C LEANUP Soil sample extracts, obtained with any of the methods described earlier, generally

8.2.3 C LEANUP Soil sample extracts, obtained with any of the methods described earlier, generally

contain a considerable amount of other components that may interfere in the subsequent analysis. Therefore, the determination of pesticides at residue level frequently requires a further cleanup of soil extracts. Liquid –liquid partition (LLP) between an aqueous and an organic phase, at modulated pH in some cases, has been the most common first step in the cleanup of extracts. An alternative cleanup technique is column chromatography, using reverse or normal phases, in which pesticides are separated from interferences by elution with a solvent of adequate polarity. Tables 8.4 through 8.6 summarize the cleanup procedures employed in the determination of pesticides in soil.

8.2.3.1 Herbicides Phenoxy acid herbicides are normally formulated as amine salts or esters, which

are rapidly hydrolyzed in soil to the acidic form. Cleanup techniques for the

TABLE 8.4 Cleanup Techniques Used in the Analysis of Herbicides

Class Technique

References Phenoxy acids

Solvent

[3] LLP, SPE-florisil

LLP, pH 8 –9

Methylene chloride

[5] LLP-pH 2

Diethyl ether

[32] SPE-silica gel

Ether:hexane

[4,26] SPE-polymer

Dichloromethane

Benzene –hexane (1:9, v=v) [8,10] SPE-C8

[17] Phenylureas

Methanol

SPE-florisil Ethyl ether –n-hexane (1:1, v=v) [23,24] Pyrimidines

SPE-alumina Ethyl ether –n-hexane (1:2, v=v) [15] Triazines

SPE-polymer Methanol –ethyl acetate (7:3, v=v) [35] Multiclass

LLP-SPE-florisil-alumina Dichloromethane –diethyl ether [21] LLP, liquid –liquid partition; SPE, solid-phase extraction.

214 Analysis of Pesticides in Food and Environmental Samples

TABLE 8.5 Cleanup Techniques Used in the Analysis of Insecticides and Fungicides

Class Technique

References Insecticides

Solvent

Organochlorines SPE-alumina Hexane –ethyl acetate (7:3, v=v) [44] SPE-carbon

Hexane –ethyl acetate (80:20, v=v) [57] SPE-florisil

Heptane –ethyl acetate (1:1, v=v) [58] Organophosphorus

SPE-florisil Hexane –ethyl acetate (2:1, v=v) [60,61] Multiclass

LLP

[42] SPE-C18

Methylene chloride

[43] Fungicides Strobilurins

Methanol

SPE-florisil Toluene-ethyl acetate (20:1, v=v) [39] LLP, liquid –liquid partition; SPE, solid-phase extraction; MISPE, molecularly imprinted solid-phase

extraction.

purification of soil extracts include liquid –liquid partitioning, at basic or acidic pH, and column chromatography using various adsorbents (Florisil, alumina, or silica gel). These cleanup processes are time consuming and large quantities of solvents are generally required. Therefore, minicolumns and cartridges, which reduce the solvent consumption and the analysis time, have replaced conventional chromatographic columns. Various organic solvents with different polarity, such as methanol, dichloromethane, or other intermediate polarity solvents, have been used to elute phenoxy acid herbicides from cleanup columns. In recent years, new polymeric packing materials have been developed.

The cleanup of triazine herbicides in soil extracts has been carried out by SPE with alumina or Florisil and various mixtures of organic solvents have been used for eluting these compounds.

TABLE 8.6 Cleanup Techniques Used in the Multiresidue Analysis of Pesticides Class

References H, I, F

Technique

Solvent

LLP Petroleum ether-diethyl ether (1:1, v=v) [68] I, F

[73] H, I, F

LLP

Dichloromethane

[78] H, I, F, A

SPE-C18

Acetone-hexane (20:80, v=v)

SPE-polymer Dichloromethane –methanol (1:1, v=v) [74] H, herbicides; I, insecticides; F, fungicides; A, acaricides; LLP, liquid –liquid partition; SPE, solid-phase

extraction.

Determination of Pesticides in Soil 215 In the analysis of multiclass herbicide mixtures, the cleanup of soil extracts has

been carried out by SPE on Florisil or alumina, after LLP.

8.2.3.2 Insecticides and Fungicides In general, extracts from soil samples have been cleaned up by means of chromato-

graphic columns filled with alumina or Florisil as adsorbents and pesticides have been eluted with nonpolar or low polarity solvents (hexane, ethyl acetate). In some cases, more hydrophobic sorbents, such as carbon, have been used for low polarity insecticides. In addition, LLP of soil extracts between immiscible solvents is a method sometimes used. Moreover, solid-phase extraction with molecularly imprinted polymers (MISPE) is a novel selective method that has been used for the analysis of OPs in soil and proved to be a good tool for their selective extraction.

In the analysis of multiclass insecticide mixtures, good recoveries have been obtained using reversed-phase C18 cartridges and methanol as eluting solvent.

8.2.3.3 Multiresidue Analysis of complex mixtures of pesticides in soil is a difficult problem because of

the presence of a wide variety of compounds with different physical –chemical properties.

In modern analytical techniques, the classical methodology for the cleanup of extracts, based on LLP, has been replaced by miniaturized techniques for residue analysis that are less solvent consuming. SPE is a technique widely used to determine pesticide residues in soil after their extraction with water or aqueous mixtures of organic solvents. Octyl and octadecyl-bonded silica sorbents have been frequently used in the analysis of nonpolar and medium polarity pesticides in soil extracts.