less of the polar plant matrix compounds, but more lipids and waxes.

Reference #

p157, Analysis of Pesticides in Food and Environmental Samples, Jose L. Tadeo