Determination of multi-class herbicides in soil by liquid–solid extraction coupled with headspace solid phase microextraction method J. Serb. Chem. Soc. 81 (8) 923–934 (2016) UDC 631.4+632.954:543.544.3:543.51+66.061 JSCS–4897 Original scientific paper 923 Determination of multi-class herbicides in soil by liquid–solid extraction coupled with headspace solid phase microextraction method RADA ĐUROVIĆ-PEJČEV1*#, TIJANA ĐORĐEVIĆ1 and VOJISLAVA BURSIĆ2# 1Institute of Pesticides and Environmental Protection, Banatska 31b, P. O. Box 163, 11080 Belgrade, Serbia and 2Faculty of Agriculture, University of Novi Sad, Trg Dositeja Obradovića 8, 21000 Novi Sad, Serbia (Received 23 November 2015, revised 21 April, accepted 13 May 2016) Abstract: Described is a method for simultaneous determination of five herb- icides (metribuzin, acetochlor, clomazone, oxyfluorfen and dimethenamid) belonging to different pesticide groups in soil samples. Developed headspace solid phase microextraction method (HS-SPME) in combination with liquid– –solid sample preparation was optimized and applied for the analysis of agri- cultural samples. Optimization of microextraction conditions, such as tempe- rature, extraction time and sodium chloride content was performed using 100 μm polydimethyl-siloxane (PDMS) fiber. The extraction efficiencies of meth- anol, methanol:acetone and methanol:acetone:hexane in 1:1 and 2:2:1 volume ratios, respectively, and the optimal number of extraction steps during the sample preparation, were tested as well. Gas chromatography–mass spectro- metry was used for detection and quantification, obtaining relative standard deviation (RSD) below 13 %, and recovery values higher than 83 % for mul- tiple analyses of soil samples fortified at 30 μg kg-1 of each herbicide. Limits of detection (LOD) were less than 1.2 μg kg-1 for all the studied herbicides. Keywords: pesticides; soil matrix; multiresidue method; gas chromatography mass spectrometry. INTRODUCTION In modern agricultural production, the use of herbicides for weed control is necessary and essential. United States Environmental Protection Agency (EPA) indicated that in 2006 and 2007 the worldwide pesticide usage was approx- imately 5.2 billion pounds, of which herbicides constituted the majority at 40 %.1 Usually, crops are showing larger sensitivity to weeds at the beginning of the growing period, due to their slower growth and lower density during that stage. * Corresponding author. E-mails: rajcica76@gmail.com; Rada.Djurovic@pesting.org.rs # Serbian Chemical Society member. doi: 10.2298/JSC151123044D _________________________________________________________________________________________________________________________ (CC) 2016 SCS. All rights reserved. Available on line at www.shd.org.rs/JSCS/ 924 ĐUROVIĆ-PEJČEV et al. Therefore, application of herbicides is required. Various pre-emergence weed control herbicides can be applied to different crops, including widely used: dimethenamid (used in corn, soybeans, sunflower, sugar beet and potatoes), ace- tochlor (in corn, sunflower, soybeans and potatoes), metribuzin (in soybeans, pot- atoes, tomatoes and peppers), clomazone (in soybeans, tobacco and rapeseed) and oxyfluorfen (in sunflower). Soil-applied pre-emergence residual herbicides, especially those used prior to sowing, are usually incorporated into the soil. Slow degradation of pesticides in the environment and extensive or inappropriate usage by farmers could lead to soil contamination. Due to the outstanding concern for human health, and con- sidering the manner and amount of herbicide application, programs monitoring soil contamination by agrochemicals have been established throughout the world, as well in Serbia. Since the herbicides are a very heterogeneous group of chemicals with dif- ferent physicochemical properties, the current trend in residue analysis of these compounds is developing multi-residual methods that would provide for simul- taneous determination of large number of compounds. In addition, these methods should overcome the drawbacks of the traditional approaches, which are labor- ious, time consuming, expensive, require large amounts