Rapid Detection of Atrazine and Metolachlor in Farm Soils: Gas

Jul 11, 2014 - Rapid Detection of Atrazine and Metolachlor in Farm Soils: Gas ... University of Johannesburg, P.O. Box 524, Auckland Park 2006 South A...
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Rapid Detection of Atrazine and Metolachlor in Farm Soils: Gas Chromatography−Mass Spectrometry-Based Analysis Using the Bubble-in-Drop Single Drop Microextraction Enrichment Method D. Bradley G. Williams,*,†,‡ Mosotho J. George,†,§ and Ljiljana Marjanovic† †

Department of Chemistry, University of Johannesburg, P.O. Box 524, Auckland Park 2006 South Africa Ferrier Research Institute, Victoria University of Wellington, Gracefield Research Centre, 69 Gracefield Road, Lower Hutt 5010, New Zealand § Department of Chemistry and Chemical Technology, National University of Lesotho, Roma 180, Lesotho ‡

S Supporting Information *

ABSTRACT: Tracking of metolachlor and atrazine herbicides in agricultural soils, from spraying through to harvest, was conducted using our recently reported “bubble-in-drop single-drop microextraction” method. The method showed good linearity (R2 = 0.999 and 0.999) in the concentration range of 0.01−1.0 ng/mL with LOD values of 0.01 and 0.02 ng/mL for atrazine and metolachlor, respectively. Sonication methods were poor at releasing these herbicides from the soil matrixes, while hot water extraction readily liberated them, providing an efficient accessible alternative to sonication techniques. Good recoveries of 97% and 105% were shown for atrazine and metolachlor, respectively, from the soil. The spiking protocol was also investigated, resulting in a traceless spiking method. We demonstrate a very sensitive technique by which to assess, for example, the length of residence of pesticides in given soils and thus risk of exposure. KEYWORDS: herbicide, microextraction, spiking method, agriculture, recovery

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INTRODUCTION Pesticide monitoring in agricultural soils is vital not only in relation to environmental or health concerns but also to provide useful information to farmers for crop rotation planning. The use of metolachlor as a pre-emergent herbicide for grass and broad-leaf weed control is widespread, while atrazine, often applied postemergence in the control of broadleaf weeds, is equally popular as a farming aid. These two herbicides are the subjects of several health concerns. The US EPA classifies metolachlor as a possible human carcinogen, and is similarly reflected by the WHO to be responsible for proliferative liver lesions (combined neoplastic nodules and carcinomas) in rats.1 This compound shows moderate toxicity in various fish species.2 The US EPA considers atrazine and its metabolites desethylatrazine and desisopropylatrazine to be endocrine disruptors.3 Atrazine is also suspected to cause hermaphroditism in male frogs, leading to lower reproduction and posing a threat to biodiversity.4 Tracking such materials in the environment and in farmed areas is of clear importance. Some reports on the monitoring of these herbicides in agricultural soils are contradictory. Hartzler5 notes that atrazine binds more strongly to soils than metolachlor, which contrasts a report by Singh6 in which the opposite is claimed. To complicate matters, leaching of pesticides from soils depends on a number of factors, including organic matter content, pH, cation exchange capacity, and mineral composition of the soil.7 Degradation of these herbicides under different conditions has also been studied.8−12 Most of the detection and quantification methods on which these reports rely are chromatography-based hyphenated methods such as HPLC−MS or GC−MS. Such methods © 2014 American Chemical Society

combine the ability of the detector not only to quantify the molecules in question but to also provide structural information necessary for identification, and the ability of the chromatographic technique to separate complex mixtures of substances. The extraction and preconcentration protocols used in conjunction with these methods are essential to achieving high levels of confidence in data related to low levels of pesticides and their breakdown products. To this end, the miniaturization of extraction methods has led to the development and use of solid-phase microextraction,13 dispersive solid-phase microextraction,14 the QuEChERS method,15 liquid-phase microextraction,16 dispersive liquid−liquid microextraction,17 single-drop microextraction,18 and an elegant variation of a gas−liquid droplet system recently detailed by Cao,19 in various applications. Recently, we introduced a modified version of single-drop microextraction termed “bubble-in-drop single-drop microextraction” (BIDSDME) in which a known volume of air is introduced into the micro droplet resulting in a bubble therein, which dramatically improves the efficiency of extraction of triazines.20 The application of this new BID-SDME technique in the detection and tracking of metolachlor and atrazine from agricultural soils is reported here. Additionally, hot water and superheated water extractions were also used for the effective release of these herbicides from the soil particles prior to BID-SMDE, where widely reported sonication21,22 methods provide poorer release. Received: Revised: Accepted: Published: 7676

