научная статья по теме LOW DENSITY SOLVENT BASED DISPERSIVE LIQUID–LIQUID MICROEXTRACTION AND PRECONCENTRATION OF MULTIRESIDUE PESTICIDES IN ENVIRONMENTAL WATERS FOR LIQUID CHROMATOGRAPHIC ANALYSIS Химия

Текст научной статьи на тему «LOW DENSITY SOLVENT BASED DISPERSIVE LIQUID–LIQUID MICROEXTRACTION AND PRECONCENTRATION OF MULTIRESIDUE PESTICIDES IN ENVIRONMENTAL WATERS FOR LIQUID CHROMATOGRAPHIC ANALYSIS»

ЖУРНАЛ АНАЛИТИЧЕСКОМ ХИМИИ, 2015, том 70, № 10, с. 1040-1048

ОРИГИНАЛЬНЫЕ СТАТЬИ

УДК 543

LOW DENSITY SOLVENT BASED DISPERSIVE LIQUID-LIQUID MICROEXTRACTION AND PRECONCENTRATION OF MULTIRESIDUE PESTICIDES IN ENVIRONMENTAL WATERS FOR LIQUID CHROMATOGRAPHIC ANALYSIS © 2015 Tesfa Bedassa*, Abera Gure*, **, Negussie Megersa*, 1

*Department of Chemistry, Addis Ababa University P.O. Box 1176; Addis Ababa, Ethiopia 1E-mail: megersane@yahoo.com, negussie.megersa@aau.edu.et **Department of Chemistry, Jimma University P.O. Box 378; Jimma, Ethiopia Received 28.03.2014; in final form 14.03.2015

A simple, efficient and selective sample preparation technique using low density based dispersive liquid-liquid microextraction has been developed for the analysis of nine multiresidue pesticides inculding six sulfonylurea and three organophosphorus pesticides in environmental waters by HPLC-diod array detector. Various experimental parameters affecting the extraction efficiency were investigated. Under the optimum experimental conditions, matrix-matched calibration curves were established in groundwater and good linearities were obtained with coefficient of determination (r2) of 0.990 or better. The limits of detection and quantification were in the ranges 0.8-3.3 and 2.5-11.0 p.g/L, at a signal-to-noise ratio of 3 and 10, respectively. The relative standard deviations of the precision studies were varied over the range of 0.2-13%. The proposed method was successfully applied to selective extraction of the target pesticide residues in different environmental waters and acceptable recoveries, in the range 81-121%, were obtained.

Keywords: LD-DLLME, multiresidue pesticides, environmental waters, trace level enrichment, liquid chro-matographic analysis.

DOI: 10.7868/S0044450215100187

Pesticides comprise a large number of substances that belong to varying chemical groups with common characteristics, used to control pests and prevent diseases in order to increase agricultural yields [1]. Only small proportion of the applied pesticides, i.e., less than 0.1% of the applied pesticides actually reach the targeted pests, while the remaining could get their ways to reach the other environmental compartments, including environmental waters (ground and surface waters) [2, 3]. Consequently, these compounds have been detected in all types of water circulating in the ecosystem, creating a potential source of exposure to the aquatic lives and health of all living organisms. Due to their toxicity and also bioaccumulation, regular monitoring of pesticides in the environmental waters is mandatory [4].

Most pesticides are volatile and thermally stable and therefore are amenable to gas chromatography [5, 6]. However, thermally unstable and polar/ionic pesticides are usually analysed by HPLC mainly with diode array detector (DAD) [6—8]. Liquid chromatography—

mass spectrometry (LC—MS) methods [9—11] offering high sensitivity and higher degree of selectivity have also been used increasingly during the last few years. However, LC—MS instruments are very expensive and unavailable in most common laboratories. Therefore, developing selective and sensitive analytical methods that utilize HPLC-DAD for routine analyses of pesticide residues seem attractive and promising, particularly in the laboratories of the developing world.

Due to the occurrence of the pesticide residues in trace levels and complexity of the real environmental water matrices, analysis of pesticide residues requires sample preparation, which is required to isolate (extract) and preconcentrate the target analytes from sample matrices prior to instrumental analysis. Traditional sample preparation techniques such as liquid—liquid extraction [12] and solid phase extraction [13—16] have been the most commonly used techniques for the determination of multiresidue pesticides in environmental waters. But, these methods have several inherent drawbacks including

longer extraction time, labor intensiveness and the requirement of large quantities of expensive and toxic organic solvents, and so on [17, 18].

During the past few years, dispersive liquid—liquid microextraction (DLLME) has replaced traditional sample preparation procedures and thus extensively employed for extraction and preconcentration of mut-liresidue pesticides in various environmental matrices, including contaminated waters [17—21]. Though, DLLME has enormous advantages such as simplicity of operation, rapidity, low cost, high recoveries, high enrichment factor and being environmentally benign [22], the choice of extraction solvents is generally limited to solvents with higher density than water such as halogenated hydrocarbons. But, these solvents are potentially toxic to human beings and the environment [17, 23]. To overcome these drawbacks, several attempts have recently been done to use organic solvents ,lighter in density than water and reported their several applications in DLLME technique, for analysis of chlorophe-nols [24—26], triazines [27], organochlorine pesticides [28—30], carbamate pesticides [31—33], carbamate and organophosphorus pesticides [34] in various aqueous matrices. However, to date there is no report on the use of low density based dispersive liquid—liquid microextraction (LD-DLLME) procedure, in combination with HPLC-DAD, for multiresidue analysis of sulfo-nylurea and organophosphorus pesticides in environmental water samples.

