application of solid phase microextraction for the determination of soil fumigants in water and soil...

J. Sep. Sci. 2005, 28, 98–103 www.jss-journal.de i 2005WILEY-VCH Verlag GmbH&Co. KGaA,Weinheim

Sonia FusterJoaquim BeltranFrancisco J. L�pezFelix Hern�ndez

Analytical Chemistry,Dept. Experimental Sciences,University Jaume I,PO Box 8029AP,12080 Castell�n, Spain

Application of solid phasemicroextraction for thedetermination of soil fumigants in water and soilsamples

The potential of solid phase microextraction (SPME) for the determination of the soilfumigants 1,3-dichloropropene (1,3-DCP) and methyl isothiocyanate (MITC) in envi-ronmental samples such as soil and water samples has been investigated. Directimmersion SPME followed by GC/ECD/NPD analysis allowed the rapid determinationof the two fumigants in water samples, with very little sample manipulation, giving anLOD of 0.5 lg L– 1. Precision, calculated as relative standard deviation (RSD) for sixreplicates at three concentration levels, was found to be lower than 20% at the con-centration levels tested. For the analysis of soil samples, headspace (HS)-SPMEcombined with GC/ECD/NPD analysis has been applied. Quantification using matrix-matched calibration curves allowed determination of both analytes (MITC and 1-3-DCP) with a LOD of 0.1 lg kg–1 (RSD a10%) for the two concentration levels assayed(0.02 and 0.2 mg kg–1). The HS-SPME procedure developed in this paper wasapplied to soil samples from experimental green house plots treated with metham-Na, a soil disinfestation agent that decomposes in soil to MITC. The absence of sam-ple manipulation as well as the low solvent consumption in SPME methodology areamong the main advantages of this analytical approach.

KeyWords:Soil fumigants; Soil; Water; Solid phasemicroextraction; Gas chromatography;

Received: July 22, 2004; revised: October 6, 2004; accepted: October 29, 2004

DOI 10.1002/jssc.200401888

1 Introduction

1,3-Dichloropropene (1,3-DCP) and metham-sodium(methylcarbamodithionic acid sodium salt) are widelyused as soil fumigants, applied prior to planting out ofcrops, for controlling soil fungi, various nematode species,soil-dwelling insects, and weed seeds [1]. Once in the soilenvironment, metham sodium quickly degrades to gas-eousmethyl isothiocyanate (MITC) which exerts the pesti-cide action. MITC is also the main metabolite (active com-pound) of the pesticide Dazomet (3,5-dimethyl-1,3,5-thia-diazinane-2-thione).

Both 1,3-DCP and MITC have low boiling points and highvapour pressures. Because of their high volatility, mostanalytical procedures are based on gaseous phase pre-separation followed by gas chromatographic determina-tion. Thus, analytical procedures proposed by the EPA for1,3-CP are based on a purge and trap GC/MS method foraqueous and sediment samples and on a closed-systempurge and trap for low level soil samples [2].

Headspace gas chromatography analysis is a goodapproach for the determination of these compounds. Ganet al. [3] proposed a static headspace gas chromatogra-phy (ECD and NPD detection) method for the determina-tion of 1,3-DCP and MITC in soil and water samples,reaching concentration levels down to 0.01 mg kg– 1 in soilsamples. Headspace gas chromatography methods havealso been proposed for the determination of 1,3-DCP andMITC in air samples [4] andmethyl bromide in food [5].

Solid phase microextraction (SPME) is a solvent-freetechnique that minimises sample preparation, allowingthe extraction and concentration steps to be focused intoa single step. SPME is now widely accepted as a rapidand reliable technique for the determination of severalorganic compounds in water samples due to its advan-tages of simplicity, low detection limits, very little samplemanipulation, and low solvent consumption [6–8]. Thistechnique is of increasing interest in the field of pesticideresidue analysis, especially when dealing with water sam-ples [9–11]. However, the determination of pesticides insoil samples by SPME has received only limited attention.Most applications are based on direct immersion (DI-SPME) of the fibre in a soil/water suspension [12–14] or,less frequently, in an aqueous or organic extract obtainedafter conventional solvent extraction [10, 11, 15].

98 Fuster, Beltran, L�pez,Hern�ndez

Correspondence: Joaquim Beltran, Analytical Chemistry, Dept.Experimental Sciences, University Jaume I, PO Box 8029AP,12080 Castell�n, Spain. Phone: +34 964 72 80 96.Fax: +34 964 72 80 66. E-mail: [email protected].

Abbreviations: 1,3-DCP, 1,3-dichloro propene; MITC, methylisothiocyanate; DI, Direct immersion.

SPME in determination of soil fumigants 99

Headspace SPME (HS-SPME) has also been used todetermine pesticide compounds in soil samples. Sam-pling in the headspace mode represents a significantadvantage in terms of selectivity because only volatile andsemivolatile organic compounds can be released in theheadspace. Since the fibre is not in contact with the sam-ple, background adsorption and matrix effects arereduced, which also enhances the life expectancy ofSPME fibres. HS-SPME technique has been applied tothe analysis of soil samples for the determination of orga-nochlorine pesticides [16], the herbicide oxadiazon [17,18], and a variety of volatile organic compounds (VOC)[19, 20], chlorobenzenes [21], chlorophenols [22], andchemical warfare agents as bis(2-chloroethyl)sulphide[23]. The potential of HS-SPME for the determination ofMITC has been proved by Gandini and Riguzzi [24] whoexploited the capabilities of this technique for the determi-nation of MITC in wine samples.