of organic solvents and usually involve many steps, leading to loss of analyte quantity. Solid phase mic- roextraction (SPME), as a technique that combines extraction, purification and concentration processes into a single step, is an example of such development. Until recently, most of the SPME applications for determination of herbicide residues in soil were based on preparation of soil mixtures with distilled water and subsequent immersion of the SPME fiber in thus prepared slurry (DM-SPME)2–6 or its exposing to a gas phase above the slurry (HS-SPME)7–9. Some researchers have suggested that combination of liquid–solid (L–S) soil preparation followed by DM-SPME determination of herbicides in obtained extracts is the most reliable soil SPME method.10–15 However, previous studies based on a combination of L–S extraction and HS-SPME determination of herbicides in soil samples were done only with triazines (simazine, atrazine and prometryn)16,17 and mixture of the two pyridazinones (chloridazon and fluorochloridone) and pendimethalin as dinit- roaniline herbicide.18 In those studies, methanol–acetone combination was used for the extraction of pesticides from the soil matrix. Regarding determination of herbicides belonging to various pesticide groups, there are no published methods based on a combination of L–S procedure fol- lowed by simultaneous HS-SPME herbicides determination. Therefore, the aim of this study was to develop a rapid and simple HS-SPME method combined with L–S sample preparation for simultaneous determination of five compounds (metribuzin, acetochlor, clomazone, oxyfluorfen and dimethenamid) having dis- tinct chemical structures and belonging to different herbicide groups. Microext- _________________________________________________________________________________________________________________________ (CC) 2016 SCS. All rights reserved. Available on line at www.shd.org.rs/JSCS/ MULTI-CLASS HERBICIDES IN SOIL 925 raction temperature, time and NaCl content, main parameters affecting SPME, were tested and optimized using a 100 µm long PDMS fiber. The extraction effi- ciencies of pure methanol, methanol:acetone and methanol:acetone:hexane mix- tures (1:1 and 2:2:1 volume ratios, respectively) were optimized, as well as the optimal number of extraction steps within sample preparation stage. The pro- posed method was applied in the analysis of selected agricultural soil samples. EXPERIMENTAL Reagents and materials The herbicides chosen for this study were: metribuzin, acetochlor, clomazone, oxy- fluorfen and dimethenamid (Dr Ehrenstorfer, Germany, Table I). Standard stock solutions containing 1 g dm-3 of each herbicide were prepared in acetone (J. T. Baker, Holland), and stored at –18 °C. Standard working mixed solutions were prepared weekly by diluting the individual stock solutions with acetone and stored at 4 °C. Sodium chloride of 99.5 % purity was purchased from Merck (Germany) and methanol from J. T. Baker (Holland). TABLE I. Physicochemical properties of herbicides studied;19,20 Mr, molecular weight; Sw, water solubility; log Kow, partition coefficient between n-octanol and water; H, Henry’s constant Herbicide Chemical class Mr / g mol -1 Sw / mg dm -3 log Kow H / Pa m3 mol-1 Metribuzin Triazinone 214.3 1050 1.6 1×10-5 Acetochlor Chloroacetamide 269.8 223 4.14 0.383 Clomazone Isoxayolidinone 239.7 1100 2.5 4.19×10-3 Oxyfluorfen Diphenyl ether 361.7 0.116 4.47 9.40×10-2 Dimethenamid Chloroacetamide 275.8 1200 2.15 8.32×10-3 PDMS fibers (Supelco, USA), 100 µm long, were used for SPME measurements. Extraction, along with constant mixing was performed in 4 cm3 vials (Supelco, USA). An uncontaminated soil sample originating from region of the town of Kikinda (Serbia) was used in the study. The main physicochemical properties of the soil were: pH (in H2O) 8.39, organic matter content, 3.17 %, sand content, 73.96 %, silt content, 22.60 % and clay content, 3.44 % (all as mass %). The soil was air-dried and sieved (2 mm pores) before use. Polypropylene centrifuge tubes with caps (50 cm3, Sarstedt, Germany), filter papers 1PS, 150 mm diameter (Watman, UK) and a centrifuge (UZ 4, Slovenia) were