E XTRACTION The liquid –solid extraction (LSE) of pesticides from soil is generally carried out by

8.2.2 E XTRACTION The liquid –solid extraction (LSE) of pesticides from soil is generally carried out by

organic solvents. Two techniques have been widely used, the shaking and filter method and the Soxhlet extraction method. These classical analytical techniques have the advantage of being simple and low cost methods, but they are time consuming, laborious, difficult to automate, and nonselective methods. In addition, they suffer from various disadvantages, such as the use of large volume of organic solvents and the need of cleanup steps.

Several modern analytical techniques have been developed to overcome these problems. Accelerated solvent extraction (ASE), also named pressurized liquid extrac- tion (PLE), is a fast technique that uses low volumes of solvents and can be automated, although the high temperatures used to accelerate the process may degrade some pesticides. Supercritical fluid extraction (SFE) uses fluids above their critical tempera- ture and pressure. In these conditions, supercritical fluids behave similar to liquids,

CO 2 being widely employed because of its reduced cost and low critical temperature (318C) and pressure (73 atm). Microwave-assisted extraction (MAE) is also a fast technique that is able to extract multiple samples at the same time, but the extraction vessels are expensive and must be cooled at room temperature before opening. Ultrasonic or sonication assisted extraction with various organic solvents has also been employed to extract pesticides from soil. A miniaturized technique based on the sonication assisted extraction in small columns (SAESC) has been recently developed in our laboratory. In this method, the soil sample located in a small column is placed in an ultrasonic water bath, wherein pesticides are extracted with a low solvent volume, assisted by sonication. Tables 8.1 through 8.3 summarize representative published papers on the analysis of pesticides in soil using those extraction techniques.

8.2.2.1 Herbicides Analyses of herbicide residues in soil have been frequently performed because of

the wide application of these compounds. Initially, polar herbicides, such as benzonitriles and phenoxy acids, were extracted from soil with organic solvents of

210 Analysis of Pesticides in Food and Environmental Samples

TABLE 8.1 Extraction Methods of Herbicides from Soil

Technique Class Solvent References Shaking

Benzonitriles, phenoxy acids Low –medium polarity, acidic pH [3 –6] Dinitroanilines

Acetonitrile –water (99:1, v=v) [7] Phenoxy acids, glyphosate

[8 –10] Phenylureas, triazines

Water, basic pH

[11 –16] Sulfonylureas

Methanol

Methanol, acidic pH [17] Multiclass

Ethyl acetate

Triazines, benzonitriles

Methanol

[26,27] Triazines

Phenoxy acids, pyrimidines

Water, basic pH

Hexane –acetone (2:1, v=v) [28] Multiclass

Cyclohexane –acetone (3:1, v=v) [29] SAESC

[30,31] PLE

Ethyl acetate

Phenoxy acids

Phenoxy acids Water –methanol, pH 7 [34] Triazines

Water –methanol (1:1, v=v) pH 7 [35] Multiclass

[36,37] SPME

Acetonitrile

Triazines [36] SAESC, sonication assisted extraction in small columns; PLE, pressurized liquid extraction; MAE,

microwave-assisted extraction; SPME, solid-phase microextraction.

low –medium polarity at acidic pH, using manual or mechanical shaking or sonica- tion. For less polar herbicides, such as triazines, chloroacetamides, and dinitroani- lines, organic solvents such as acetone, ethyl acetate, methanol, and acetonitrile, alone or in mixtures with water, were commonly used.

More recently, a considerable reduction in solvent consumption has been achieved by miniaturizing the scale of sample extraction. In addition, MAE and SPME have been successfully applied to the extraction of various herbicides from soil. MAE is a technique with a reduced consumption of solvent, which is normally acetonitrile or methanol, alone or in mixtures with water, and solid-phase microextraction (SPME) eliminates the need of solvent and an ulterior cleanup step is not needed.

In multiclass herbicide analysis, soil samples were generally extracted with a polar or medium polarity solvent, such as acetone or acetonitrile. PLE is a new technique used successfully for the extraction of herbicides, such as triazines and phenoxy acids, using water and acetone as solvents.

