Sun, Lee 67

Lei Sun,Hian Kee Lee

Department of Chemistry,National University ofSingapore, 3 Science Drive 3,Republic of Singapore, 117543

Microwave-assisted extraction behavior of non-polar and polar pollutants in soil with analysis byhigh-performance liquid chromatography

Microwave-assisted extraction (MAE) was applied for the first time to study the ther-mal degradation of five carbamates (propoxur, thiuram, propham, methiocarb, chlor-propham) after heating for 6 min at 958C with different extractants. Analytes weredetermined by high performance liquid chromatography (HPLC) with UV detection. Itwas found that significant thermal degradation of the five targets occurred under theextraction conditions. The break down percentage ranged from 10% to 100%, depen-ding on polarities of both analytes and extractants. The greater the polarity of theextractant, the less degradation the analyte underwent. Lower recoveries were foundfor all carbamates with less polar extractants, such as hexane-acetone (4:1, v/v) anddichloromethane. With more polar extractants (methanol, hexane-acetone 1:4 and1:1), higher recoveries of three carbamates (propoxur, methiocarb, and chlorpro-pham) were obtained, whereas the recoveries of the other two remained low. Somenon-polar and polar pollutants spiked in soil, such as polynuclear aromatic hydrocar-bons (PAHs), polychlorobiphenyls (PCBs), triazines (atrazine, simazine), and carba-mates (propoxur, methiocarb, chlorpropham), subjected to MAE in a closed-vesselmicrowave system under 80% magnetron power output (1200 W) at 1158C, werealso studied. The recoveries of such pollutants ranged between 70% and 99% withexcellent reproducibility, except for carbamates. The effects of the soil matrix, the soilmoisture, and aging on recoveries were also investigated.

Key Words: Microwave-assisted extraction; Thermal degradation; Carbamates; Soil;

Received: June 14, 2001; revised: October 18, 2001; accepted: October 25, 2001

1 Introduction

The determination of pesticides in environmental matricesis receiving increasing attention nowadays because oftheir toxicity [1]. In order to measure the low levels allowedin food as well as in drinking water, an extraction and pre-concentration step is necessary prior to their instrumentaldetermination [2]. Common extraction techniques for solidmatrices include mainly Soxhlet extraction and sonication.Other novel extraction techniques such as supercriticalfluid extraction (SFE), microwave-assisted extraction(MAE), and accelerated-solvent extraction (ASE) havealso been used [3, 4].

Conventional Soxhlet extraction has some advantages: itallows use of large amounts of sample; no filtration isrequired after the extraction. However, significantamounts of solvents, which are often toxic and flammable,are needed. Also the procedure is tedious and time con-

suming. Sonication is faster than Soxhlet extraction andallows extraction of large amounts of sample at relativelylow cost. However, it still uses about as much solvent asSoxhlet extraction, is labor-intensive, and filtration isrequired after extraction [5].

More recently introduced extraction techniques, such asSFE and microwave-assisted extraction (MAE), are veryattractive because they are less time-consuming andenvironmentally friendlier and use much smaller amountsof solvents [6, 7]. The main drawbacks that SFE has tocope with are difficulties in extracting polar analytes aswell as different efficiencies obtained for spiked and nat-ural samples [8, 9].

In recent years, MAE has attracted growing interest as itallows rapid extractions of organic micro pollutants fromsolid matrices with extraction efficiencies comparable tothose of the classical techniques [10–13]. In comparisonto conventional extraction procedures the benefits of MAEare enhanced extraction times, low solvent consumption,and improved extraction efficiencies. Moreover, simulta-neous extraction of multiple samples is possible. In com-parison to SFE, the most significant advantages of theMAE technique are the preconcentration effect, safety,

J. Sep. Sci. 2002, 25, 67–76

Correspondence: Prof. Hian Kee Lee, Department ofChemistry, National University of Singapore, 3 Science Drive3, Republic of Singapore, 117543.E-mail: chmleehk@nus.edu.sgFax: + 65 779 1691

i WILEY-VCH Verlag GmbH, 69469 Weinheim 2002 1615-9306/2002/0101–0067$17.50+.50/0