used in the soil preparation procedure. Instrumentation A gas chromatograph–mass spectrometer (GC–MS), model CP-3800/Saturn 2200 (Var- ian, Australia) was used for separation and detection. Column VF-5ms having dimensions 30 m×0.25 mm×0.25 µm by Varian was used. The thermal desorption of analytes from PDMS fiber was performed for 7 min at injector temperature of 270 °C. The GC was programmed as follows: initial temperature was 120 °C, followed by increase to 170 °C at 8 °C min-1 rate, kept constant for 4.5 min, increased to 280 °C at 9 °C min-1 rate and kept at the same temperature for 5.5 min. Helium was used as a carrier gas and its flow rate was 1.1 mL min-1. The ion trap mass spectrometer operated in the electron impact/selected ion monitoring (EI/SIM) mode. The ion trap and transferline temperatures were set to 220 °C and 250 °C, respectively. One specific herbicide ion was selected for detection and quantification, while _________________________________________________________________________________________________________________________ (CC) 2016 SCS. All rights reserved. Available on line at www.shd.org.rs/JSCS/ 926 ĐUROVIĆ-PEJČEV et al. the second one was used for confirmation. The ions inspected were as follows: 198 (215) for metribuzin, 223 (146) for acetochlor, 204 (125) for clomazone, 252 (317) for oxyfluorfen and 154 (230) for dimethenamid. Optimization of HS–SPME analysis HS–SPME conditions, such as temperature, extraction time and NaCl content were tested and optimized using 100 µm PDMS fiber. Optimization was done using 2.5 cm3 of aqueous solution containing 25 µg dm-3 of each herbicide. Desorption parameters (temperature and time) for this study were initially selected according to previous research.13 Optimization of soil sample preparation The efficiency of the HS-SPME method, optimized for aqueous solutions, was tested using analysis of soil samples. For that part of the study, 10 g of sub-samples were placed in the polypropylene centrifuge tubes and fortified to concentration of 30 µg kg-1 for each herb- icide using 1 mg dm-3 mixed standard solution. The spiked samples were homogenized for 15 min using a rotary stirrer and left to rest for 24 hours prior to further analysis. The extraction efficiencies of the pure methanol, methanol:acetone (1:1 volume ratio) and methanol:ace- tone:hexane (2:2:1 volume ratio) mixtures and the optimal number of extraction steps were determined by the following procedure: soil samples were extracted by dissolving in 15 cm3 of solvent for 30 min using a rotary stirrer and then centrifuged for 15 min at 4000 rpm. The extract was filtered and evaporated to dryness at 35 °C using a rotary evaporator. The residues were redissolved in 1 cm3 of acetone, and 0.2 cm3 of those solutions were diluted with water to 10 cm3 for HS-SPME measurements. RESULTS AND DİSCUSSİON HS-SPME optimization Selected 100 µm PDMS fiber for SPME measurements in this multiresidue herbicides analysis, as well as the desorption time of 7 min and 270 °C desorp- tion temperature as optimal conditions for the used fiber, were chosen according to the results presented in detail in previous study.13 Optimal microextraction temperature, NaCl content and microextraction time, as experimental parameters affecting HS-SPME measurements, were optimized by a well-structured step-by- step approach, using spiked water samples. Microextraction temperature It is well known that the increase of microextraction temperature leads to increase of analyte vapor pressure, resulting in improvement of HS-SPME effi- ciency.21,22 Therefore, the temperature effect in the range of 23–90 °C on the HS- -SPME efficiency was analyzed. The obtained extraction-temperature profiles for each of the herbicide studied are shown in Fig. 1. The results presented, clearly indicate that the temperature increase leads to the enhancement of the overall sorbed mass on the fiber, while other experimental conditions were kept constant. This effect could be explained by the increase of analyte vapor pressure, i.e., higher concentration in the gas phase. However, for temperatures above 75 °C for oxyfluorfen, further increase in temperature results in a reduction of herbicide _________________________________________________________________________________________________________________________ (CC) 2016 SCS. All rights reserved. Available on line at www.shd.org.rs/JSCS/ MULTI-CLASS HERBICIDES IN SOIL 927 amount sorbed on the fiber (Fig. 1). Explanation for such a behaviour lies in the exothermic nature of the sorption process (enhanced analyte desorption from the fiber at high temperatures) and in the very low solubility of the oxyfluorfen in water (weak solvent–analyte interactions cannot be further weakened by the tem- perature increase). 