Therefore, in this study, a LD-DLLME combined with HPLC-DAD has been proposed for trace level enrichment of nine multiresidue pesticides: six sulfonylurea including prosulfuron (PS), metsulfuron-me-thyl (MSM), nicosulfuron (NS), flazasulfuron (FS), chlorimuron-ethyl (CSE) and rimsulfuron (RS), and three organophosphorus insecticides such as methi-dathion (Meth), diazinon (Diaz) and chlorpyrifos (Chlor) in environmental waters. Various parameters affecting the extraction efficiency and separation of the target analytes were studied and the optimal conditions have been established. The analytical performances and possible applications in environmental waters were also investigated in order to confirm reliability of the method with the target analytes.

EXPERIMENTAL

Chemicals and reagents. All chemicals used in this study are of analytical grade reagents while the solvents were of HPLC grade. Methanol and acetonitrile were purchased from Carlo Erba Reactifs-SDS (Val de Revil, France) and Ashland chemical (S. Giuliano MI, Italy), respectively. Potassium dihydrogen phosphate (KH2PO4) was obtained from Fulka (Buchs, Switzerland). Dipotassium hydrogen orthophosphate, anhy-

drous (K2HPO4); sodium hydroxide (NaOH); sodium chloride (NaCl) and glacial acetic acid (CH3COOH) were purchased from BDH Laboratory supplies (Poole, England). Hydrochloric acid (HCl) was obtained from Sigma-Aldrich (St. Louris, MO, USA). Ultrapure water obtained after purification with double distiller, A8000 Aquatron water Still (Bibby Scientific Ltd., Staffordshire, United Kingdom) and deionizer Thermo Scientific Barnstead E-Pure™ (Thermo Fisher Scientific Inc., Italy) was used throughout the study. Whatman® filter paper, grade 1 and size 8.5 cm obtained from Whatman International Ltd. (Maidstone, England) was used for filtration of the water samples.

Analytical standards of prosulfuron (97.9%), metsul-furon-methyl (99.9%), nicosulfuron (99.7%), methi-dathion (97.9%), diazinon (99.9%) and chlorpyrifos (99.7%) were purchased from Sigma-Aldrich (St. Louis, MO, USA) while Flazasulfuron (99.3%) and chlorimu-ron-ethyl (98%) were from ChemService Inc. (West Chester, USA). Rimsulfuron (99%) was obtained from Dr. Ehrenstorfer (Augusburg, Germany). Methidathion (97.9%), diazinon (99.9%) and chlorpyrifos (99.7%) were purchased from Sigma-Aldrich (St. Louis, MO, USA). The chemical structures, common names, abbreviations and the octanol—water partition coefficient (logP; at pH 7 and 20°C) of the target pesticides are given in the Scheme. Individual stock standard solutions containing 1000 mg/L and intermediate working solution containing 20 mg/L of each analyte were prepared in acetonitrile. All standard solutions were stored in a refrigerator below 4°C.

Instruments and equipment. Chromatographic analyses were performed using HPLC system, Agilent 1200 series (Agilent Technologies, Waldbronn, Germany) equipped with quaternary pump (flow range 0.2—10 mL/min), vacuum degasser, standard and preparative autosampler, thermostated column compartment, autosampler thermostat and diode array multiple wavelength detector. Sample processing and data acquisitions were performed using LC Chemstation software, (B.02,01-SR1). Chromatographic separations were carried out using Nucleosil C18 column (250 x 4.6 mm i.d., 5 ^m particle size and 10 nm pore size) obtained from Phenomenex (Torrence, CA, USA).

A pH meter, Adwa, model 1020, Adwa Hungary Kft. (Szeged, Hungary) was used for measurement of pH. An ultrasonic heater, Dacon®, Dacon laboratories Ltd. (St. Hove, East Sussex); centrifuge, Model 800, Jiangsu Zhenji insturuments Co., Ltd. (Jiangsu, China) and 15 mL centrifuge tube, corning incorporated (Corning, NY, Mexico) were used during sample preparation.

3 XyPHAtf AHAtfHTH^ECKOH XHMHH tom 70 № 10 2015

Chlorimuron-ethyl (CSE): pKa 4.2, logP 0.11

Metsulfuron-methyl (MSM): pKa 3.3, logP-1.7

CF3O

Г O O N

S* O Л

'/ NH NH N

O

Flazasulfuron (FS): pKa 4.37, log P -0.06

CF3

O O

S. A . O NH NH N O

N^N

ЛЛ

Prosulfuron (PS): pKa 3.76, log P 1.5

CH

Çs

O

,O

O

A.

N

Л

с II NH NH N O

// Vi

O O

Rimsulfuron (RS): pKa 4.0, log P-1.46

O

O

O

N

ii o H

N NH NH \O

OS

Nicosulfuron (NS): pKa 4.6, logP 0.61

Cl>

Cl

r

sYY

O,

)

Chloropyrifos (Chlor): pKa 4.55, logP 4.7

N о

>

N

S

Diazinon(Diaz): pKa 2.6, logP 3.69

s-f

XOA ,N.

O N

O

/V

S

Methidathion (Meth): logP 2.57

Scheme. The chemical structures, common names, abbreviations and octanol—water partition coefficients (log P at pH 7 and 20°C) of the target

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