2 Sample Handling of Pesticides in Food and Environmental Samples Esther Turiel and Antonio Martín-Esteban CONTENTS 2.1 Introduction 36 2.2 Sample Pretreatment 36 2.2.1 Drying 37 2.2.2 Homogenization 37 2.3 Extraction and Purification 38 2.3.1 Solid–Liquid Extraction 38 2.3.1.1 Shaking 39 2.3.1.2 Soxhlet Extraction 41 2.3.1.3 Microwave-Assisted Extraction 42 2.3.1.4 Pressurized Solvent Extraction 42 2.3.2 Supercritical Fluid Extraction 43 2.3.3 Liquid–Liquid Extraction 45 2.3.4 Solid-Phase Extraction 45 2.3.4.1 Polar Sorbents 46 2.3.4.2 Nonpolar Sorbents 48 2.3.4.3 Ion-Exchange Sorbents 49 2.3.4.4 Affinity Sorbents 49 2.3.5 Solid-Phase Microextraction 52 2.3.5.1 Extraction 53 2.3.5.2 Desorption 54 2.3.6 Solid–Solid Extraction: Matrix Solid-Phase Dispersion 54 2.3.7 Other Treatments 55 2.3.7.1 Stir Bar Sorptive Extraction 55 2.3.7.2 Liquid Membrane Extraction Techniques 55 2.4 Future Trends 56 References 56 ß 2007 by Taylor & Francis Group, LLC. 2.1 INTRODUCTION The determination of pesticides in food and environmental samples at low concen- trations is always a challenge. Ideally, the analyte to be determined would be already in solution and at a concentration level high enough to be detected and quantified by the selected final determination technique (i.e., HPLC or GC). Unfortunately, the reality is far from this ideal situation. Firstly, the restrictive legislations from European Union and World Health Organization devoted to prevent contamination of food and environmental compartments by pesticides make necessary the develop- ment of analytical methods suitable for d etecting target analytes at very low concen- tration levels. Besides, from a practical point of view, even when the analyte is already in solution (i.e., water or juice), there are several difficulties related to the required sensitivity and selectivity of the selected determination technique that must be overcome, since the concentrati on of matrix-interfering compounds is much higher than that of the analyte of interest. Consequently, the development of an appropriate sample preparation procedure involving extraction, enrichment, and cleanup steps becomes mandatory to obtain a final extract concentrated on target analytes and as free as possible of matrix compounds. In this chapter, the different sample treatment techniques currently available and most commonly used in analytical laboratories for the analysis of pesticides in food and environmental samples are described. Depending on the kind of sample (solid or liquid) and the specific application (type of pesticide, concentration level, multiresidue analysis), the final procedure might involve the use of only one or the combination of several of the different techniques described later. 2.2 SAMPLE PRETREATMENT Generally, sampling techniques provide amounts of sample much higher (2–10 L of liquid samples and 1–2 kg of solid samples) than those needed for the final analysis (just few milligrams). Thus, it is always necessary to carry out some pretreatments to get a homogeneous and representative subsample. Even if the sample is apparently homogeneous, that is, an aqueous sample, it will be at least necessary to perform a filtration step to remove suspended particles, which could affect the final determin- ation of target analytes. However, some hydrophobic analyt es (i.e., organochlorine pesticides) could be adsorbed onto particles surface and thus, depending on the objective of the analysis, might be necessary to analyze such particles. This simple example demonstrates the necessity of establishing clearly the objective of the ana- lysis, since it will determine the sample pretreatments to be carried out, and high- lights the importance of this typically underrated analytical step. Usually, environmental water samples just require filtration, whereas liquid food samples might be subjected to other kinds of pretreatments depending on the objective of the analysis. However, solid samples (both environmental and food samples) need to be more extensively pretreated to get a homogeneous subsample. The wide variety of solid samples prevents an exhaustive description of the different procedures in this chapter; however, some general common procedures will be described later. ß 2007 by Taylor & Francis Group, LLC. 2.2.1 DRYING The presence of water or moisture in solid samples