AnalyticalMethods

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aSchool of Geographical Sciences, Universit

1SS, UK. E-mail: Becky.Harrison@bristol.ac

(0)117 331 7317bOrganic Geochemistry Unit, School of Che

1TS, UK

Cite this: Anal. Methods, 2013, 5, 2053

Received 16th November 2012Accepted 27th February 2013

DOI: 10.1039/c3ay26413k

www.rsc.org/methods

This journal is ª The Royal Society of

A method for the simultaneous extraction of sevenpesticides from soil and sediment

Rebecca Harrison,*a Ian Bullb and Katerina Michaelidesa

Organic pesticides are difficult compounds to extract from soils and sediments due to their strong affinity

for soil particulates. Different pesticide compounds migrate to varying extents in the environment

depending on their chemical structures, with effects ranging from strong adsorption to soil particles to

rapid dissolution in water. Published methodologies report procedures for extracting and analysing

discrete compounds or chemical classes but there is a lack of previous work concerned with

methodologies for the simultaneous extraction and analysis of organic pesticides from different

chemical classes. The soil environment is a complex matrix and farmers use a variety of compounds for

pest control; with regular crop rotation a diverse mixture of chemicals is likely to be present in arable

fields. Mindful of a legacy of pesticide contamination in water and the requirement for management of

water resources at catchment scales there is a need to quantitatively assess the storage and transport of

a variety of organic pesticides in different phases; soil, sediment and water. This paper describes a

methodology for analysing seven different organic pesticides representative of five different classes of

pesticide using a single methodology. Prior to use all glassware was deactivated using

dimethyldichlorosilane to prevent adsorption effects. Soils then underwent a triple ultrasonication in

acetone before being methylated with trimethylsilyldiazomethane. Final extracts were dissolved in

hexane and analysed by GC/MS. Recoveries from the soil were determined to range between 70%

and 114%.

Introduction

It has been estimated that 34% of global crop production can belost on an annual basis to pests including insects, plant path-ogens and weeds.1,2 To prevent crop losses large quantities ofpesticides are applied annually to arable land pre- and post-cropemergence. With more than nine hundred active ingredientsavailable for pest control, there is a signicant and risingconcern about soil and water quality, especially where extra-neous compounds persist in the environment for very longperiods of time.3,4

The distribution of a pesticide between the particulate andwater phase depends upon the soil characteristics, particularlysoil type, soil structure, moisture, temperature and availabilityof oxygen,5 and the chemical structure of the compound alongwith its half-life, toxicity, aqueous solubility and soil–waterpartition coefficient.6 These chemical characteristics are alsoinuenced by local environmental conditions.7 At the eld orcatchment scale the fate of pesticides is directly inuenced byland-use management such as crop rotation, tillage and

y of Bristol, University Road, Bristol BS8

.uk; Fax: +44 (0)117 928 7878; Tel: +44

mistry, University of Bristol, Bristol BS8

Chemistry 2013

irrigation. Differences in soil type and the potential for pesti-cides to travel longer distances over time can create a greaterspatial distribution of chemicals. Crop rotation is practised tomitigate against persistent pests and to improve soil structureand fertility thereby maximising benets from the soil.Different pesticides are applied on the varying crops which canlead to a mixture of chemicals in catchment soils. Approxi-mately 500 different pesticide products are available worldwide;comprising a mix of some of the 908 active compounds.8 Theseven pesticides selected for this study are frequently used byUK arable farmers and have been reported as causing acontamination threat to raw water supplies.9 This is due tochemical persistence, ability to adsorb to soil particulates andtheir potential for leaching.10 The pesticides represent vedifferent chemical groups which have not been previouslyreported as being extracted and analysed simultaneously.

Establishing the links between hydrological processes, soilcharacteristics, erosion dynamics and pesticide applications, iskey to understanding the fate of pesticides, including theirchemical alteration, transport mechanisms and storage time-scales in soils and sediments. However, there is currentlylimited understanding of the role of soil- and sediment-boundpesticides in surface and subsurface water contamination overdifferent timescales. Sediments on slopes, oodplains, instreams, lakes or reservoirs can act as a long-term sink for

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pesticides.11,12 which can be released episodically to watercourses long aer the original application took place.Contaminated soils or sediments have the potential for remo-bilisation and release of pesticides to water bodies during stormevents. Compounds which have a particularly high affinity forsolids are found preferentially bound to sediment particles andtend to follow slower and more convoluted pathways fromsource to sink.13,14