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Peer-review of papers inthe section “MicrocolumnSeparations” was super-vised by Milton L. Lee andPat Sandra. Their editor-ial support is gratefullyacknowledged.

and simplicity [14]. Additionally, it can be used to extractnot only non-polar analytes from solid matrices but alsopolar and ionic compounds [9]. In particular, numerousapplications of this technique have been developed forthe extraction of pollutants from environmental samplessuch as soils and marine sediments. Optimised MAE con-ditions for the extraction of PAHs have been reported byChee et al. [15].

Carbamate pesticides, as substitutes for the more envir-onmentally stable organohalogen pesticides, are widelyused for the control of insects in the agricultural industry[16]. They belong to a relatively polar group of substancesand are thermally unstable. When heated, they have atendency to break down to the corresponding phenols andamines [17]. In the past few years, numerous compoundshave been extracted by microwave-assisted extractionfrom various types of matrices with special emphasis onenvironmental applications, such as PAHs [18–27],PCBs [28–31], organochlorine pesticides [32–34], phe-nols [35–37], herbicides [38–40], triazines [41–44], andorganic mercury compounds [45–49]. But hitherto, littlework has been reported on the MAE of carbamates [50].

In this work, MAE was applied for the first time to studycarbamates. Our focus is first on the thermal instability ofcarbamate in different extraction solvents under MAEconditions, and secondly on the comparison of MAE effi-ciency of relatively more polar organic pollutants, such ascarbamates (propoxur, methiocarb, propham, thiuram,chlopropham) and triazines (atrazine, simazine) with thatof relatively non-polar substrates, such as PCBs(PCB1242, PCB1248) and PAHs (naphthalene, phenan-threne). Furthermore, we also pay particular attention tothe optimisation of the parameters that influence MAE effi-ciency, including the polarities of the solvents, extractiontemperature, extraction pressure, and heating duration.

2 Experimental

2.1 Reagents and solvents

Analytical-grade PAHs (naphthalene and phenanthrene)were purchased from Supelco (Bellefonte, PA, USA) orUltra-Scientific (North Kingston, RI, USA). The stock stan-dard solutions were prepared in acetone at concentrationsof 2.0 mg/mL for each compound and stored at –48C.Working solutions were prepared by diluting the stocksolutions with acetone.

PCB-1242 and PCB-1248 were purchased as individualstandard stock solutions containing a nominal concentra-tion of 100 lg/mL in methanol from Ultra-Scientific.

Atrazine (purity 98%) and simazine (purity 99%) were pur-chased from Supelco. Stock solutions (1000 lg/mL) wereprepared by dissolving the solid standards in acetone and

stored under refrigeration. Working solutions wereobtained by diluting with acetone.

Carbamates: propoxur (purity 99%), methiocarb (purity99%), propham (purity 99.5%), thiuram (purity 98%), andchlorpropham (purity 99.5%) were supplied by ChemSer-vice (West Chester, PA, USA). The stock solutions con-taining each compound (1000 lg/mL) were prepared inmethanol and diluted with methanol to obtain workingsolutions at various concentrations.

All solvents used in this study were either pesticide-gradeor HPLC-grade and obtained from Fischer Scientific (FairLawn, NJ, USA). The ultrapure water was purified on aMilli-Q water purification system (Millipore, Bedford, MA,USA).

2.2 Preparation of soil samples

2.2.1 Blank soils

Three soil samples were collected from local sites (soil-1,soil-2, and soil-3, respectively). They were air-dried, pul-verized, and sieved through a 60-mesh sieve. In order toremove possible traces of PAHs, PCBs, triazines, carba-mates, and other organic contaminants, 100 g of each ofthe soil sample was immersed consecutively in 200 mL ofmethanol, acetone, dichloromethane, and n-hexane for atleast 24 h. The treated soil was spread out on a tray andair-dried for 8 h in a fume hood to remove as much solventas possible. Finally, it was determined that there were nodetectable levels of the target analysts in soil samplesbefore spiking.