has to be taken into account since it might produce alterations (i.e., hydrolysis) of the matrix and=or analytes, which will obviously affect the final analytical results. Besides, water content varies depending on atmospheric conditions and thus, it is recommended to refer the content of target analytes to the mass of dry sample. Sample drying uses to be carried out before crushing and sieving steps, although it is recommended drying again before final determination since rehydration process might occur. Typically, sample is dried inside an oven at temperatures about 1008C. It is important to stress that higher temperatures can be used to decrease the time devoted to this step but losses of volatile analyt es might occur. In this sense, it is important to know a priori the physicochemical properties of target analytes to preserve the integrity of the sample. A more conservative approach, using low temperatures, can be followed but it will unnecessarily increase the drying time. Alternatively, lyophilization is recommended if a high risk of analytes loss es exists and it is an appropriate procedure for food, biological material, and plant samples drying. However, even following this procedure, losses of analytes might occur depending on their physical proper ties (i.e., solubility, volatil ity). The results are evident that it is not possible to establish a general rule on how to perform sample drying. Thus, studies on stability of target analytes in spiked samples should be carried out to guarantee the integrity of the sample before final determin- ation of the analytes. 2.2.2 HOMOGENIZATION As mentioned earlier, samples are heterogeneous in nature and thus, they must be treated to get a homogeneous distribution of target analytes. Generally, soil samples are crushed, grinded, and sieved through 2 mm mesh. Grinding can be done manually or automatically using specially designed equip- ments (i.e., ball mills). It is important to stress that this procedure might provoke the local heating of the sample and thus, thermolabile or volatile compounds might be affected. In this sense, it is recommended to grind the sample at short time intervals to minimize sample heating. In addition, due to heating, water content may vary making necessary to recal culate sample moisture. Food samples use to be cut down to small pieces with a laboratory knife before further homogenization with au tomatic instruments (i.e., blender). Sample freezing is a general practice to ease blending, especially recommended for samples with high fat content (i.e., cheese) and for soft samples with high risk of phase separation during blending (i.e., liver, citrus fruits). Apart from these general guidelines, especially in food analysis, the determin- ation of pesticides might be restricted to the edible part of the sample or to samples previously cooked and thus, sample pretreatments will vary depending on the objective of the analysis. Finally, it is important to point out that, in most of the cases, samples need to be stored for certain periods of time before performing the analysis. In this sense, although sample storage cannot be considered a sample pretreatment, the addition ß 2007 by Taylor & Francis Group, LLC. of preservatives as well as the establishment of the right conditions of storage (i.e., at room temperature or in the fridge) to minimize analyte=sample degradation are typical procedures carried out at this stage of the analyt ical process and need to be taken into account to guarantee the accuracy of the final result. 2.3 EXTRACTION AND PURIFICATION The main aim of any extraction process is the isolation of analytes of interest from the selected sample by using an appropriate extracting phase. Pesticides from liquid samples (i.e., environmental waters) are preferably extracted using solid phases by solid-phase extraction (SPE) or solid-phase microextraction (SPME) procedures, although for low volume samples, liquid–liquid extraction (LLE) can also be carried out. Extraction of pesticides from environmental or food solid samples is usually performed by mixing the sample with an appropriate extracting solution, where the mixture is subjected to some process (agitation, microwaves, etc.) to assist migration of analytes from sample matrix to the extracting solution. For certain applications, matrix solid-phase dispe rsion (MSPD) can also be a good alternative. In all cases, once a liquid extract has been obtained, it is subsequently subjected to a purification step (namely cleanup), which is usually performed by SPE or LLE. In some cases, extraction and cleanup procedures can be performed in a unique step (i.e., SPE with selective sorbents), which enormously simplifies the sample preparation procedure. 