Previous communications describe how to extract andanalyse different organic pesticides from water15,16 and soilwhere several reviews are available.13,17–19 Techniques ofextracting pesticides from soil range from traditional Soxh-lets20–22 which use large quantities of solvent and are very timeconsuming to more environmentally friendly methods ofultrasonication,22–29 accelerated solvent extraction (ASE),30–32

supercritical uid extraction (SFE),20 microwave assistedextraction (MAE)20,22,33–35 and subcritical water extraction.36

These latter procedures are less time-consuming and requirelower volumes of solvent, thereby reducing cost.18 The tech-niques of MAE, ASE and SFE reduce solvent volume andincrease efficiency of extraction through enhanced solu-bility19,22 however, specialist instrumentation is required forthe extraction and a clean-up step is oen required using solid-phase extraction (SPE) or solid-phase microextraction (SPME).An ultrasonic probe has been tested for sonication of soilsamples increasing efficiency of the ultrasonication method,however this can only be used with non-volatile or semi-volatilecompounds.37 The majority of these previously reportedexperiments are for single compounds or a single chemicalclass. There is a general lack of understanding of the storageand transport of pesticides bound to sediment particles,primarily due to the cost and complexity of simultaneousextraction and analysis of multiple compounds. Analysis ofcompounds with different functional groups is complex as theymay undergo different chemical reactions within the soil/sediment and during extraction and subsequent analysis.Furthermore, compounds possessing different functionalgroups may also express diverse physical properties during theextraction process, such as adsorption or evaporation, whichmay obfuscate the results of any analysis of multiplecompounds.

The majority of previous pesticide studies concentrate onpesticides solely in the soluble phase in the water environment,primarily due to the lack of a single technique which hashindered the systematic analysis of sediment-bound pesticides.Herein, a new method developed for the concurrent extractionand analysis of seven different organic pesticides, representa-tive of ve distinct chemical classes, from soils and sediments isreported. The method optimizes the use of solvents and reducesthe time taken for extraction to ensure an efficient, accurate andprecise quantitative analysis. These protocols are unique topreviously published methodologies as they incorporatesimultaneous extraction followed by analysis using GC/MS forseven key pesticides which have not been previously analysedtogether. The described protocol is cheap and cost effective,enabling the analysis for large numbers of soil samples wheremultiple compounds are present.

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Chemicals

Pesticide standards were purchased as solids, from Sigma-Aldrichwith a purity of >98%. Compounds used in the analysis were:2,4,6,8-tetramethyl-1,3,5,7-tetraoxocanemetacetaldehyde (metal-dehyde), 3-(3-chloro-4-methylphenyl)-1,1-dimethylurea (chlor-otoluron), 3-(4-isopropylphenyl)-1,1-dimethylurea (isoproturon),(R,S)-2-(4-chloro-2-methylphenoxy)propanoic acid (mecoprop), 4-(4-chloro-2-methylphenoxy)butanoic acid (MCPB), 1-chloro-3-ethyl-amino-5-isopropylamino-2,4,6-triazine (atrazine) and 2-chloro-N-(2,6-dimethylphenyl)-N-(1H-pyrazol-1-ylmethyl)acetamide (metaza-chlor). Internal standards d5-atrazine andmethyl palmitate werepurchased from LGC Standards and Sigma-Aldrich, respectively.Stock pesticide standard solutions were prepared at a concen-tration of 1mgml�1 usingmethanol and refrigerated at <5 �C forno more than one year. All pesticide standards in powder formand trimethylsilyldiazomethane (TMSDAM) were stored at �20�C. Solvents (methanol, acetone, hexane and toluene) used in theprocess were HPLC grade and purchased from RathburnChemicals Ltd. TMSDAM and dimethyldichlorosilane (DMDCS)at 5% in toluene were purchased from Sigma-Aldrich.

MethodologyExperimental preparation

Prior to extraction the soils were prepared for analysis asfollows: wet soil was frozen and dried using a Thermo ScienticHeto PowerDry LL3000 freeze dryer for up to 5 days dependingupon soil moisture content. Soil was then ground using a pestleand mortar and passed through a 2 mm sieve to remove stones,vegetation and other extraneous objects. The resulting soil orsediment sample was then stored, frozen, in sealed glass vialsprior to extraction. Soil and sediment samples can be storedfrozen for up 450 days without major degradation.38

A silylation procedure was undertaken for all glassware usedin the experiments to prevent adsorption of compounds duringthe extraction and methylation processes thereby maximisingrecovery of all compounds.