2.2.2 Spiked soils

Freshly spiked soil samples were prepared by adding anappropriate volume of spiking solutions into the soils pre-pared above. To ensure that the analytes were well dis-tributed, a reasonable amount of acetone was added tomoisten the soil and careful agitation was performed.These standards were prepared 10–14 days prior to soilanalysis.

Aged spiked soil samples were obtained by storing theabove spiked soil in capped bottles in the dark and agedfor 60 days prior to analysis.

2.3 MAE procedure and treatment of extracts

Microwave-assisted extraction was carried out using aMarsX (1200-W) laboratory microwave extraction system(CEM, Matthews, NC, USA) equipped with a solventdetector. The instrument is able to extract concurrentlyfourteen solid samples in Teflon extraction vessels underidentical extraction conditions. It controls either pressure

68 Sun, Lee J. Sep. Sci. 2002, 25, 67–76

or temperature depending on which parameter reaches itscontrol set point first.

In this study, 30 mL extractant was added into the MAEvessel, which contained 2.0 g of spiked soil sample.Extraction was performed at 1158C with heating time of6 min for PAHs and PCBs or 4 min for triazines at 80%power. After the extraction, the vessels were cooled downto room temperature before they were opened. Sampleextracts were further clarified by centrifugation at4000 rpm for 15 min to separate out the fine particulate.The supernatant was then evaporated to dryness in arotary evaporator. Finally, 1 mL of methanol was added todissolve the residue, which was directly analyzed byHPLC.

In order to examine the thermal degradation behavior ofcarbamates the following was performed: 2 mL of stan-dard solution containing 2 ppm of propoxur, thiuram, prop-ham, methiocarb, and chlorpropham was evaporated todryness and then 30 mL of the appropriate extractant wasadded to dissolve the residue. The solution was trans-ferred to the extraction vessel (no soil inside) and MAEwas performed at 958C for 6 min at 80% power. After theextraction, the vessel was allowed to cool down to roomtemperature before it was opened.

2.4 HPLC measurements

The HPLC system consisted of a Shimadzu (Kyoto,Japan) LC-6A pump, a Rheodyne (Cotati, CA, USA) 7010injector equipped with a 20-lL loop, a Shimadzu SPD-6AV UV-VIS detector, and a Shimadzu C-R6A integrator.

For triazines: A Phenomenex (Torrance, CA, USA) ODS25064.6 mm column was used. Detection wavelengthwas 254 nm. Mobile phase: Methanol-water (65 :35) at aflow rate of 0.8 mL/min.

For carbamates: A Phenomenex ODS 15063.2 mm col-umn was used. Detection wavelength was 225 nm. Mobilephase: Acetonitrile-water (40 :60) at a flow rate of 0.8 mL/min.

PAHs analysis was performed on a Waters (Milford, Mas-sachusetts, USA) 600E HPLC system equipped with aWaters 700 autosampler, a Waters 486 UV-VIS detectorand Millennium 2.15 version software. A PhenomenexODS 25064.6 mm column was used. Detection was at254 nm.The mobile phase was acetonitrile-methanol-water (78 :7 :15) for the first 8 min, then the compositionwas changed to 100% acetonitrile over 25 min at a flowrate of 0.8 mL/min.

2.5 GC measurements

PCB analysis was performed on a Hewlett-Packard (PaloAlto, CA, USA) 5890 Series II gas chromatograph

equipped with a 63Ni electron-capture detector. Separa-tions were conducted using a DB-5, 30 m60.32 m capil-lary column coated with a 0.25 lm stationary phase film (J& W, Folsom, CA, USA). The carrier gas was purifiednitrogen at a flow rate of 1.5 mL/min.

The GC conditions for PCBs analysis were as follows:injector temperature 2608C; detector temperature 3008C;initial oven temperature 808C for 1 min, increased to2008C at a rate of 25 8/min, then maintained at 2008C for1 min; a second ramp 2308C at rate of 5 8/min, increasedto 2808C at a rate of 20 8/min; The final temperature of2808C was held for 4 min.