2.3.1 SOLID–LIQUID EXTRACTION As mentioned earlier, solid–liquid extra ction is probably the most widely used procedure in the analysis of pesticides in solid samples. Solid–liquid extraction includes various extraction techniques based on the contact of a certain amount of sample with an appropriate solvent. Figure 2.1 shows a scheme of the different steps Solvent Organic matter 5 4 1 2 3 A 6 FIGURE 2.1 Scheme of the different steps involved in the extraction of a target analyte A from a solid particle. ß 2007 by Taylor & Francis Group, LLC. that take place in a solid –liqu id extra ction procedu re and will infl uence the fi nal extractio n ef ficiency . In the fi rst stage (step 1), the solve nt must pene trate inside the pores of the samp le particula tes to achiev e desorp tion of the analyt es bound to mat rix active sites (ste p 2). Su bsequen tly, analytes ha ve to diff use throu gh the mat rix (step 3) to be d issolved in the extra cting solve nt (step 4). Again, the analytes must diffuse through the solve nt to leave the samp le po res (step 5) an d be finally swe pt away by the exter nal solve nt (ste p 6). Obvi ously, the proper selection of the solve nt to be used is a key factor in a soli d–liquid extractio n procedu re. However , other parameter s such as press ure and temperat ure have an imp ortant in fluence on the extractio n ef ficiency . Worki ng at high pressure facil itates the solvent to penetr ate sample pores (ste p 1) and, in general, incre asing temperat ure incre ases solub ility of the analytes on the solve nt. Moreo ver, high temperat ures incre ase diffusion coef fi- cients (steps 3 and 5) and the ca pacity of the solve nt to disr upt mat rix–analyt e interactions (step 2). Depending on the strength of the interaction between the analyte and the sample matrix, the extraction will be performed in soft, mild, or aggres sive condit ions. Table 2.1 shows a summary and a compa rison of drawbacks and advantages of the different solid–liquid extraction techniques (which will be described later) most commonly employed in the analysis of pesticides in food and environmental samples. 2.3.1.1 Shaking It is a very simple procedure to extract pesticides weakly bound to the sample and is very convenient for the extraction of pesticides from fruits and vegetables. It just involves shaking (manually or automatically) the sample in presence of an appro- priate solvent for a certain period of time. The most commonly used solvents are acetone and acetonitrile due to their miscibility with water making ease the diffusion of analytes from the solid sample to the solution, although immiscible solvents such as dichloromethane or hexane can also be used for the extraction depending on the properties of target analytes. In a similar manner, the use of mixtures of solvents is a typical practice when analytes of different polarity are extracted in multiresidue analysis. Once analytes have been extracted, the mixture needs to be filtered before further treatments. Besides, since volume of organic solvents used following this procedure is relatively large, it is usually necessary to evaporate the solvent before final determination. However, shaking might not be effective enough to extract analytes strongly bound to the sample. In order to achieve a more effective shaking, the use of ultrasound-assisted extraction is recommended. Ultrasound radiation provokes molecules vibration and eases the diffusion of the solvent to the sample, favoring the contact between both phases. Thanks to this improvement, both the time and the amount of solvents of the shaking process are considerable reduced. An interesting and useful modification for reducing both the amount of sample and organic solvents is the so-called ultrasound-assisted extraction in small columns proposed by Sánchez-Brunete and coworkers [1,2] for the extraction of pesticides from soils. Briefly, this procedure just involves placing