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temperature increased by 50 °C/min to 200 °C, then ramped by 10 °C/min to 280 °C and held for 5 min. The total time for one GC run was 17 min. The MS (EI 70 eV, 200 °C ion source temperature, 1.5 kV detector potential) was set up in scanning mode with mass range m/z 50−350 mass units. The ions of interest were extracted out of the total ion chromatogram as follows: m/z 162 (166, 4 × 2H), 169, 172, 187, and 200 (205, 5 × 2H) for metolachlor, diphenylamine, desethylaztrazine, desisopropylatrazine, and atrazine, respectively (the values in the parentheses represent the 2H analogues). For quantitative analysis at trace level (lower ng/mL range), the MS was set to the selected ion monitoring (SIM) mode using the same ions as listed above together with their qualifying ions (m/z 215 and 238 for atrazine and metolachlor, respectively); otherwise, the MS was always used on scanning mode using the extracted ion monitoring (EIM) facility. Microextraction Procedure. The setup for the BID-SDME extraction protocol was reported in detail in our earlier paper.20 Briefly, 1 μL of the extracting solution was drawn into the syringe, followed by 0.5 μL of air. These contents were introduced into the aqueous solution by gentle depression of the plunger, causing the air to form a bubble contained within the microdroplet. Following a period of 20 min (under static conditions), the total solvent volume was carefully retracted into the syringe, and injected into the GC−MS. Performance of the Analytical Method for Metolachlor. Only a limited amount of development and validation work was carried out as part of the present study, given the backdrop of our prior work.20 The main thrust included an assessment of the chromatographic separation of metolachlor in the same TP-619 mixture of 10 triazines reported in our earlier study.20 The extraction efficiency was evaluated using three aqueous solutions (prepared in 10% aqueous NaCl) at concentrations of 1, 50, and 100 ng/mL for each pesticide together with the corresponding organic solutions. Linearity and LOD were determined using aqueous mixtures of atrazine and metolachlor in the concentration range 0.01−1.0 ng/mL. Evaluation of Extraction Recovery. Soil samples were air-dried, ground, and sieved through a 0.0035 in. (0.089 mm) mesh. Subsamples (2 g) were wet with 1 mL of HPLC grade water and spiked with 50 μL of a 1 μg/mL solution of the 2H-standard so as to prepare concentrations of 25 ng/g of the pesticide standards in the soil samples. The suspension was allowed to air-dry. Following complete drying, the soil residue was suspended in 2 mL of hot water to dissolve and release the spiked herbicides. The extracts were then subjected to the BID-SDME extraction protocol as described above. The effect of organic solvents in spiking was also evaluated using different spiking solutions, being 100% water, 3:1 water/MeOH, 1:1 water/MeOH, 1:3 water/MeOH, and pure MeOH, respectively. The extraction values from these solutions, especially those from the herbicides existing in the soil from the farming activities (metolachlor and atrazine), were compared to those obtained with the spiked HPLC grade water (spiked to the same concentration as the samples). All soil samples were sampled in triplicate and each sample analyzed in triplicate, so every analysis was carried out for n = 9 as a minimum. Following assessment of recovery, quantitation of the amounts of the herbicides in the soil was calculated from the calibration curve using the extraction efficiency factor.

Finally, a traceless method of spiking is investigated, after a finding that solvent-based spiking of standards led to significant influences on the release of the analytes from the soil.