3 Results and discussion

3.1 General aspects

MAE consists of heating the extractants (mostly liquidorganic solvents) in contact with the sample with micro-wave energy. The partitioning of the analytes of interestfrom the sample matrix to the extractant depends on thetemperature and the nature of the extractant.

In order to absorb microwave energy, the solvent shouldexhibit dielectric polarization. The greater the dielectricconstant, the more thermal energy is released and themore rapid is the heating for a given frequency. Besidesthe absorption of microwave energy, the efficiency of thesolvent converting it into heat is also important, which isexpressed by the dielectric loss factor. In fact, the dielec-tric constant and the dielectric loss factor of a certain sol-vent may differ widely. Therefore, the overall efficiency ofheating using microwave energy is always given by thedissipation factor, which is the ratio of the sample’s dielec-tric loss (i.e. the loss factor e 99) to its dielectric constant(e 9). The dielectric constant is a measure of the sample’sability to absorb microwave energy and the loss factor isthe ability to dissipate the absorbed energy. It must berealized that, unlike classical heating, microwaves heatthe entire sample simultaneously without heating the ves-sel. Therefore, the solution reaches its boiling point veryrapidly, leading to very short extraction times [51].

3.2 Optimization of the solvents, microwaveheating time, and temperature

With MAE, temperature, extraction time, and suitable sol-vents appear to be the major parameters affecting theextraction efficiency of the pollutants. Employing informa-tion that is available from the literature and our own experi-ence on the extraction of some pollutants from soil suchas heating for 6 min at 1158C is sufficient to obtain highrecoveries of PAHs and PCBs, and heating for 4 min isenough for triazines. However, as far as carbamates were

J. Sep. Sci. 2002, 25, 67–76 Study on microwave-assisted extraction 69

concerned, more attention should be paid to the extractiontemperature because of their thermal instability. Since thedegradation temperature for most of carbamates studiedhere is around 1108C, especially for propham with adegradation temperature of 1008C, heating up to only958C was applied to the five carbamates in this study.

When selecting solvent, consideration should be given tothe microwave-absorbing properties of the solvent andthe analyte solubility in the solvent. In order to enhancethe absorption of microwave energy, solvents with a highdielectric constant such as water, methanol, and acetoneare preferably applied. The dielectric constants of somesolvents and the temperatures of different solventsachievable with microwave heating time under MAE con-ditions are shown in Table 1 and Figure 1, respectively.From Figure 1, it is clear that the larger the dielectric con-stant of the solvent, the more rapid the heating is undermicrowave irradiation. In some cases, however, some sol-vents with high dielectric constants make the concentra-tion step laborious after extraction because of their inher-ently high boiling points. They also have poor extractionselectivities due to polar co-extractives. Thus, a mixture ofhexane-acetone was selected as the ideal solvent forcompounds of environmental significance. As Figure 1shows, hexane with a dielectric constant of 1.8 will not beheated; however, on mixing it with acetone heating will

take place in a few seconds. Furthermore, due to the goodsolubility of the carbamates in this solvent mixture the for-mation of permanent dipoles assures absorption of micro-wave radiation, which is considered to be favorable toaccelerate the extraction. Based on this consideration,three different ratios of hexane-acetone mixtures for MAEwere applied in this study.

3.3 Thermal degradation of carbamates underMAE conditions in different extraction solventsystems

The structures of the carbamates studied are shown inFigure 2.

The extent of thermal degradation of each carbamate indifferent extractants under MAE conditions was deter-mined in the absence of soil. From Figure 3, it can beseen that thermal degradation of all five carbamatesoccurred in almost all solvents considered, especiallythiuram and propham, which underwent degradation in allextractants, except methanol. The other three carba-mates (propoxur, methiocarb, and chlorpropham)degraded more significantly in dichloromethane and hex-ane-acetone (4 :1) than in methanol, acetone, and hex-ane-acetone (1 :4). The percentage of thermal degrada-tion ranged from 10% to 100%, depending on the polarityof solvent. Based on the results, some very interestingconclusions were drawn: (1) The thermal degradation ofthe five carbamates studied took place in varying degreesunder MAE conditions of heating 6 min at 958C. (2) The