the sample (~5 g) in a glass column equipped with a polyethylene frit. Subsequently, samples are extracted with ß 2007 by Taylor & Francis Group, LLC. TABLE 2.1 Solid–Liquid Extraction Techniques Technique Description Advantages Drawbacks Shaking Samples and solvent are placed in a glass vessel. Shaking can be done manually or mechanically . Simple . Filtration of the extract is necessary . Fast (15–30 min) . Dependent of kind of matrix . Low cost . Moderate solvent consumption (25–100 mL) Soxhlet Sample is placed in a porous cartridge and solvent recirculates continuously by distillation–condensation cycles . Standard method . Time-consuming (12–48 h) . No further filtration of the extract necessary . High solvent volumes (300–500 mL) . Independent of kind of matrix . Solvent evaporation needed . Low cost USE Samples and solvent are placed in a glass vessel and introduced in an ultrasonic bath . Fast (15–30 min) . Filtration of the extract is necessary . Low solvent consumption (5–30 mL) . Dependent of kind of matrix . Bath temperature can be adjusted . Low cost MAE Sample and solvent are placed in a reaction vessel. Microwave energy is used to heat the mixture . Fast (~15 min) . Filtration of the extract is necessary . Low solvent consumption (15–40 mL) . Addition of a polar solvent is required . Easily programmable . Moderate cost PSE Sample is placed in a cartridge and pressurized with a high temperature solvent . Fast (20–30 min) . Initial high cost . Low solvent consumption (30 mL) . Dependent on the kind of matrix . Easy control of extraction parameters (temperature, pressure) . High temperatures achieved . High sample processing Note: USE, Ultrasound-assisted extraction; MAE, microwave-assisted extraction; PSE, pressurized solvent extraction. ß 2007 by Taylor & Francis Group, LLC. around 5–10 mL of an appropriate organic solvent in an ultrasonic water bath. After extraction, columns are placed on a multiport vacuum manifold where the solvent is filtered and collected for further analysis. 2.3.1.2 Soxhlet Extraction As indicated earlier, in some cases shaking is not enough for disrupting interactions between analytes and matrix components. In this regard, an increase of the tempera- ture of the extraction is recommended. The more simple approach to isolate analytes bound to solid matrices at high temperatures is the Soxhlet extraction, introduced by Soxhlet in 1879, which is still the more used technique and of reference of the new techniques introduced during the last few years. Sample is placed in an apparatus (Soxhlet extractor) and extraction of analytes is achieved by means of a hot condensate of a solvent distilling in a closed circuit. Distillation in a closed circuit allows the sample to be extracted many times with fresh portions of solvent, and exhaustive extraction can be performed. Its weak points are the long time required for the extraction and the large amount of organic solvents used. In order to minimize the mentioned drawbacks, several attempts toward auto- mation of the process have been proposed. Among them, Soxtec systems (Foss, Hillerød, Denmark) are the most extensively accepted and used in analytical labora- tories and allow reducing the extraction times about five times compared with the classical Soxhlet extraction. Table 2.2 shows a comparison of the recoveries obtained for several pesticides in soils after extraction using different techniques. In this case, it is clear that ultrasound-assisted extraction allows the isolation of target analytes, whereas the TABLE 2.2 Recoveries (%) of Pesticides in Soils Obtained by Different Extraction Techniques Pesticide Concentration (mg=mL) Ultrasound-Assisted Extraction Soxhlet Extraction Shaking Atrazine 0.04 103.5 Æ 2.8 201.9 Æ 14.6 108.3 Æ 6.2 Pyropham 0.05 79.7 Æ 6.3 143.0 Æ 18.6 65.1 Æ 9.3 Chlorpropham 0.05 93.6 Æ 7.9 155.6 Æ 20.4 88.1 Æ 10.0 a-Cypermethrin 0.12 97.2 Æ 4.4 128.4 Æ 16.4 90.1 Æ 9.1 Tetrametrin 0.26 83.4 Æ 4.2 64.3 Æ 16.0 52.0 Æ 8.3 Diflubenzuron 0.02 92.8 Æ 4.0 182.5 Æ 17.4 98.1 Æ 8.9 Source: Reproduced from Babic, S., Petrovic, M., and Kastelan, M., J. Chromatogr. A, 823, 3, 1998. With permission from Elsevier. Experimental conditions: 10 g of soil sample spiked at indicated concentration level. Ultrasound-assisted extraction: 20 mL of acetone, 15 min; Soxhlet extraction: 250 mL of acetone, 4 h; Shaking: 20 mL of acetone, 2 h. ß 2007 by Taylor & Francis Group, LLC. simple shaking is not effective enough to extract the selected pesticides quantitatively. It is important to stress that recoveries after Soxhlet extraction were too high, which means that a large amount of matrix components were coextracted with target analytes. At this regard, it is clear that an exhaustive extraction is not always required and a balance between the recoveries obtained of target analytes and the amount of matrix components coextracted needs to be established. 