relevant aspects in the determination of pesticides in to the context of the analytical...
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AbstractIn this chapter we discuss some important aspects
related to the context of the analytical determination of pesticides in food by chromatographic techniques. Top-ics explored a brief introduction to the topic, emphasizing the importance of monitoring of pesticides in foods. Next we discuss the major sample preparation methods used, the gas chromatography technique (GC) and liquid (LC) in the detection and quantification and finally, the prob-lem of pesticide metabolites and breakdown products.
IntroductionThe determination of pesticides, especially in food
matrices, is a topic of great relevance. Since hundreds of pesticides are applied in several crops to combat pos-sible pests that may affect agricultural production [1-3]. Whereas that the presence of amounts in levels of traces, of both pesticide residues and their degradation products are potential causes of health risks, these must be con-trolled and monitored. Therefore, many countries include these analytes as hazardous pollutants to human health. Therefore, monitoring of pesticide residues in food is of great interest to ensure food security. For this reason, nu-merous regulations of various organizations such as the European Union directives established maximum residue limits (MRL) of pesticides in fruits, vegetables, cereals, water and other foods [2-5].
Usually, we can refer to pesticides through a common name or a business name. The formulated product, which
Relevant Aspects in the Determination of Pesticides in FoodsRonaldo Ferreira do Nascimento*, Ftima Itana Chaves Custdio Martins, Jhonyson Arruda Carvalho Guedes, Vtor Paulo Andrade da Silva and Pablo Gordiano Alexandre Barbosa
Department of Chemical analytical and physical-chemical, Federal University of Cear, Brazil
*Corresponding Author: Ronaldo Ferreira do Nascimento, Department of Chemical analytical and physical-chemical, Federal University of Cear, Brazil, Email: email@example.com
First Published July 02, 2016
Copyright: 2016 Ronaldo Ferreira do Nascimento, et al.
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is marketed, comprises one or more active ingredients and formulates or excipient substances.The active ingredient is the main component of the product and whose effect is attached to the pesticide. That is, the substance that has an effect on the target organism. The formulates substances are other compounds added as solvents to dilute the ac-tive ingredient function.They also serve to improve some physical and chemical characteristics of the active ingredi-ent, such as the solubility (by cosolvence), spraying capac-ity or stability.
Pesticides can be applied in the form of powder (sol-id), liquid (spray, emulsion, solution) or in gas form (fu-migant). Some are sold ready for use, others require prior preparation (e.g.: dilution).
Pesticides can be classified based on several criteria, including the chemical nature, the chemical group to which it belongs, the target organism, the level of toxicity, time or mode of action, persistence, etc.
As for chemical nature, pesticides can be classified into two major groups: inorganic and organic.
The inorganic usually present as fine powder of crys-talline appearance are stable in the environment and can easily dissolve in water. Are derived from minerals, may be made based on boron, antimony, lime, sulfur, lead or cadmium, for example. The use of inorganic pesticides re-mote antiquity. Its use has been reduced considerably with the rise of organic.
Organic pesticides are characterized by the presence of carbon in their molecular structure and can be natu-rally occurring (extracted from sources like plants) or synthetic (produced in our laboratory).
According to the chemical structures, these com-pounds can be divided into chemical groups such as: tria-zines, carbamates, neonicotinoid, organochlorines, or-ganophosphates, pyrethrins, acetanilides, dinitroanilines, phenylureas, etc. An example for organophosphates and triazines can be seen in Figure 1.
Figure 1: Example for triazines and organophosphates.Organic pesticides have varied and relatively complex
molecular structures, and may present in the same mole-cule many functional groups. Consequently, it is expected
a great variability in physicochemical properties and how these compounds interact with the components that sur-round (matrix).
Usually, the physical and chemical properties most used to characterize a pesticide are: vapor pressure, wa-ter solubility (S), octanol/water partition coefficient (Kow), dissociation constant (pKa) and DT 50 (degradation time) [7,8].
In the table below (Table 1) we can see the wide varia-tion in these characteristics between pesticides.
Table 1: Physical and chemical properties of some pesticides.
VP: Vapour pressure at 25 C; S: Solubility in water at 20 C; Kow: Octanol/water partition coefficient at pH7,20 C; pKa:dissociation constant at 25 C; DT 50: typical degra-
dation time (aerobic).
Taking as an example the solubility, we observed val-ues ranging from 9x10-3 to 6,2x104mg L-1corresponding to paraquat and cypermethrin respectively. In some cases, the pesticide is liable suffer dissociation and not in others.
Because of great diversity of structures and phys-icochemical characteristics, the nature of intermolecular interactions involved with matrix may also vary widely. These interactions can be ionic, hydrogen bond, covalent bonds, dipole, and van der Waal forces, hydrophobic in-teractions or partitioning. Two or more types of interac-tion can simultaneously occur between the same molecule and the matrix .
These are particular characteristics and often so di-vergent that hinder the laboratory analysis of monitoring pesticide residues in food.
Another factor is the very different chemical compo-sition among the many types of food (e.g.: are each acids, other fatty, others have many pigments) that interferes with the extraction and analysis by matrix effect.
Thus, methods of analysis should be assessed and ap-propriate case by case in order to obtain reliable results.
Methods of Sample Preparation for Determination of Pesticide Residues in Food Matrices
The determination of pesticides residues in food ma-trices is a difficult task because of the analytes usually
Pesticide Group VP (mPa) S (mg mL-1)
Kow at pH 7, 20 C
pKa (25C) DT 50 in soil (day)
Atrazine Triazine 0.039 35 5.01x102 1.7 752,4-dichlorophenoxya-cetic acid
24300 1.51x10-1 3.40 4.4
Paraquat Bipyridylium 0.01
Methiocarb Carbamate 1.50x10-2 27 1.51x103 No dissoci-ation
Chlorpyrifos Organophosphate 1.43
Malathion Organophosphate 3.1
148 5.62x102 No dissoci-ation
MCPA Aryloxyalkanoicacid 0.4 29390 1.55x10-1 3.73 24Cypermethrin Pyrethroid 0.00023 0.009 2.00x105 No dissoci-
present very low concentrations, distinct chemical prop-erties, complexity of matrices. Thus is necessary a prelimi-nary stage of sample preparation. Moreover, given the fact that the measurements are typically made at low concen-tration levels, interferences are frequent issues that should be considered[10-12] Thus, samples typically cannot be directly analyzed by analytical instruments requiring a prior sample preparation. Therefore, the sample prepara-tion aims to isolate/concentrate/extract analytes in a given matrix, moreover, it is important to obtain a fraction of the sample interfering free.
Mills and co-workers developed the first method for pesticides multiresidue extraction in 1963, in the labora-tory of the Food and Drug Administration (FDA). The method is based on an extraction with acetonitrile is used basically in the determination of nonpolar organochlorine compounds in non-greasy samples[13-14]. Through the years, there were many sample preparation procedures for pesticide extraction, among the most common include: Storherr Method[10,13], Luke method [10,13,15], mini-Luke extraction method, this last one has a miniaturiza-tion of the original Luke method .
Many of sample preparation procedures are carried out by conventional techniques such as liquid-liquid ex-traction (LLE) and other techniques mentioned above. However, they have the disadvantages of being expensive and uses large amounts of organic solvents, which are tox-
ic for the analyst and may contaminate the environment, since they are generated large amounts of waste. These limitations leading to the development of new and more convenient techniques, as they consume less organic sol-vents and have the ability to detect analytes at low concen-tration levels [10,16-18]. Thus, during the 1990s due to the strong environmental pressures and the factors associated with the human health, efforts in the fie