70 Sun, Lee J. Sep. Sci. 2002, 25, 67–76

Table 1. Dielectric constant (e9) of different materials at fre-quency of 3 GHz and temperature of 258C (from ref. [2])

Material e 9 Material e 9

Ice 3.2 Heptane 1.9Water (258C) 76.7 Dichloromethane 8.93Methanol 32.63 Hexane 1.89n-Propanol 3.7 Carbon tetrachloride 2.2Ethylene glycol 12.0 Acetone 21.01Hexane-acetone (1 : 4) 13.4 Hexane-acetone

(1 : 1)11.5

Hexane-acetone (4 : 1) 3.3

Figure 1. Temperatures of some solvents with heating timeunder MAE conditions.

Figure 2. Names and structural formulae of carbamatesused in this study.

percentage of thermal degradation of carbamates in morepolar solvents, such as methanol, acetone, and hexane-acetone (1:4, 1 :1), was much less than in less polar sol-vents, such as dichloromethane and hexane-acetone(4 :1). Evidence is presented in Figure 4, which shows astandard chromatogram and some typical chromato-grams of mixtures of the five carbamates various inextractant solvents under the stated MAE conditions. Thefigure shows that thermal degradation occurred seriouslyand was dependent on the polarity of the extractant. Theobservation may be explained by the principle of “like dis-solves like” during MAE. Thus, polar analytes are moresoluble in polar extractants than in less polar ones. This isdue to the protective effects of extractants against ana-lytes. Therefore, careful selection of extractants toaddress this issue must be considered in MAE. Moreover,although the polarity of water is the strongest among thesolvents used, complete degradation of the five carba-mates occurred, which indicates that hydrolysis of the car-bamates occurred seriously in this extractant under theapplied MAE conditions.

3.4 The extraction behavior of carbamates spikedin soil

Since propham and thiuram was found to degrade ser-iously, propoxur, methiocarb, and chlorpropham wereselected to investigate the recoveries of carbamatesspiked in soil under the applied MAE conditions. Theresults are shown in Table 2 and Figure 5. The recov-eries of carbamates (propoxur, methiocarb, and chlor-propham) were low in all extractants used except inmethanol. Furthermore, the recoveries increased with

increasing extractant polarity. For example, the recoveryof propoxur increased from 20% to >65% when methanolwas used as extractant instead of hexane-acetone (4:1).The principle “like dissolves like” is still applicable toexplain these results. As carbamates are polar pesticides,their solubility in polar extractants is much higher than inless polar extractants. In addition, the dipole-dipole inter-action between the analytes and polar extractants ishigher than that with the less polar extractants. At thesame time, polar extractants absorb microwave energywith high efficiency. As a result the efficiency of micro-wave extraction will be higher in polar extractants thanless polar extractants. What is surprising is that in lesspolar extractants, unusually high recoveries of the targetswere observed in soil, considering the high thermal degra-dation in the same solvents without the matrix (previous

J. Sep. Sci. 2002, 25, 67–76 Study on microwave-assisted extraction 71

Figure 3. Plots of percentage of thermal degradation of five carbamates in different extrac-tion solvents: 1) methanol; 2) acetone; 3) hexane-acetone (1 :4, v/v); 4) hexane-acetone(1 :1); 5) dichloromethane; 6) hexane-acetone (4 :1); 7) water.

Table 2. Effects of using different extraction solvents onrecoveries of carbamates spikeda) in soil under MAE condi-tions.