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MATERIALS AND METHODS

General. Apparatus used in this study includes: Zx3 vortexer (Velp Scientifica, Italy); sonicator (FungiLab, Barcelona, Spain); centrifuge (Eppendorf centrifuge 5415D, Hamburg, Germany); gastight Hamilton GC syringes (Seelze, Germany); gas chromatograph−mass spectrometer (Shimadzu QP2010, Kyoto, Japan) equipped with a Zebron 35MS column with 30 m × 0.25 mm × 0.25 μm dimensions (Phenomenex, Torrance, U.S.). The pesticides metolachlor (Met), atrazine (Atrz), and their deuterated (2H) analogues (each being 100 μg/mL in 1 mL ampules) were obtained from Dr. Ehrenstorfer GmbH (Augsburg, Germany). A 10-component standard mixture of triazines, mixture TP-619, was obtained from Chem Service (PA) (prometron, atraton, atrazine, propazine, simazine, terbuthylazine, prometryn, ametryn, simetryn, and terbutryn with individual triazine concentrations of 500 mg/L). Diphenylamine, desethylatrazine, and desisopropylatrazine (100 μg/ mL) were purchased from Chem Service (PA), while chloroform, methanol, and water (all HPLC grade) were purchased from Riedel de Haën (Seelze, Germany). Sodium chloride (AR grade) was obtained from Sigma-Aldrich (Seelze, Germany). Helium (99.999%) was from Afrox, South Africa. The agricultural soil samples were collected from a farming area in the North West Province near the small town of Derby about 150 km West of Johannesburg, South Africa. In each case, collections were made at various depths (0−5 cm, 20−25 cm cm), and three samples of each were collected from a single agricultural field at distances 15−20 m apart. The soil in the field is classified as red-brown humic sandy soil with approximately 20% silt content, pH 6, cation exchange capacity of around 70 mmol/kg, and bulk density around 1300 kg/m3.23 Standard Solutions. The H-standards of 100 μg/mL atrazine and metolachlor were combined, diluted to 1 μg/mL in MeOH, and kept in a freezer at −18 °C. The deuterated (2H) standards for the same herbicides were prepared and stored similarly. The atrazine breakdown products desethylatrazine and desisopropylatrazine were also mixed with atrazine and metolachlor to prepare a 1 μg/mL standard mixture in MeOH, and the mixture was kept in the same freezer. The working solutions were prepared by dilution of these stock solutions. Reference solutions were prepared by dilution of the stock solution with methanol and were injected directly (1 μL) into the instrument. For the extraction procedure, 10% NaCl aqueous solutions were used, in light of the efficiencies gained using this medium as determined in our previous work.20 A 100 ng/mL diphenylamine solution was prepared by dilution from a 100 μg/mL solution with chloroform. This was used as the extracting solvent for the BID-SDME method with the diphenylamine employed as internal standard. Preparation of Soil Samples. Soil suspensions were prepared by adding 2 g of air-dried soil to 2 mL of water (1 g/mL). This suspension was treated by vortexing it at 30 Hz, followed by sonication for 15 min, and thereafter centrifugation at 8165g. Given portions of the clear supernatant liquid were transferred into a gas chromatograph (GC) vial containing 0.1 g of NaCl, after which BID-SDME extraction was applied as previously described.20 Different extraction methods were used to solubilize the pesticides from the soil, and the results were compared to room temperature sonication. These techniques included heating the suspension, or using boiling/hot water to prepare the suspension, as well as superheating the suspension in a closed vessel. GC−MS Setup. A calibrated gastight syringe (10 μL) was used for sampling and injections. Analyses were carried out using a gas chromatograph coupled to a mass spectrometer (GC−MS). Pure helium at a constant flow rate of 1 mL/min was used as the carrier gas. Injections (1 μL) were carried out in the splitless mode; after 2 min, a split ratio of 1:10 was maintained throughout the runs. The injector and transfer line were maintained at 250 °C. The oven programming included an initial temperature of 100 °C (held for 4 min), the

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RESULTS AND DISCUSSION Assessment of Linearity of the Extraction Method for Metolachlor and Atrazine. The linearity of the method was determined in the concentration range 0.01−1 ng/mL by BIDSDME extraction under optimum conditions established for triazines20 and analysis using GC−MS: 1 mL of a 10% aqueous NaCl solution of the herbicides was extracted for 20 min with a 1 μL chloroform droplet containing diphenylamine as an internal standard, and a 0.5 μL air bubble, with no stirring. The calibration curves give the following data for atrazine, R2 0.999 and LOD 0.013 ng/L, and for metolachlor, R2 0.999, LOD 0.024 ng/L. LOD values were determined using LOD = 3 × 7677