2.3.1.3 Microwave-Assisted Extraction Microwave-assisted extraction (MAE) has appeared during the last few years as a clear alternative to Soxhlet extraction due to the ability of microwave radiation of heating the sample–solvent mixture in a fast and efficient manner. Besides, the existence of several instruments commercially available able to perform the sequen- tial extra ction of several samples (up to 14 samples in some instruments), allowing extraction parameters (pressure, temperature, and power) to be perfectly controlled, has made MAE a very popular technique. Microwave energy is absorb ed by molecules with high dielectric constant. In this regard, hexane, a solvent with a very low dielectric constant, is transparent to microwave radiation whereas acetone will be heated in few seconds due to its high dielectric constant. However, solvents with low dielectric constant can be used if the compounds contained in the sample (i.e., water) absorb microwave energy. A typical practice is the use of solvent mixtures (especially for the extra ction of pesticides of different polarity) combining the ability of heating of one of the components (i.e., acetone) with the solubility of the more hydrophobic compounds in the other solvent of the mixture (i.e., hexane). As an example, a mixture of acetone:hexane (1:1) was used for the MAE of atrazine, parathion- methyl, chlorpy- riphos, fenamiphos, and methidathion in orange peel with quantitative recoveries in <10 min [3]. As a summary, in general, the recoveries obtained are quite similar to those obtained by Soxhlet extraction but the important decrease of the extraction time (~15 min) and of the volume of organic solvents (25–50 mL) have made MA E to be extensively used in analytical laboratories. 2.3.1.4 Pressurized Solvent Extraction Pressurized solvent extraction (PSE), also known as accelerated solvent extraction (ASE), pressurized liquid extra ction (PLE), and pressurized fluid extraction (PFE), uses solvents at high temperatures and pressures to accelerate the extraction process. The higher temperature increases the extraction kinetics, whereas the elevated pressure keeps the solvent in liquid phase above its boiling point leading to rapid and safe extractions [4]. Figure 2. 2 shows a schem e of the inst rumentat ion and the procedu re used in PSE. Experimentally, sample (~10 g) is placed in an extraction cell and filled up with an appropriate solvent (15–40 mL ). Subsequently, the cell is heated in a furnace to the temperatures below 2008C, increasing the pressure of the system (up to a 20 Mpa) to perform the extraction. After a certain period of time (10–15 min), ß 2007 by Taylor & Francis Group, LLC. the extract is directly transferred to a vial without the necessity of subsequent filtration of the obtained extract. Then, the sample is rinsed with a portion of pure solvent and finally, the remaining solvent is transferred to the vial with a stream of nitrogen. The whole process is automated and each step can be programmed, allowing the sequential unattended extraction of up to 24 samples. This technique is easily applicable for the extraction of pesticides from any kind of sample and the high temperature used allows to perform very efficient extraction in a short time. In addition, the considerable reduction in the amount of organic solvents used makes PSE a very attractive technique for the extrac- tion of pesticides. The main limitations of this technique are the high cost of the apparatus and the unavoidable necessity of purifying obtained extracts, which is common to other efficient extraction techniques based on the use of organic solvents as mentioned earlier. 