All glassware was furnaced for 4 hours at 450 �C and le tocool before being treated. DMDCS in 5% toluene was applied tothe glassware (including pipette tips, vials and asks) for 2minutes. The glassware was then double rinsed in toluene andthen methanol before being le in a fume cupboard to air dryovernight. Tests showed a consistently higher average recoveryfor all compounds from silylated (81% �22) rather than non-silylated (45% �3) glassware used in the experiments. Theresults indicate that a considerable quantity of pesticide isadsorbed to glassware if it is not deactivated before being used.

Extraction and methylation

A solvent extraction was undertaken which enables the sepa-ration of the bound constituents from the soil particles withoutchemically altering the target compounds. Previous methodshave used a range of individual or mixed solvent systems forextraction including methanol, acetonitrile, acetone andsulfuric acid.37,39,40 In the method reported herein, pesticideextraction was carried out by ultrasonication with methanol or

This journal is ª The Royal Society of Chemistry 2013

Table 1 Physical properties of the reference soil used in the recovery test

Property Percentage

Clay 10%Sand 50%Silt 40%pH 8.4Organic matter 1.85%

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acetone as these solvents have been previously proven to effi-ciently extract pesticides from soil particles. Approximately 2 gof soil was weighed into a 28 ml silylated vial and 8 ml of HPLCgrade solvent added. The vial was then sonicated for 20minutes. Extracts were separated from soil by centrifugation for5 minutes at 3000 rpm and the solution removed using a sily-lated glass pipette and stored in another silylated vial. Theultrasonication step was repeated twice more resulting in a nalvolume of solution of 24 ml. This was then dried under a gentlestream of nitrogen for a minimum time period and stored readyfor analysis.

Ultrasonic extraction uses a lower volume of solvent thanSoxhlet extractions and, although it is not as exhaustive aprocess, it is substantially less costly.13 Pesticides are also lesslikely to degrade due to the lower temperatures used. Extractiontests using Soxhlet apparatus and rotary evaporation showedhigh losses of the more volatile compounds (average loss:metaldehyde 33%, chlorotoluron 30%, mecoprop 18%, atrazine43%, MCPB 35%, isoproturon 21% andmetazachlor 3%). Theselosses could have occurred due to the high temperatures andlong time period of extraction using Soxhlet extractors and,subsequently, rotary evaporation at the end of the procedure toremove large volumes (�250 ml) of solvent. Tests betweendrying using rotary evaporation and nitrogen blow-downapparatus showed losses were on average 5% less using thenitrogen blow-down. Aer extraction a test was conductedwhereby the samples were subjected to a clean-up procedureusing Supelclean� ENVI-Carb� SPE tubes. These were used toincrease pesticide recovery by avoiding losses using dryingtechniques alone. The tests showed percentage recovery torange between 76% and 106% which was similar to those notsubjected to this procedure (70% to 114% using acetone). Inaddition, the chlorotoluron was not detected on the GC-MS aerthe SPE phase. Therefore this was deemed an unnecessary stepand abandoned, thereby reducing overall cost and time foranalysis.

To quantify compounds and account for changes in theinstrument over time and between sample runs an internalstandard was used. Initial tests were conducted using adeuterated version of one of the compounds of interest, (d5-atrazine) as this will exhibit almost identical chemical andphysical properties to the target analogue. However, GC/MSrevealed that atrazine co-eluted and exhibited identical ions,albeit at much lower abundance, to those generated by theatrazine standard, hence it was impossible to select between thetwo thereby preventing any reliable quantication to be made.Further work utilised methyl palmitate at a concentration of40 ng ml�1, an n-alkanoic acid methyl ester dissimilar to any ofthe pesticides and consequently unlikely to co-elute or producethe same diagnostic ions as the targets facilitating a moreaccurate quantication. The methyl palmitate eluted in thesame range as the targets and produced sufficiently selectiveions to enable the use of SIM-GC/MS. Therefore, it is recom-mended that methyl palmitate or some similar standard beused as an internal standard.

Prior to analysis a derivatisation procedure was used toconvert compounds to less polar, more volatile derivatives and

This journal is ª The Royal Society of Chemistry 2013

produce a more chromatographically resolved distribution.41

Previous studies42,43 have used diazomethane for methylatingacid herbicides prior to analysis but this is explosive. Due to thehazardous nature of diazomethane the safer alternative,TMSDAM was used for this procedure44–47 Addition of theTMSDAM converts the chemical compounds to their methylesters. 10 ml of TMSDAM was added to a 1 ml mixture of tolueneand methanol at a ratio of 4 : 1. Approximately, 200 ml of theTMSDAM and solvent solution was added to the dry sample andle in a fume hood to react. 1 ml aliquots of standard solutionat a concentration of 50 ng ml�1 were le in contact withTMSDAM for periods between 10 minutes and 24 hours.Analysis showed there to be no signicant difference (ANOVAF ¼ 7.08, P ¼ <0.05) in percentage of compound recovery andtherefore the actual time period for methylation proved unim-portant. One hour was used in this study for consistencybetween experiments. The reaction with the TMSDAM wasquenched using a few drops of acetic acid. Solvent and excessTMSDAM were then removed by evaporation under a gentlestream of nitrogen just to the point of complete solvent evap-oration then re-dissolved in 50 ml hexane prior to analysis byGC/MS.