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standard deviationIntercept/slope24 (see also Supporting Information, Figure S3). For purposes of comparison, LOD values for the US FDA Method 302 for metolachlor and atrazine are both 0.010 μg/mL,25 while the LOD values applied in the EPA Method 507 from water samples are 0.19 and 0.015 ng/mL for metolachlor and atrazine, respectively.26 The data above indicate that the present approach falls well within the FDA and EPA method requirements for testing these analytes, while following an easy-to-use protocol. Indirectly, the data in Table 1 also fall within these parameters: these were generated from liquid extracts of the soils and converted into mass-basis information.

explored: the use of vortexing, sonication, hot water (adding freshly boiled water to the sample), heating the sample to boiling, hot water + sonication, and superheated water. The superheated water scenario was achieved by heating the sample in a sealed small stainless steel autoclave to a temperature of 140 °C. In all cases, the allocated time for extraction was 20 min. The results are given below (Figure 2).

Table 1. Hot Water Extraction and Analysis of Herbicides at 25 cm Soil Depth for Different Dates week no.

metolachlor (ng/g)a

atrazine (ng/g)

9 10 11 15 26

n.d.b n.d. 4 4 2

n.d. n.d. 3 1.5 n.d.

Figure 2. Evaluation of different extraction methods for a given soil sample.

As can be seen in Figure 2, only metolachlor was detected using vortexing or sonication-assisted extraction followed by BID-SDME and GC−MS analysis. However, when using the other more vigorous extraction methods with higher temperatures, atrazine was detected. As can be seen in Figure 2, hot water (96 °C) and heating the sample in water to boiling do not offer much difference in extraction efficiency. Also, sonication is no longer important when using hot water extraction. Although superheated water extraction gave higher extraction of both analytes (Figure 2), this method was not considered useful for wide application due to the difficulty in sample handling and the high pressure equipment required. Having shown quite good improvements to the solubilization protocols when employing hot water (96 °C) extraction, it was decided to revisit the analysis of the soil samples with the view to improving sensitivity for the detection of metolachlor in the latter part of the sampling period and reassessing the samples for the presence of atrazine. Figure 1 shows the dissipation profiles obtained for the two extraction methods, room temperature sonication versus hot water (96 °C) extraction. Room temperature sonication had extracted only metolachlor (between weeks 2 and 9), while the hot water (denoted in Figure 1 legend as HWE) extracted both analytes, thereby allowing their detection. The improvements secured with hot water extraction are clearly evident with respect to metolachlor with extractability rising from about 20 to 90 in relative abundance (representing about a 4.5-fold improvement in the amount extracted). Given the improvements achieved in detecting both metolachlor and atrazine, the subsurface layers of soil were also assessed for the presence of the two herbicides of interest there. Table 1 below shows results obtained using samples lying about 25 cm deep and using different time periods following spraying. Evidently, neither herbicide suffers significant leaching from the top layer into lower soil strata as an intact entity, because they were present only at low levels in the subsurface soil. This is a contrast to prior work, which describes metolachlor as “transient” (meaning substantial leaching, in the context of the cited work) in soils, leading to diminished herbicidal efficacy.29 One of the two major breakdown products of atrazine, desethylatrazine, could be detected at low levels when

a

The calculation is based on the outcomes of the later section on quantification. bn.d. means not detected.

Analysis of Soil Samples. Monitoring of Soil Pesticide Concentration over Time. Following method refinement, the dissipation of the herbicides over time in agricultural soil samples was investigated, making use of samples collected from the upper 5 cm of the soil. Interviews with the farmer had revealed that metolachlor had been sprayed on the farmland in week 1 while atrazine had been sprayed on the farmland during week 4 (the fourth week postspraying of metolachlor). The collected samples were sonicated27,28 for 15 min, after which they were subjected to BID-SDME. GC−MS analysis of extracts made from farm soils over a period of 15 weeks postspraying showed only the presence of metolachlor (Figure 1: the “sonication” curve shows the depletion of metolachlor

Figure 1. Dissipation−time profile of the herbicides in farmland soil over a 15 week period using sonication and hot water extraction (HWE).

over time). Discovering that neither atrazine nor its breakdown products desethylatrizine and desisopropylatrazine could be detected, we investigated more forcing dissolution methods than sonication (see below). Throughout and for consistency, subsamples of a given soil sample were employed. Different extraction conditions were 7678