8.2.2.2 Insecticides and Fungicides Conventional methods have been widely used in the extraction of organochlorine

(OC) insecticides from soil, although the use of new extraction techniques has

Determination of Pesticides in Soil 211

TABLE 8.2 Extraction Methods of Insecticides and Fungicides from Soil

Technique Class

References Shaking

Ethyl acetate

Organochlorines Petroleum ether –acetone (1.1, v=v) [44] Organophosphorus

Water, acetone

Isooctane –Dichloromethane (15:85, v=v) [47] Multiclass-fungicides

[48] SAESC

Water, acetone

Ethyl acetate

[51] SFE

Ethyl acetate

Carbamates, Pyrethroids

[52,53] Organochlorines

CO 2 –3%methanol

CO 2 [54] Multiclass-insecticides

[55] PLE

CO 2 –3%methanol

Organochlorines Acetone –hexane (1:1, v=v) [56 –58] MAE

Acetone –hexane (1:1, v=v) [59] Pyrethroids

[60,61] SPME

Toluene

Organochlorines [62,63] Organophosphorus

[64,65] Multiclass-fungicides

[66,67] SAESC, sonication assisted extraction in small columns; SFE, solid-phase extraction; PLE, pressurized

liquid extraction; MAE, microwave-assisted extraction; SPME, solid-phase microextraction.

increased during the last years. In the PLE, the soil sample is placed in a cartridge and extracted with mixtures of acetone and hexane. The use of MAE has also increased because of the good recoveries obtained. Moreover, headspace SPME has been successfully used to determine OC insecticides in soil with limits of detection (LOD) similar to other extraction techniques.

Organophosphorus (OP) pesticides are compounds highly polar and soluble in water that have been extracted from soil by shaking with organic solvents such as methanol. Other new techniques, such as SPME, are now frequently used for the extraction of these compounds in soil samples.

Carbamates were initially extracted from soil by conventional methods using mechanical shaking with different solvents. SFE and MAE were afterwards successfully applied to soil as a practical alternative to traditional methods. In recent years, analysis by means of SAESC has obtained good results.

212 Analysis of Pesticides in Food and Environmental Samples

TABLE 8.3 Multiresidue Methods of Pesticide Extraction from Soil

Technique Class

References Shaking

Solvent

H, I, F Acetonitrile –water (70:30, v=v) [68]

[69] Soxhlet

Ethyl acetate

[70] H, I

I, A

Hexane –acetone (1:1, v=v)

[71] H, I

Acetone

Methylene chloride –acetone (1:1, v=v) [72] Sonication

F, I

[73] H, F, I, A

Acetonitrile –water (2:1, v=v)

[74] H, I, A

Methanol –water (4:1, v=v)

[75] SAESC

Ethyl acetate

[76,77] SFE

H, I, F, A

Ethyl acetate

[78,79] PLE

H, I, F

CO 2 –3%methanol

H, I [80] H, herbicides; I, insecticides; F, fungicides; A, acaricides; SAESC, sonication assisted extraction

small columns; SFE, solid-phase extraction; PLE, pressurized liquid extraction; SPME, solid-phase microextraction.

Pyrethroid insecticides are a class of natural and synthetic compounds that are retained in soils because of their high lipophility and low water solubility and extracted from soil samples by sonication with organic solvents, alone or in binary mixtures. Investigations with fortified samples showed that good and similar recoveries of these compounds were obtained with MAE and SFE.

The analysis of multiclass mixtures of insecticides was initially carried out by Soxhlet or shaking methods with low or medium polarity solvents. SFE with CO 2 modified with methanol and SAESC with ethyl acetate are other techniques used more recently.

The analysis of fungicides in soil was initially accomplished by classical extraction methods, such as the shaking and filter method using acetone or ethyl acetate. The ultrasonic assisted extraction and SPME have been other techniques used more recently for the determination of fungicides in soil samples.

8.2.2.3 Multiresidue Reliable multiresidue analytical methods are needed for monitoring programs of

pesticide residues in soil. The classical procedure for pesticide extraction from soil was to shake soil samples with an organic solvent, ethyl acetate or acetonitrile, alone or in mixtures with water, being the most widely used solvents.

SFE with carbon dioxide containing 3% methanol, as a modifier used to improve recoveries of polar pesticides, has been employed for the multiresidue extraction of pesticides having a wide range of polarities and molecular weights. SFE using CO 2 is essentially a solvent-free extraction wherein the carbon dioxide is easily removed at atmospheric pressure.

Determination of Pesticides in Soil 213 Recently, a modification of the SAESC has been used for the simultaneous

determination of different classes of pesticides. The good reproducibility and detection limits achieved with this method allow its application to the monitoring of pesticide residues in soil [76].

SPME has been mainly used for the extraction of pesticides from aqueous samples; however, head space SPME has been recently used for the determination of pesticides volatilized from soil. The application of MAE for the extraction of pesticide residues is increasing in the last years and together with other modern techniques, such as sonication and PLE, are the most widely used methods at present.