20 40 60 80 0 7000 14000 21000 31500 Pe ak a re a, c ou nt s t / 0C Clomazone Dimethenamid Metribuzin Acetochlor Oxyfluorfen Fig. 1. HS-SPME temperature pro- files for herbicides studied. Since satisfactory sensitivity for each of the individual compounds studied was obtained at 75 °C, this temperature was chosen as optimal for the mixture of herbicides tested. Effect of ionic strength As previously reported, an addition of salt to the sample could decrease the solubility of some analytes in the aqueous solution, stimulating their movement into the gas phase and consequently to the fiber coating.22 This is especially pro- nounced in the case of hydrophobic compounds that have low affinity for the PDMS fibers. Therefore, the ionic strength was a well studied experimental para- meter influencing the HS-SPME measurements. Ionic strength was adjusted by adding different amounts of NaCl to the standard herbicide aqueous solutions (0, 25, 50, 100, 150, 200, 250 and 300 g dm–3). As shown in Fig. 2, the obtained results indicate that the ionic strength inc- reases the SPME efficiency for all the herbicides studied. Also, it is evident that for the most hydrophobic herbicide studied (oxyfluorfen) the enhancement of mass sorbed on the fiber at higher NaCl concentrations is significantly less pro- nounced. A possible explanation for this behaviour of oxyfluorfen could be the presence of a strong competition between this analyte and the more polar ones for PDMS fiber sorption that finally results in a minor increase in its extraction efficacy. _________________________________________________________________________________________________________________________ (CC) 2016 SCS. All rights reserved. Available on line at www.shd.org.rs/JSCS/ 928 ĐUROVIĆ-PEJČEV et al. Fig. 2. Effect of ionic strength on the analytical signal for studied herbicides. Finally, based on the results obtained for all the herbicides studied, NaCl concentration of 300 g dm–3 was chosen for further work. Microextraction time Some theoretical models proposed for explanation of the HS-SPME process recommended shortening duration time of the analysis by indicating that quan- tification is possible before a sorption equilibrium could be reached.21,23 Although microextraction using equilibrium time is advised, for practical reasons an effi- cient half-hour microextraction (enough to provide sufficient analytical sensi- tivity for all the compounds studied and in accordance with the chromatographic run time of 28.47 min), was compared only to 20 min procedure. The results obtained (Fig. 3) indicate that the time period of 30 min was a better choice for all the herbicides studied, and therefore it was chosen for further work. Overall, considering the results obtained for all the parameters optimized for HS-SPME determination of studied herbicides, the following SPME conditions _________________________________________________________________________________________________________________________ (CC) 2016 SCS. All rights reserved. Available on line at www.shd.org.rs/JSCS/ MULTI-CLASS HERBICIDES IN SOIL 929 were found to be the most efficient: temperature of 75 °C, 300 g dm–3 NaCl con- tent and 30 min extraction time. 1 2 3 4 5 0 225000 450000 675000 900000 Pe ak a re a, c ou nt s Herbicide 20 min 30 min Fig. 3. Effect of microextraction time on HS-SPME determination of clomazone (1), dimethenamid (2), metribuzin (3), acetochlor (4) and oxyfluorfen (5). Soil extraction optimization Optimized HS-SPME method was tested by analyzing soil