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reported in literature is carried out using an organic solvent in which the surrogates are dissolved, and the influence of the spiking solvent is often overlooked. Traceless Spiking Method. The composition of the spiking solution was found to influence the extractability of analytes. To avoid this variable, it was decided to use strictly aqueous solutions even for spiking. For this purpose, a 1 μg/mL aqueous spiking solution containing the surrogate deuterated samples was prepared by dilution of 100 μg/mL solutions into 1 mL with HPLC grade water. This solution (50 μL) was used to spike the soil samples (2 g, wet with 1 mL of HPLC grade water), vortexing to ensure equal distribution of the spiking solution over the soil sample and air drying as described above. The application of hot water extraction, BID-SDME, and GC− MS analysis afforded the set of results contained in Table 2, which demonstrate the recovery of the both the analyte and the surrogate herbicides from differently treated soils and from water used as a reference solution.

combining BID-SDME preconcentration with GC−MS analysis. No breakdown products for metolachlor could be identified in the extracts. Additional future work will provide robust data on the usefulness of the methods described here to the monitoring of breakdown products of these herbicides. Recovery of Herbicides from the Soil. While the data generated above provide good relative information, absolute data based on recovery experiments are important for the quantification of analytes. To assess the level of recovery, labeled analogues of the analytes of interest are usually spiked into the sample.30 Such labeled surrogate materials are readily quantified as separate entities by detectors such as mass spectrometers.31 Given their physical characteristics that are essentially identical to those of the analyte in question, this approach is widely used to establish quantitative recovery of analytes from samples.32 The spiking solution is usually added with the organic analyte dissolved in an appropriate organic solvent such as methanol,33,34 and the influence of the spiking solvent is ignored. In other instances, aqueous spiking solutions are used.35,36 Figure 3 shows the effect of the spiking solvent to recovery of the herbicides (see also Supporting Information, section S3).

Table 2. Recoveries of the Analytes (ng/g) from Different Samples Spiked with Aqueous Solutions atrazine water (ref) control soilb control treated soilc spiked soild % recovery

metolachlor

2

H-atrazine 34 (16)

3 (6)a 2.8 (8)a

13 (8)a 12.8 (5)a

3 (9)a 100%e

13 (5)a 100%e

a

33 (10)a 97%f

2

H-metolachlor 21 (12)a

22 (8)a 105%f

a

Values in parentheses are %RSD based on n = 9. bThe soil sample was used without spiking or suspension in water. cThe soil sample was wet with water to mimic spiking but without the use of the spiking solution. dThe spiking solution contained 2H-atrazine and 2Hmetolachlor. eThe % recovery was calculated on the basis of the control soil sample as the reference. fThe % recovery was calculated on the basis of the water sample as the reference.

These results show that water indeed provided a suitable vehicle for the deuterated surrogates, as the control, treated control, and spiked samples gave the same extractability for the H-analytes. The recoveries obtained for the two deuterated herbicides are 97% and 105%, respectively, for atrazine and metolachlor using the spiked water sample set at 100% for comparison. Quantitation of the Amounts of the Herbicides in the Soil. Having determined the recoveries of the analytes from the soil samples, the actual concentrations observed at different times were calculated as shown in Table 3. The soil samples were collected over several months from the upper 5 cm layer of soil, right up to the time of harvesting. The data clearly demonstrate that the method can be used to track the concentration of the herbicides in question in agricultural soils. In summary, this work demonstrates the applicability of the BID-SDME preconcentration technique as an efficient tool for application to the environmental monitoring of the herbicides atrazine and metolachlor using a method that demonstrates sub ng/mL LOD values (0.013 and 0.024 ng/mL, respectively). We identify highly influential effects of the spiking solvent used to prepare the surrogate solutions of the analytes of interest and the care that should be taken to ensure that traceless spiking is employed. Finally, the beneficial effect of hot water extraction over standard techniques such as sonication has been demonstrated. The data generated in this study provide a

Figure 3. Effect of solvent composition of the spiking solution on extraction.