Recovery tests were carried out using spiked samples withultrasonic extraction to validate the method. Reference soil waspurchased for recovery testing from LGC Standards certiedfree from most pesticides (below limit of detection). The refer-ence soil used for recovery testing was classied as a clay loamand has the properties described in Table 1. This soil wasselected based on the soil characteristics for a small agriculturalcatchment in south west England where eld based monitoringof pesticides in soil and sediments is taking place. 500 ml of theworking solution of the pesticide mixture was applied to the soilbeing tested to make a concentration of 25 mg g�1 soil (equiv-alent to 50 ng ml�1 in solution) and le for 24 hours. Theextraction, methylation and analysis procedures were thenconducted and recoveries established.

Instrumentation

Analysis was conducted by GC/MS using a ThermoQuestTraceMS (Thermo Fisher Scientic, Hemel Hempstead, UK). Aspecialised Zebron ZB-MultiResidue-2 column (30 m length �0.32 mm internal diameter � 0.25 mm lm thickness) forpesticide analysis was supplied by Phenomenex. All analyseswere made on a 1 ml injection volume. The oven temperatureprogram was 70 �C for 1 min; this was then ramped to 160 �C at7 �Cmin�1 and held for 2 min, then increased to 300 �C at 10 �Cmin�1 and held for 20 minutes. The mass spectrometer was

Anal. Methods, 2013, 5, 2053–2058 | 2055

Table 2 Molecular ions used in GC-MS analysis in the SIM mode

Retentiontime(minutes)

Targetions(m/z)

InstrumentLoDa (ng ml�1)

InstrumentLoQa (ng ml�1)

MethodLoDa (ng ml�1)

MethodLoQa (ng ml�1)

Molluscicides Metaldehyde 11.63 89 117 1 5 100 500Phenyl ureas Chlorotoluron 12.3 132 167 0.5 3 50 300

Isoproturon 22.58 146 206 1 5 100 500Chlorophenoxy acids Mecoprop 17.78 142 169 0.5 3 50 300

MCPB 20.81 101 142 0.5 3 50 300Triazines Atrazine 20.81 200 215 0.5 3 50 300Pyrazoles Metazachlor 24.05 133 209 0.5 3 50 300Internal standard Methyl palmitate 20.79 74 143 n/a n/a n/a n/a

a LoD – Limit of Detection, LoQ – Limit of Quantication.

Fig. 1 Chromatogram for SIM-GC-MS analysis of seven selected pesticidecompounds (a) metaldehyde (b) chlorotoluron (c) mecoprop (d) atrazine (e) MCPB(f) methyl palmitate (g) isoproturon (h) metazachlor.

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operated in Selected Ion Monitoring (SIM) mode using thetarget m/z values as specied in Table 2 and shown in theexample chromatogram in Fig. 1. For calibration a workingsolution of a mixture of all seven pesticides was prepared fromthe 1 mg ml�1 standard solutions to make a range of differentconcentrations from 0.5 ng ml�1 to 200 ng ml�1. Analytes werequantied using a linear calibration on an eight point

Table 3 Recovery tests results for seven pesticide compounds using ultrasonicationreference soils. 25 mg of each pesticide added to 2 g reference soil. Experiment rep

Pesticide compound

Methanol extraction recovery (%)

Loss tests Soil recoveryStandard desoil recover

Metaldehyde 79 59 6Chlorotoluron 69 71 10Isoproturon 95 89 27Mecoprop 102 96 22MCPB 104 95 29Atrazine 105 96 9Metazachlor 70 61 13

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calibration graph. The Limit of Detection (LOD) and Limit ofQuantication48 varied slightly for each of the compounds(Table 2).

Results and discussion

Loss tests for the entire chemical extraction process with no soilwere conducted using both methanol and acetone to determinewhether compounds are lost, chemically altered or degradedduring the extraction. The working solution was added to a vialand three extractions were undertaken using the same methodas for soil. Some losses for metaldehyde, chlorotoluron andmetazachlor were observed during the extraction process,however, the remaining four chemicals isoproturon, mecoprop,MCPB and atrazine showed no losses during the extractionphase (Table 3).