samples. Lite- rature reports on the analysis of herbicide residues from soil samples, using DM- SPME of a soil organic extract obtained by L–S extraction of previously diluted samples is a more efficient method than direct immersion of the SPME fiber in the slurry of soil sample and water.10–15 Previous study, based on the combination of conventional L–S procedure followed by DM-SPME determination of the sel- ected herbicides, showed that among different solvents tested (water, hexane, acetonitrile, acetone and methanol), two successive extractions with methanol (Met) as the extraction solvent seemed to be the optimal sample preparation choice.13 In the same study, somewhat lower recovery obtained for oxyfluorfen (62.82 %) was explained by insufficient power of methanol as an extraction sol- vent in the sample preparation step and/or the strong influence of soil matrix on this herbicide. Considering the obtained results and aiming for an improvement of the sample preparation step, the extraction efficiency of methanol was compared to the efficiencies obtained by combining it with solvents of different polarity (methanol:acetone (Met:Ac) and methanol:acetone:hexane (Met:Ac:Hex) and employing a single extraction procedure as described in Experimental – Optimiz- ation of soil sample preparation section. The results (Table II) show that both solvent mixtures considerably amended the sample preparation step. The highest recoveries for majority of the tested herbicides were obtained after extraction with Met:Ac, therefore this solvent mixture was selected for future experiments. _________________________________________________________________________________________________________________________ (CC) 2016 SCS. All rights reserved. Available on line at www.shd.org.rs/JSCS/ 930 ĐUROVIĆ-PEJČEV et al. TABLE II. Dependence of liquid-solid (LS) extraction efficiency on type of organic solvent (Met: methanol, Ac: acetone, Hex: hexane) and number of extraction steps (I-IV), using the most efficient solvent (Met:Ac) Herbicide Met I Met:Ac I Met:Ac:Hex I Met:Ac II Met:Ac III Met:Ac IV Metribuzin 98.41 99.33 99.02 102.94 101.62 102.09 Acetochlor 91.39 94.68 95.24 98.61 98.86 96.02 Clomazone 80.13 87.76 81.32 91.65 91.03 89.78 Oxyfluorfen 59.36 79.68 65.78 84.26 81.38 82.69 Dimethenamid 71.31 80.22 75.64 83.91 83.06 83.11 After selection of extraction solvent, the next step was to determine the opti- mal number of extraction steps. For that purpose, the extraction of spiked soil samples with methanol-acetone mixture was repeated up to four times using the same procedure. The results presented in Table II show that, for majority of the herbicides studied, the best recovery was achieved after two extraction steps. According to the results obtained in those two sets of experiments, two suc- cessive extractions with Met:Ac as the extraction solvent were chosen as the optimal sample preparation procedure. As clearly indicated in Fig. 4, the additional SPME step in our sample pre- paration method has real advantages as shown by comparative chromatograms obtained by direct injection (Fig. 4C) of the extract (after liquid-solid extraction) and after additional HS-SPME purification and concentration (Fig. 4A) of the same extract. The chromatogram obtained after DM-SPME of the selected herbi- cides in soil extract is presented in Fig. 4B, too. The choice of experimental con- ditions for DM-SPME determination was based on our previous investigation.13 Evidently, compared to L–S method (Fig. 4C), both additional SPME steps (HS- -SPME (Fig. 4A) and DM-SPME (Fig. 4B)) provide higher sensitivity in deter- mination of all herbicides, and HS-SPME approach is slightly better. Since the SPME fiber was not in direct contact with unwanted impurities (compounds that were co-extracted from the soil matrix during LS sample preparation) for HS-SPME application, but was for DM-SPME application, the obtained results were expected. Validation of the proposed method The most important analytical parameters, such as linearity, limit of detec- tion (LOD), precision and confidence of the presented method were determined for the optimized LS extraction procedure