Clearly, the spiking solvent has an effect on the release of the pre-existing herbicides as demonstrated by the varying extractabilities thereof. Concomitantly, the deuterated spiking surrogate materials showed little changes to their levels of extraction (with the exception of the use of 100% MeOH). Bearing in mind that only 50 μL of spiking solution was used with 2 g of soil, the influence is rather dramatic. Given the limited scope of our work in terms of soil types, it is not yet known whether this is a general phenomenon or not. Nevertheless, the implication of this exercise is that the suspension of the soils in methanol-containing solutions enhances the extractability of the analytes in question, despite the samples being air-dried and putatively solvent-free at the time of analysis. This implies that the method of choice for the application of spiking solutions to soil samples is to use aqueous spiking solutions where practicable and to allow the samples to dry completely before further manipulation. This organic solvent effect is consistent with the results obtained by Merini et al.,37 who reported recoveries of 49% when using an aqueous surrogate and 81% when using methanol surrogate for 2,4-dichlorophenoxyacetic acid. This observation is very important because most of the spiking 7679

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Table 3. Quantitative Data for Different Samplesa Obtained from the Calibration Curves Using Recovery Values week no.

atrazine

(4) Hayes, T.; Haston, K.; Tsui, M.; Hoang, A.; Haeffele, C.; Vonk, A. Atrazine-induced hermaphroditism at 0.1 ppb in American leopard frogs (Rana pipiens): laboratory and field evidence. Environ. Health Perspect. 2003, 111, 568−575. (5) Hartzler, B. Absorption of Soil-Applied Herbicides, Department of Agronomy; Iowa State University, 2002; http://www.weeds.iastate. edu/mgmt/2002/soilabsorption.htm (09 April 2014). (6) Singh, N.; Kloeppel, H.; Klein, W. Movement of metolachlor and terbuthylazine in core and packed soil columns. Chemosphere 2002, 47, 409−415. (7) Zambonin, C. G.; Palmisano, F. Determination of triazines in soil leachates by solid-phase microextraction coupled to gas chromatography-mass spectrometry. J. Chromatogr., A 2000, 874, 247−255. (8) Accinelli, C.; Dinelli, G.; Vicari, A.; Catizone, P. Atrazine and metolachlor degradation in subsoils. Biol. Fertil. Soils 2001, 495−500. (9) Kochany, J.; Maguire, R. J. Sunlight photodegradation of metolachlor in water. J. Agric. Food Chem. 1994, 42, 406−412. (10) Munoz, A.; Koskinen, W. C.; Cox, L.; Sadowsky, M. J. Biodegradation and mineralization of metolachlor and alachlor by Candida xestobii. J. Agric. Food Chem. 2011, 59, 619−627. (11) Krause, A.; Hancock, W. G.; Minard, R. D.; Freyer, A. J.; Honeycutt, R. C.; Lebaron, H. M.; Paulson, D. L.; Liu, S. Y.; Bollag, J. M. Microbial transformation of the herbicide metolachlor by a soil actinomycete. Environ. Toxicol. Water Qual. 1985, 33, 584−589. (12) Liu, D.; Maguire, R. J.; Pacepavicius, G. J.; Aoyama, I.; Okamura, H. Microbial transformation of metolachlor. Environ. Toxicol. Water Qual. 1995, 10, 249−258. (13) Frías, S.; Rodríquez, M. S.; Conde, J. E.; Pérez-Trujillo, J. P. Optimisation of a solid-phase microextraction procedure for the determination of triazines in water with gas chromatography-mass spectrometry detection. J. Chromatogr., A 2003, 1007, 127−135. (14) Di Muccio, A.; Pelosi, P.; Camoni, I.; Barbini, D. A.; Dommarco, R.; Generali, T.; Ausili, A. Selective, solid-matrix dispersion extraction of organophosphate pesticide residues from milk. J. Chromatogr., A 1996, 754, 497−506. (15) Payá, P.; Anastassiades, M.; Mack, D.; Sigalova, I.; Tasdelen, B.; Oliva, J.; Barba, A. Analysis of pesticide residues using the Quick Easy Cheap Effective Rugged and Safe (QuEChERS) pesticide multiresidue method in combination with gas and liquid chromatography and tandem mass spectrometric detection. Anal. Bioanal. Chem. 2007, 389, 1697−1714. (16) Rasmussen, K. E.; Petersen-Bjergaard, S.; Krogh, M.; Ugland, H. G.; Gronhaug, T. Development of a simple in-vial liquid-phase microextraction device for drug analysis compatible with capillary gas chromatography, capillary electrophoresis and high-performance liquid chromatography. J. Chromatogr., A 2000, 873, 3−9. (17) Rezaee, M.; Assadi, Y.; Hosseini, M.-R. M.; Aghaee, E.; Ahmadi, F.; Berijani, S. Determination of organic compounds in water using dispersive liquid−liquid microextraction. J. Chromatogr., A 2006, 1116, 1−9. (18) Jeannot, M. J.; Cantwell, F. F. Solvent microextraction into a single drop. Anal. Chem. 1996, 68, 2236−2240. (19) Xie, H.-Y.; Yan, J.; Jahan, S.; Zhong, R.; Fan, L.-Y.; Xiao, H.; Jin, H.-Q.; Cao, C.-X. A new strategy for highly efficient single-drop microextraction with a liquid-gas compound pendant drop. Analyst 2014, 139, 2545−2550. (20) Williams, D. B. G.; George, M. J.; Meyer, R.; Marjanovic, L. Bubbles in solvent microextraction: the influence of intentionally introduced bubbles on extraction efficiency. Anal. Chem. 2011, 83, 6713−6716. (21) Ozcan, S.; Tor, A.; Aydin, M. M. Application of miniaturised ultrasonic extraction to the analysis of organochlorine pesticides in soil. Anal. Chim. Acta 2009, 640, 52−57. (22) Lambropoulou, D. A.; Albanis, T. A. Determination of the fungicides vinclozolin and dicloran in soils using ultrasonic extraction coupled with solid-phase microextraction. Anal. Chim. Acta 2004, 514, 125−130.