Extraction solvents Recoveries (%) l RSD (%) n = 6

Propoxur Methiocarb Chlor-propham

Methanol 66.0 l 6.2 80.0 l 5.6 65.7 l 5.1Hexane-acetone (1 :4) 48.3 l 4.0 29.4 l 2.9 35.0 l 3.9Hexane-acetone (1 :1) 41.0 l 3.8 18.4 l 4.0 24.0 l 3.0Dichloromethane 26.0 l 2.8 5.2 l 3.0 –Hexane-acetone (4 :1) 20.0 l 4.2 8.6 l 3.3 5.6 l 3.2Water – – –

a) Spiked level of 4 lg kg – 1 each carbamate.– No recovery.

section). For example, the recovery of propoxur from soilwas around 29% when dichloromethane was used asextractant, whereas thermal degradation (no soil

involved) was nearly 100% in the same extractant. Thismay be due to the protective effects of the matrix affordedto the analytes. The recoveries were nearly zero when

72 Sun, Lee J. Sep. Sci. 2002, 25, 67–76

Figure 4. (a) Chromatography of standard mixture of five carbamates (b–f) chromatographyobtained after simulated MAE (6 min heating at 958C, 80% microwave power) in the variousextractants. Peak identities: 1) propoxur; 2) thiuram; 3) propham; 4) methiocarb; 5) chlor-propham; a) unknown.

water was used as extractant, which appears to confirmthat carbamates undergo significant hydrolysis in waterunder the MAE conditions used.

3.5 MAE of different pollutants from spiked soil

Although there are some studies on the extraction beha-vior of a certain pollutant extracted from soil by using dif-ferent extractants under MAE conditions, until now therehas been little comparison of extraction behaviors of dif-ferent pollutants in soil extracted simultaneously in thesame extractant by using MAE. This was one aspect ofour present work. The MAE recoveries of some analytesfrom soil-3 with different solvents are shown in Table 3. Itindicates that in each tested solvent high recoveries ofPAHs, PCBs, and triazines were obtained and rangedfrom 70% to 99% with excellent reproducibility, except forPAHs extracted in methanol. Although methanol cancause dipole-induced dipole interactions with the numer-ous p-electrons on the PAHs because of its permanentdipole, the poor solubility of PAHs in methanol results inlower recoveries. The low recoveries of carbamates arepartially due to their thermal degradation, as determinedpreviously. Figure 6 shows the dependence of recoveriesof some classes of analytes on different extractants. Aninteresting result observed from Figure 6 is that in thesame solvent, the recovery increases from PAHs toPCBs, and to triazines with increasing polarity of analyte.The recovery is probably affected by the interactionbetween the analyte and the matrix. The higher the polar-ity of the analyte, the larger its dipole. Thus it is moreeasily reoriented and finally desorbed from the soil matrixin the applied microwave field. Moreover, when the sol-vent is rapidly heated in a reproducible way under micro-

wave irradiation, the selective interaction with polar mole-cules allows local heating and an improvement of extrac-tion efficiency. From Figure 6, it is also observed that thedifference in recoveries is reduced with decreasing polari-ties of extractants. For example, the recovery differencebetween PCBs and triazines when methanol is used asextractant (14.7%) is larger than that when dichloro-methane is used as extractant (2.35%). Thus suggeststhat the analyte solubility in the solvent is another impor-tant factor on recovery. Based on the principle of “like dis-solves like”, triazine as a type of polar pesticides is moreeasily dissolved in methanol than in dichloromethane sothe recovery decreases when dichloromethane is usedinstead of methanol. However, the less polar PCBs aremore preferentially dissolved in dichloromethane; there-fore, the recovery is slightly better when this solvent isused.

J. Sep. Sci. 2002, 25, 67–76 Study on microwave-assisted extraction 73

Figure 5.Variation of the carbamates from spiked soil withdifferent extractants after heating 6 min at 958C. Extractionsolvents: 1) methanol; 2) hexane-acetone (1 :4, v/v); 3) hex-ane-acetone (1 :1); 4) dichloromethane; 5) hexane-acetone(4 :1); 6) water.

Figure 6. Plots of recovery of PAHs, PCBs and triazinesfrom spiked soil with different solvents after heating 6 min at115 8C. Extraction solvents: 1) dichloromethane; 2) hexane-acetone (1 :1, v/v); 3) methanol.

Table 3. The MAE recoveries of different analytes in spikedsoila) with different solvents.