followed by HS-SPME measurement. Concentration, ranging from 2 to 600 μg kg–1 was used for linearity testing of the developed method. The obtained arrangements and correlation coefficients (R) for all the herbicides under study are presented in Table III. The acquired correlation coefficients exceeded 0.99 for all the compounds tested, indicating good linearity. _________________________________________________________________________________________________________________________ (CC) 2016 SCS. All rights reserved. Available on line at www.shd.org.rs/JSCS/ MULTI-CLASS HERBICIDES IN SOIL 931 Fig. 4. GC–MS chromatograms of soil sample fortified at 30 µg kg-1 level of each herbicide (cloma- zone (1), dimethenamid (2), ace- tochlor (3), metribuzin (4) and oxyfluorfen (5)), obtained by applying: A) L–S–HS-SPME method, B) L–S–DM-SPME method and C) L–S method (dir- ect injection without additional SPME step). _________________________________________________________________________________________________________________________ (CC) 2016 SCS. All rights reserved. Available on line at www.shd.org.rs/JSCS/ 932 ĐUROVIĆ-PEJČEV et al. The limit of detection (LOD) was determined as 3.29×sB (where sB is the blank standard deviation), according to IUPAC recommendations.24 Obtained LOD for all the herbicides studied were less than 1.2 µg kg-1 (Table III). TABLE III. Analytical characteristics of the proposed LS–HS-SPME method for all the herbicides studied; R, correlation coefficient; LOD, limit of detection; RSD, relative standard deviation; Linearity range: 2–600 µg kg-1 Herbicide R LOD / µg kg-1 RSD / % Recovery, % Metribuzin 0.995 1.16 8.1 102.94 Acetochlor 0.992 0.34 12.1 98.61 Clomazone 0.996 0.10 6.3 91.65 Oxyfluorfen 0.993 0.59 3.8 84.26 Dimethenamid 0.998 0.87 10.2 83.91 Confidence and precision of the method were determined by performing four consecutive measurements of the soil samples fortified at 30 µg kg-1 level. RSD and recovery values are presented in Table III. As shown, RSD for all herbicides were below 13 %. As RSD below 20 % are considered acceptable in trace anal- ysis,25 the proposed method can be satisfactory in terms of precision. For all the herbicides studied, the recovery values were above 83 %, indicating that the pro- posed method could be used for efficient determination of the selected herbicides from complex matrix samples such as soil. Application of the L–S–HS-SPME method L–S–HS-SPME method proposed in this study, as well as L–S–DM-SPME method proposed in our previous study,13 were used for analysis of twelve soil samples from Belgrade agricultural area. Quantification was done using spiked soil samples that were used in the optimization procedure. The obtained results showed that with the exception of acetochlor, detected in only one sample (con- centration of 13.0 µg kg-1 determined using L–S–HS-SPME and 13.2 µg kg-1 by L–S–DM-SPME method), all the other herbicides remained below detection limits. The effectiveness of both L–S–HS-SPME and L–S–DM-SPME methods for routine analysis of real samples were confirmed by excellent agreement with the obtained results. However, as soil represents extremely complex matrix, it seems that HS-SPME approach is more appropriate for herbicide residue analysis, con- sidering that in this method the fiber is not in direct contact with the sample, which enables an extension of its lifespan and reduces the matrix effects. Accept- able precision and results repeatability obtained with the same SPME fiber during experiments presented in this paper showed that a single PDMS fiber could be utilized for more than 130 measurements. On the other hand, our pre- vious studies showed that the same PDMS fiber could be used for fewer inject- ions (about 70–80 times) when DM-SPME mode was used for determination of pesticides in the soil matrix.13 _________________________________________________________________________________________________________________________ (CC) 2016 SCS. All rights reserved. Available on line at www.shd.org.rs/JSCS/ MULTI-CLASS HERBICIDES IN SOIL 933 CONCLUSION The multi-residue method based on a combination of liquid–solid sample preparation