2.3.2 SUPERCRITICAL FLUID EXTRACTION Supercritical fluid extraction (SFE) has been widely used for the isolation of a great variety of organic c ompounds from almost any kind of solid samples. Supercritical fluids can be considered as a hybrid between liquids and gases, and possess ideal properties for the extraction of pesticides from solid samples. Supercritical fluids have in common with gases the ability to diffuse through the sample, which facilitates the extraction of analytes located in not easily accessible pores. In add- ition, the solvation power of supercritical flu ids is similar to that of liquids, allowing the release of target analytes from the sample to the fluid. Carbon dioxide has been widely used in SFE because it can be obtained with high purity, it is chemically inert, and its critical point (31.1 8C and 71.8 atm) is easily Oven Collection vial Extraction cell Solvent Pump Static valve Purge valve Nitrogen Load sample into cell. Fill cell with solvent. Heat and pressurize cell. Hold sample at pressure and temperature. Pump clean solvent into sample cell. Purge solvent from cell with N 2 gas. Extract ready for analysis. ASE ® Schematic 1-2 0.5 5 5 Total 12-14 Time (min) 0.5-1 FIGURE 2.2 Pressurized solvent extraction equipment. (Courtesy of Dionex Corporation. With permission.) ß 2007 by Taylor & Francis Group, LLC. accessible. Its main drawback is its apolar character, limiting its applicability to the extraction of hydrophobic compounds. In order to overcome, at least to a certain extent, this drawback, the addition of a small amount of an organic solvent modifier (i.e., methanol) has been proposed and permits varying the polarity of the fluid, thus increasing the range of extractable compounds. However, the role of the modifier during the extraction is not well understood. Figure 2.3 shows schematically the possible mechanisms taking place during the SFE of the herbicide diuron form soil samples using CO 2 as supercritical fluid modified with methanol [5]. Some authors propose that methanol molecules are able to establish hydrogen bonds with the phenolic moieties of the humic and fulvic acids present in soil samples and thus, diuron is displaced from active sites. However, other authors consider that the modifier is able to interact with target analyte releasing it from the sample. Once target analytes are in the supercritical fluid phase, they have to be isolated for further analysis, which is accomplished by decompression of the fluid through a restrictor by getting analytes trapped on a liquid trap or a solid surface. With a liquid trap, the restrictor is immersed in a suitable liquid and thus, the analyte is gradually dissolved in the solvent while CO 2 is discharged into the atmosphere. In the solid surface method, analytes are trapped on a solid surface (i.e., glass vial, glass beads, solid-phase sorbents) cryogenically cooled directly by the expansion of the super- critical fluid or with the aid of liquid N 2 . Alternatively, SFE can be directly coupled to gas chromatography or to supercritical fluid chromatography and is successful of such online coupling dependent of the interface used, which determines the quanti- tative transfer of target analytes to the analytical column [6]. As mentioned earlier, SFE has been widely used for the extraction of pesticides from solid samples; thanks to the effectiveness and selec tivity of the extraction and to the possibility of online coupling to chrom atographic techniques. However, H H O O O H H H + + O O O H H Cl Cl NNC CH 3 CH 3 CO 2 + CH 3 OH CH 3 OH CH 3 OH H O O O O H Supercritical fluid H H O O O H Cl Cl NNC CH 3 CH 3 O Cl Cl NNC CH 3 CH 3 CH 3 CH 3 CH 3 O FIGURE 2.3 Mechanisms of the extraction of the herbicide diuron from sediments by SFE (CO 2 þ methanol). (Reproduced from Martin-Esteban, A. and Fernandez-Hernando, P., Toma y tratamiento de muestra, Cámara, C., ed., Editorial Síntesis S.A., Madrid, 2002, Chap. 6. With permission from Editorial Síntesis.) ß 2007 by Taylor & Francis Group, LLC. [...]... 0.70–0.73 ß 20 07 by Taylor & Francis Group, LLC The number of developed methods based on SPE using polar sorbents for the determination of pesticides in food and environmental solid samples is huge, and thus, for specific examples, the interested reader should consult Chapters 6 through 8 of this book 2. 3.4 .2 Nonpolar Sorbents This kind of sorbent is appropriate for the trace-enrichment and cleanup of pesticides. .. polymerisation and its application to the clean-up of fenuron in plant samples, Anal Chim Acta, 4 82, 165, 20 03 12 Hennion, M.