S TANDARD LLE LLE is one of the earliest methods used for analyzing pesticides in water samples.

9.2.1 S TANDARD LLE LLE is one of the earliest methods used for analyzing pesticides in water samples.

Because of its simplicity and also its inclusion in EPA methods, LLE is still probably

234 Analysis of Pesticides in Food and Environmental Samples the most used method for pesticide analysis in water. Depending on the types of

analytes, different solvents or other conditions may be used. In the United States, LLE procedures for different classes of pesticides are given in different EPA methods and are routinely used by commercial laboratories. The following method is a brief description of EPA method 8141, using separatory funnels for preparation of water samples containing organophosphate or carbamate residues.

9.2.1.1 General Procedures

Measure out 1000 mL water sample using a 1 L graduated cylinder or by weighing in a container.

Spike 100 mL of the surrogate spiking solution into each sample and mix well.

For the sample in each batch selected for use as a matrix spike sample, add 100 mL of the matrix-spiking standard.

Quantitatively transfer the sample to a 2 L glass separatory funnel, adding

50 g of sodium chloride. Use 100 mL of methylene chloride to rinse the sample container and transfer this rinse solvent to the separatory funnel.

Seal and shake the separatory funnel vigorously for 1 –2 min with periodic venting to release excess pressure.

Allow the organic layer to separate from the water phase for a minimum of

10 min. If the emulsion interface between layers is more than one-third the size of the solvent layer, the analyst must employ mechanical techniques to complete the phase separation. The optimum technique depends upon the sample and may include stirring, filtration of the emulsion through glass wool, centrifugation, or other physical methods. Dry the extract by passing it through a drying funnel containing about 50 g of anhydrous sodium sulfate. Collect the solvent extract in a round bottom flask.

Repeat the extraction two more times using fresh portions of solvent. Combine the three solvent extracts.

Rinse the separation flask, which contained the solvent extract, with 20 –30 mL of methylene chloride and add it to the drying column to complete the quantitative transfer.

Perform the concentration, if necessary, using a vacuum evaporator. For further concentration, nitrogen blow down technique is used to adjust the extract to the final volume required.

The extract may now be analyzed for the target analytes using the appro- priate determinative technique(s).

9.2.1.2 Advantages Standard LLE is a mature method that has been well used and tested. Its advantages

include relatively minimal requirements for equipment and low demand on the analyst’s skills, compatibility for a broad range of pesticides, and reliability. Variations in analyte recovery may be addressed by using a surrogate prior to the

extraction. The surrogate can be either a similar compound or a stable-isotope labeled form of the target analyte, if detection is to be made by a selective detector such as mass spectrometry (MS).

Determination of Pesticides in Water 235

9.2.1.3 Disadvantages

A number of drawbacks may be easily iterated regarding the standard LLE; Most notable is the consumption of large quantities of organic solvents, which makes LLE methods less environment-friendly. Analysis of a 1 L water sample typically needs about 300 –500 mL solvent. The heavy use of solvents in LLE may pose a health concern to the analyst, and also produce large amounts of wastes. LLE is generally labor intensive, time consuming, and physically demanding. Extraction and prepar- ation of 6 –8 samples may easily take one day of the analyst’s time. LLE is generally not suitable for analysis of polar pesticide compounds. LLE can also be less effective for water samples containing high levels of organic matter or suspended particles, such as runoff effluents and other surface water samples, because heavy emulsion often forms between the aqueous and solvent phases. This may prolong phase separation and make recovery variable.