followed by HS-SPME herbicide determination was used for the simultaneous determination of five herbicides belonging to different pesticide groups. Investigation and optimization of microextraction conditions, such as temperature, extraction time and NaCl content was performed using 100 μm polydimethyl–siloxane (PDMS) fiber. During the optimization, the extraction efficiencies of several solvents and the optimal number of extraction steps for sample preparation were tested as well. Results indicated that two successive extractions with mixture of methanol:acetone (1:1 volume ratio) as the extraction solvent could be used as optimal sample preparation procedure. Subsequently for the HS-SPME method, temperature of 75 °C, 300 g dm–3 NaCl content and 30 min microextraction time could be set as microextraction conditions to yield most effective analysis. Comparing the results obtained by application of the developed HS-SPME method and those published for application of DM-SPME method for the same set of herbicides and soils under study, we inferred that both methods are suitable for the routine determination of selected herbicides in the soil samples. However, since the proposed HS-SPME mode secures an extended fiber lifetime, compared to the DM-SPME mode, it could be more appropriate for analyzing complex soil matrix. Acknowledgment. This study was carried out as part of the projects No TR31043 and III43005, supported by the Ministry of Education, Science and Technological Development of the Republic of Serbia. И З В О Д МЕТОДА МИКРОЕКСТРАКЦИЈА У ЧВРСТОЈ ФАЗИ–УЗОРКОВАЊЕ ИЗ ГАСОВИТЕ ФАЗЕ У ОДРЕЂИВАЊУ ХЕРБИЦИДА ИЗ РАЗЛИЧИТИХ ХЕМИЈСКИХ ГРУПА У УЗОРЦИМА ЗЕМЉИШТА РАДА ЂУРОВИЋ-ПЕЈЧЕВ1, ТИЈАНА ЂОРЂЕВИЋ1 и ВОЈИСЛАВА БУРСИЋ2 1Институт за пестициде и заштиту животне средине, Банатска 31б,11080 Београд, 2Пољопривредни факултет, Универзитет у Новом Саду, Трг Доситеја Обрадовића 8, 21000 Нови Сад У раду је представљена метода за истовремено одређивање пет хербицида (метри- бузин, ацетохлор, кломазон, оксифлуорфен и диметенамид) у земљишту, који на основу своје структуре припадају различитим хемијским групама пестицида. Предложена метода микроекстракција у чврстој фази-узорковање из гасовите фазе (HS-SPME) у ком- бинацији са течно–чврстом припремом узорака земљишта (L–S) је оптимизована и при- мењена за анализу реалних узорака пољопривредног земљишта. Ортимизовање микро- екстракционих услова, као што су температура, екстракционо време и садржај натри- јум-хлорида је извршена употребом 100 μm полидиметил-силоксанског (PDMS) влакна. Испитиване су такође екстракционе ефикасности различитих растварача (метанол, метанол:ацетон и метанол:ацетон:хексан у запреминским односима 1:1 и 2:2:1, редом), као и оптималан број екстракционих корака у току припреме узорака земљишта. Дет- _________________________________________________________________________________________________________________________ (CC) 2016 SCS. All rights reserved. Available on line at www.shd.org.rs/JSCS/ 934 ĐUROVIĆ-PEJČEV et al. екција и квантификација испитиваних хербицида су извршени методом гасно–масене спектрометрије (GC–MS). Вредности релативних стандардних девијација и приноса одређивања хербицида у узорцима земљишта обогаћеним до концентрација од 30 μg kg-1 сваког једињења су биле испод 13 %, односно изнад 83 %, редом, док су границе детек- ције биле ниже од 1,2 μg kg-1. (Примљено 23. новембра 2015, ревидирано 21. априла, прихваћено 13. маја 2016) REFERENCES 1. United States Environmental Protection Agency (US EPA), http://www.epa.gov/pes- ticides/pestsales/07pestsales/usage2007.htm (accessed on 23/09/2015) 2. M. Möder, P. Popp, E. Eisert, J. Pawliszyn, Fres. J. Anal. Chem. 363 (1999) 680 3. C.G. Zambonin, F. Palmisano, J. Chromatogr., A 874 (2000) 247 4. S. Bengtsson, T. Berglöf, J. Agric. Food Chem. 44 (1996) 2260 5. H. Prosen, S. Fingler, L. Zupančiċ-Kralj, V. Drevenkar, Chemosphere 66 (2007) 1580 6. A.A. Boyd-Boland, S. Magdic, J.B. Pawliszyn, Analyst 121 (1996) 929 7. J. Castro, R.A. Perez, C. Sanchez-Brunete, J.L. Tadeo, Chromatographia 53 (2001) S-361 8. M. Fernandez-Alvarez, M. Llompart, J. 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