metolachlor

Calculated Concentrations (ng/g)b,c 18 0.144 (2.9)c 1.585 (6.2)c 22 0.087 (8.4)c 1.295 (2.0)c c 26 0.048 (3.5) 1.231 (0.8)c c 30 0.031 (7.3) 0.930 (6.8)c Adjusted Concentration (ng/g) Using the Recovery Datad 18 0.15 ± 0.01 1.51 ± 0.13 22 0.09 ± 0.01 1.23 ± 0.04 26 0.05 ± 0.01 1.17 ± 0.02 30 0.03 ± 0.01 0.89 ± 0.11 a Soil samples were collected from the upper 5 cm of soil. bCalculated using the calibration equations: yMet = 47.966x + 3.8069 and yAtrz = 56.147x + 0.2193. cThe values in parentheses depict the %RSD. d Recovery values used: 97% (atrazine) and 105% (metolachlor).

good platform from which decision making as far as herbicide residues in the soils is concerned (environmental and farming perspective) can be performed. In the agricultural context, crop rotation is routinely performed, for example, for maintenance of soil productivity. Because crops show varied sensitivities toward given herbicides, intimate knowledge relating to herbicide residues may be critical information in deciding which crops may or should not be planted in a particular field. This work expands the BID-SDME and associated extraction protocols, which now underpin this method, to become more widely applicable in assessing the transport and fate of pesticides in the environment.

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ASSOCIATED CONTENT

S Supporting Information *

Miscellaneous data covering chromatographic and recovery experiments. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: (644) 463-0065. Fax: (644) 931-3055. E-mail: bradley. [email protected]. Funding

We gratefully acknowledge the University of Johannesburg, NRF, and THRIP for funding of this project. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the Derby farmer for allowing access to his farm and for information related to his spraying program. M.J.G. thanks the National University of Lesotho for study leave.

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REFERENCES

(1) WHO. Guidelines for Drinking-Water Quality, 2nd ed.; World Health Organization: Geneva, 1993; Chapter 3, Vol. 1. (2) Extoxnet, Extension Toxicology Network: Pesticide Information Profiles-Metolachlor, Oregon State University, http://extoxnet.orst. edu/pips/metolach.htm (09 April 2014). (3) EPA. Special Report on Environmental Endocrine Disruption: An effects Assessment Analysis, 1997 (EPA document number EPA/630/ R-96/012). 7680

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dx.doi.org/10.1021/jf502411t | J. Agric. Food Chem. 2014, 62, 7676−7681