The experiments were replicated eight times using methanolor acetone as the solvent. Atalay and Hwang40 showed methanolto be an efficient solvent to extract a high proportion of pesti-cides, however, it has a low volatility and evaporation of solventwith nitrogen can take more than 8 hours in this case. Acetoneexhibits a similar efficiency but is more volatile (boiling point of56 �C vs. 65 �C for methanol) and evaporates more rapidly whenle under nitrogen (�2 hours). Recovery tests indicate acetoneextractions to be more consistent with lower variability betweenreplicate samples compared with using methanol (Table 3). Dueto previously reported losses and degradation of chlorophenoxyacid compounds during extraction17 both single and triple

extraction in either methanol or acetone without soil for loss tests and with cleanlicated eight times in each solvent

Acetone extraction recovery (%)

viation fory Loss tests Soil recovery

Standard deviationfor soil recovery

77 76 1185 87 1999 70 1691 75 486 75 8

112 114 674 76 3

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ultrasonic extractions were compared. Results showed a tripleextraction to be approximately 20% more efficient than a singleextraction without large pesticide loss. Experimental results aresummarised in Table 3. Results showed recovery from soilbetween 70% and 114%.

This procedure provides a potential means of analysingmultiple pesticides from soils and sediments for catchmentscale studies where a high throughput of samples is essential.The technique is efficient as it optimises the use of solvent andextraction time increasing the number of samples which can bepotentially analysed for any given study. Using these protocols,soil and sediment samples can be extracted, analysed and usedto assess storage and transport of pesticide chemicals eitherlaterally or vertically in the soil prole and in sediment boundparticles in water through runoff, stream transport and depo-sition in water bodies.

This technique has been tested for a range of soil samplestaken from a small catchment in south west England between0 and 10 cm depth in arable elds with similar soil properties tothose in the reference soil. Chlorotoluron, mecoprop, andmetazachlor were found in the soil at concentrations between400 ng g�1 and 3200 ng g�1 soil. This was consistent withexpected results where known arable activity is taking place andpesticides are used on a regular basis. The analysis did notdetect any pesticides in the areas of alternative land use, asexpected.

Conclusions

The combined use of ultrasonic extraction and GC/MS analysisis a well-established analytical protocol for this type of study.Within the context of this study the additional processes ofsilylation of glassware and the use of TMSDAM for methylationare new procedures. Tests have shown silylation increases theconsistency of recovery from soil and helps reduce chemicalloss during each stage of the process. TMSDAM is an effectivemethylation agent which is easy to use and results in derivativesthat yield diagnostic ions of high abundance under electronionisation conditions thereby aiding the application of SIM-GC/MS for detection and quantication of the targets.

Using the described procedures for preparation, extractionand analysis of pesticide chemicals from soil or sediment willenhance research on commonly used pesticides. The processesenable different compounds from ve chemical classes to beextracted and analysed simultaneously. Recoveries from soilranged between 64% and 114%. A triple sonication was shownto be 20% more efficient than a single extraction at removingpesticides from the soil. The combined use of acetone as asolvent, ultrasonic extraction with silylated glassware andmethylation using TMSDAM produces a viable method forchemical extraction of seven different but key pesticide speciesfrom soil, maximising efficiency of time and resources.

This paper has described a rapid and accurate analyticaltechnique for use in assessing and monitoring the movementand storage of pesticides in soil and sediments. This techniquecan be used in combination with established analytical tech-niques for the soluble phase to gain a more complete

This journal is ª The Royal Society of Chemistry 2013

understanding of the behaviour and impact of pesticides in theenvironment. The method reported here enables simultaneousanalysis of seven distinct pesticides from particulatesthroughout a catchment which are applied to a diverse range ofcrops in different seasons. Utilising this rapid and efficientextraction procedure maximises the number of samples whichcan be analysed throughout a catchment using a singleextraction.

Acknowledgements

This work is funded by a Natural Environment ResearchCouncil (NERC) Directed Open CASE Studentship (NE/G012415/1) in partnership with Wessex Water Services Ltd. Chemicalmethodological development and analyses were carried out atthe NERC Life Sciences Mass Spectrometry Facility (LSMSF) inthe School of Chemistry at the University of Bristol. We grate-fully acknowledge the landowners in the Durleigh Catchment,Somerset, UK for allowing us access to their land for soilsampling.

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