Analytes Recovery (%) lRSD (%) n = 4

Methanol Hexane-acetone

(1 : 1)

Dichloro-methane

PAHs Naphthalene 64.4 l 7.5 79.0 l 4.1 78.3 l 3.3Phenanthrene 60.9 l 6.2 85.1 l 5.8 83.8 l 2.9

PCBs PCB1242 82.7 l 4.0 88.1 l 3.7 84.8 l 4.5PCB1248 84.4 l 2.8 89.8 l 3.2 84.0 l 4.8

Triazine Atrazine 97.9 l 5.4 91.4 l 3.0 87.7 l 6.1Simazine 98.6 l 6.0 93.5 l 4.5 89.8 l 3.6

Carbamates Propoxur 66.0 l 6.2 41.0 l 3.8 26.0 l 2.8Methiocarb 80.0 l 5.6 18.4 l 4.0 5.2 l 3.0Chloropropham 65.7 l 5.1 24.0 l 3.0 –

a) Soil- 3, collected from campus of National University ofSingapore.

– No recovery.

3.6 Influence of matrix, moisture, aging of spikedsoil on recovery of tested pollutants

The effects of matrix, moisture, and aged spiked soil onrecovery of analytes were investigated and the results areshown in Table 4, Table 5, and Table 6, respectively.

The recovery of propoxur (a representative carbamate) indifferent spiked soils is shown in Table 4. The table indi-cates no apparent effect of different soil matrices on therecovery of propoxur. Recoveries were around 60% in alltested soils when methanol was extractant.

In order to study the influence of soil moisture on theextraction efficiency, external water was added accuratelyto the soil in the range of 0% to 20% (w/w). Table 5 showsthat soil moisture had a positive effect on the recovery ofPAHs, PCBs, and triazines. For example, the recoveriesof triazines progressively improved with an increasingdegree of moisture in the tested soil. This can beexplained by the high solubility of triazines in water andthe strong ability of water to absorb microwave energy. Itfollows from this observation that water, as a safe andenvironmentally friendly solvent, should be considered asa viable MAE solvent to extract triazines from soil insteadof organic solvents [41]. As for PAHs (naphthalene andphenanthrene) and PCBs (PCB1242, PCB1248), suitable

soil moisture (i.e., 10%) enhances their recoveries in soil.This may be due to the fact that the water present in thematrix can allow important local heating and thus couldfavour the expansion of the pores and liberate the ana-lytes into the solvent. This could accelerate the extraction[22]. When the amount of water is significant (i.e., 20%)the recovery decrease slightly. There are certainly prob-lems of miscibility with the organic solvent used for extrac-tion. In this case, the water may act as a barrier and hinderthe transfer of the analytes from the matrix to the solvent.

74 Sun, Lee J. Sep. Sci. 2002, 25, 67–76

Table 4. Influence of different matrices on recovery of carbamate (propoxur).

Extractant Recovery (%) l RSD (%) n = 6

Soil-1a Soil-2a Soil-3a Soil-4a Soil-5a

Methanol 65.4 l 5.6 64.9 l 5.6 66.1l 6.2 68.1 l 6.2 65.8 l 5.2Hexane-acetone (1 :4) 47.5 l 4.2 48.6 l 4.3 48.3 l 4.0 46.4 l 4.5 49.2 l 4.4Hexane-acetone (1 :1) 36.2 l 3.0 35.9 l 2.9 41.0l 3.8 37.2 l 3.1 40.1 l 4.2Dichloromethane 24.2 l 2.9 23.6 l 3.0 26.0 l 2.8 24.9 l4.6 26.6 l 5.5

a Soil-1, collected from Dover Vista Park, Singapore; Soil-2, collected from Redhill, Singapore; Soil-3, collected from campus ofNational University of Singapore; Soil-4, Kieselguhr (A.R.); Soil-5, Kaolin (A.R.).

Table 5. Influence of moisture of soil-3 on recoveries of PAHs, PCBs, triazines, and carbamates.