-C and Scribe, P., Sample handling strategies for the analysis of organic compounds from environmental water samples, in Environmental Analysis: Techniques, Applications and Quality Assurance, Barceló, D., Ed., Elsevier Science Publishers BV, Amsterdam, 1993, chap 2 13 Martin-Esteban,... amount of sorbent used, which will increase the number of interactions that take place A second option is the addition of salts to the sample, diminishing the solubility of target analytes (salting-out effect) and thus favoring their interactions with the sorbent Table 2. 4 shows the obtained recoveries of several triazines by the SPE of 1 L of water spiked at 1 mg=L concentration level of each analyte in. .. sorbents include TABLE 2. 4 Recoveries (R%) and Relative Standard Deviations (RSD) of Several Triazines Obtained by SPE of 1 L of LC Grade Water Spiked with 1 mg=L of Each Triazine 1 C18 Disk Without NaCl Triazine Desisopropylatrazine Desethylatrazine Simazine Atrazine 2 C18 Disk 10% NaCl Without NaCl 10% NaCl R% RSD R% RSD R% RSD R% RSD 21 .5 50.4 100 .2 94.6 18.6 9.3 6.1 8.7 42. 3 98.4 93.5 98.3 13.6 6 .2 7.8... preconcentration of organic compounds in environmental water samples [ 12] The simplest way of SPE–LC coupling is shown in Figure 2. 7, where a precolumn (1 2 cm 3 1–4.6 mm i.d.) filled with an appropriate sorbent is inserted in the loop of a six-port injection valve After sorbent conditioning, the sample is loaded by a low-cost pump and the analytes are retained in the precolumn Then, the precolumn is connected online... pesticides in polar liquid samples (i.e., environmental waters) Traditionally, n-alkyl-bonded silicas, mainly octyl- and octadecyl-silica, both in cartridges or disks, have been used due to its ability of retaining nonpolar and moderate polar pesticides from liquid samples Retention mechanism is based on van der Waals forces and hydrophobic interactions, which allows handling large sample volumes and the... P., and Cámara, C., Baker’s yeast biomass (Saccharomyces cerevisae) for selective on-line trace enrichment and liquid chromatography of polar pesticides in water, Anal Chem., 69, 326 7, 1997 14 Pichon, V., Chen, L., and Hennion, M.-C., On-line preconcentration and liquid chromatographic analysis of phenylurea pesticides in environmental water using a silica-based immunosorbent, Anal Chim Acta, 311, 429 ,... determination of atrazine and four organophosphorus pesticides in oranges by gas chromatography (GC), Fresenius J Anal Chem., 367, 29 1, 20 00 4 Björklund, E., Nilsson, T., and Bøwadt, S., Pressurised liquid extraction of persistent organic pollutants in environmental analysis, Trends Anal Chem., 19 (7), 434, 20 00 5 Martin-Esteban, A and Fernandez-Hernando, P., Preparación de la muestra para la determinación... Acetonitrile 1-Butanol n-Propyl alcohol Isopropyl alcohol Ethanol Methanol 0.00 0.00–0.01 0.01 0.04 0.17–0.18 0 .26 0 .20 –0.30 0.30–0.31 0. 32 0.38 0.36–0. 42 0.36–0.40 0.44–0.49 0.51 0.56–0.58 0.56–0.61 0.45–0. 62 0.3–0. 62 0.58–0. 62 0. 62 0.75 0. 52 0.65 0.7 0.78–0. 82 0.78–0. 82 0.88 0.95 0.00 0.00–0.01 0.01 0.03 0.11 — 0 .22 0 .23 0 .25 0.38–0.43 0. 32 0 .26 — — 0.47–0.53 0.49–0.51 0.53 0.48 0.38–0.48 — 0.50–0. 52 — —... desisopropyl-, desethyl-, and hydroxyatrazine were 16%, 46%, and 46%, respectively [7] In order to increase the efficiency and thus, the range of application, the partition coefficients may be increased by using mixtures of solvents, changing the pH (preventing ionization of acids or bases), or by adding salts (‘‘salting-out’’ effect) At this regard, the recoveries for the atrazine degradation products of the . 2 Sample Handling of Pesticides in Food and Environmental Samples Esther Turiel and Antonio Martín-Esteban CONTENTS 2. 1 Introduction 36 2. 2 Sample Pretreatment 36 2. 2.1 Drying 37 2. 2 .2 Homogenization. commonly employed in the analysis of pesticides in food and environmental samples. 2. 3.1.1 Shaking It is a very simple procedure to extract pesticides weakly bound to the sample and is very convenient. sample drying. Thus, studies on stability of target analytes in spiked samples should be carried out to guarantee the integrity of the sample before final determin- ation of the analytes. 2. 2 .2 HOMOGENIZATION As