Extractant Water% Recovery (%) l RSD (%) n = 4

PAHs PCBs Triazines Carbamatesnaphtha-

lenephenan-

threnePCB1242 PCB1248 atrazine simazine propoxur chlorpro-

pham

Hexane-acetone (1 :1) 0 79.0 l 4.1 85.1 l 5.8 88.1 l 3.7 89.8 l 3.2 91.4 l 3.0 93.5 l 4.5 41.0 l 3.8 18.4 l 4.010 80.0 l 3.6 84.2 l 4.5 89.2 l 5.0 90.2 l 4.0 93.2 l 2.9 94.6 l 3.7 36.8 l 2.8 15.2 l 2.415 79.4 l 5.0 84.6 l 3.6 86.6 l 3.6 87.9 l 4.7 94.5 l 4.5 96.2 l 4.2 30.6 l 3.8 12.1 l 3.020 81.6 l 2.9 86.1 l 4.6 82.2 l 2.8 81.8 l 3.5 95.9 l 4.0 96.8 l 5.4 18.7 l 3.0 –

Dichloromethane 0 78.3 l 3.3 83.8 l 2.9 84.8 l 4.5 84.0 l 4.8 87.7 l 6.1 89.8 l 3.6 26.0 l 2.8 5.2 l 3.010 78.5 l 6.9 80.3 l 4.7 82.9 l 5.0 83.2 l 5.4 90.9 l 3.9 91.5 l 4.5 20.9 l 3.3 2.2 l 1.615 76.8 l 6.5 78.0 l 5.5 81.5 l 6.9 79.8 l 7.3 92.6 l 4.5 93.8 l 5.2 11.5 l 2.8 –20 72.7 l 3.8 73.8 l 3.7 79.0 l 4.8 77.2 l 4.4 94.1 l 3.6 94.9 l 3.2 – –

– Not recovered.

Table 6. The influence of time-aged soil-3 on recoveries ofcarbamates.

Extractant Analyte Recovery (%) l RSD (%)

Fresh Soil Aged Soil (60 d)

Methanol propoxur 66.0 l 6.2 64.2 l 5.8methiocarb 80.0 l 5.6 78.1 l 7.1chlorpropham 65.7 l 5.1 62.1 l 5.9

Hexane-acetone propoxur 48.3 l 4.0 49.5 l 4.9(1 : 4) methiocarb 29.4 l 2.9 27.8 l 4.0

chlorpropham 35.0 l 3.9 33.7 l 5.1Hexane-acetone propoxur 41.0 l 3.8 40.5 l 4.0(1 : 1) methiocarb 18.4 l 4.0 20.0 l 2.9

chlorpropham 24.0 l 3.1 19.3 l 5.5

Although water is helpful for the improvement of theextraction efficiency, as previously suggested, hydrolysismay be responsible for the low recoveries of carbamates(propoxur, chlorpropham) due to the presence of water inthe soil.

To examine the effect of time-aged soil on the recoveriesof carbamates (propoxur, methiocarb, and chlorpropham)by MAE, freshly spiked soil and spiked soil aged for60 days were used for investigation. As shown in Table 6,there was no marked effect of this parameter on carba-mate recoveries.

4 Conclusions

For the first time, the thermal degradation of carbamates(propoxur, thiuram, propham, methiocarb, and chlorpro-pham) was studied under MAE conditions. Evidence waspresented that the thermal degradation occurred duringextraction and was largely determined by the polarity ofthe extractant. The recoveries of carbamates from soilwere also dependent on the polarity of the extractant, andthe protection afforded by the soil to the analytes wasapparent when a less polar solvent was used as extrac-tant. Hydrolysis of carbamates occurred in water underthe applied MAE conditions and led to low recoverieswhen soil moisture was present. The behavior of break-down and degradation patterns of carbamates will bestudied in a future work. In a comparison of extractionbehaviour, it was observed that recoveries of the pollu-tants from soil in the extractants considered (methanol,hexane-acetone (1 :1), dichloromethane) were dependenton the relative polarities of the class of analytes: recov-eries increase in the order of increasing polarites (tria-zines A PCBs A PAHs).

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