The aim of this work was to investigate the potential ofSPME for the determination of MITC and 1,3-DCP inwater (DI-SPME) and soil samples (HS-SPME). Finaldetermination has been carried out by gas chromatogra-phy with a dual detection system (ECD and NPD). Thedeveloped procedure has been applied to the determina-tion of MITC in soil samples collected from experimentalgreen house plots treated with metham sodium.

2 Experimental

2.1 Chemicals

Stock solutions of MITC and 1,3-DCP (mixture of two iso-mers) were prepared by dissolving 25 mg of analyticalstandards (Dr Erhenstorfer, Promochem, Wesel, Ger-many) in 50 mL acetone and were stored at –188C.Work-ing standard solutions of pesticide mixtures were pre-pared by volume dilution in acetone, methanol, or hexane,and were stored at 48C. Spiked water or soil sampleswere prepared by adding an appropriate volume of metha-nol pesticide standard solution to the sample.

Organic solvents (hexane, acetone, methanol, and ethylacetate) were of residue analysis quality (Scharlab, Bar-celona, Spain). Sodium chloride of analytical grade(Scharlab, Barcelona, Spain) was purified by overnightheating at 3008C.

2.2 Equipment

2.2.1 SPME

The SPME device for manual extraction consisted of aholder assembly and several replaceable fibres, and waspurchased from Supelco (Madrid, Spain). Three differentfibre types were compared: polyacrylate (PA, 85 lm),Carbowax/divinylbenzene (CW/DVB, 65 lm), and poly(di-methylsiloxane)/divinylbenzene Stable Flex (PDMS/DVB,

65 lm) . The fibres were conditioned prior to their first useas recommended by the manufacturer by heating them inthe injection port of the chromatographic system for 0.5–2 h at 250–3008C, depending on the fibre coating.

2.2.2 GC analysis

Chromatographic analysis was performed with a Hewlett-Packard 5890 Series II gas chromatograph (Avondale,USA) equipped with a splitless injector (2 mm ID glassliner) and a dual detection system consisting of an elec-tron capture detector (ECD) and a nitrogen phosphorusdetector (NPD). Peak areas in NPD were used for quanti-fication of MITC and those of ECD were used for 1,3-DCP(as sum of peak areas for both isomers).

The GC system was fitted with a 30 m60.25 mm ID,1.5 lm film thickness chromatographic SPB-5 column(Supelco). Injector temperature and initial oven programtemperature were investigated during the study. Finalconditions were fixed as follows:

Helium was used as carrier gas at a flow rate of1 mL min–1. The injector temperature was 1758C (5 minsplitless time) and the oven temperature was pro-grammed as follows: 358C (5.5 min); then 30 K min– 1 to708C, later 2 K min– 1 to 908C and finally 30 K min– 1 to2208C (hold time 2 min).

2.3 Extraction procedures

Recommended procedures obtained after optimisationwere as follows:

2.3.1 Solvent extraction procedure

A 10 g sample of soil wetted with 1 mL of distilled waterwere extracted with 10 mL of hexane by mechanical shak-ing for 2 h. The slurry was filtered, dried over anhydrousNa2SO4 and transferred into a GC vial for injection.

2.3.2 SPME based procedures

– SPME extraction of water was carried out by directimmersion of the PA fibre into the sample (3 mL, 30%NaCl) contained in a 4-mL clear glass vial under mag-netic stirring (120 rpm) for 30 min at 258C. Thermaldesorption of pesticides was carried out at 1758C for5 min in the hot injector in the splitless mode.

– SPME extraction of soil samples was carried out inthe headspace mode by exposing the PA fibre to theheadspace (in a 4-mL clear glass vial) over 2 g of soilwetted with 400 lL of distilled water under magneticstirring (120 rpm) for 30 min at 508C (sample pre-heating 30 min at 508C). Thermal desorption of pesti-cides was carried out at 1758C for 5 min in the hotsplit-splitless injector.


ShortCommunication


SPME quantification was performed by using externalcalibration curves obtained by extracting spiked blanksamples, as indicated in the recommended procedures (4concentration levels, n = 1).

3 Results and discussion

3.1 SPME optimisation

Initial optimisation of the SPME procedure was carried outfor the two compounds studied (MITC and 1,3-DCP) inspiked distilled water, considering parameters such astype of fibre and absorption and desorption steps sepa-rately.

Three different type of fibres (PDMS/DVB, PA, CW/DVB)were tested by fixing absorption (30 min extraction, 3 mLof water spiked at 50 lg L–1 level, ambient temperature)and desorption conditions (5 min desorption, 2708C injec-tor temperature). Although Figure 1 shows higherresponses for PDMS/DVB Stable Flex fibre, when usingthis fibre in repeated experiments, it showed a short life-time and it started cracking (seen on using an opticalmicroscope), leading to poor reproducibility. Then the PAfibre was chosen in preference to the CW/DVB fibre,because it gave better extraction efficiency for MITC witha still satisfactory response for 1,3-DCP combined with agood durability.

Optimum desorption conditions as regards time (1–10 min) and injector temperature (1758C, 2008C, and2708C) were also studied. A desorption temperature of1758C was selected as it led to higher responses. Theresults were very similar on varying the desorption time,and 5 min was selected to ensure complete desorptionand to avoid carryover effects.

Optimisation of the absorption step included severalparameters. In all experiments, thermal desorption wasperformed at 1758C for 5 min. The first variable consid-ered was ionic strength which was modified by addingdifferent NaCl concentrations [25, 26]. Both MITC and

1,3-DCP showed an improvement in extraction efficiencywith increasing NaCl concentration. Thus the addition of30% of NaCl (w/v) to the water sample was considered asthe optimum, because higher salt concentrations coulddecrease the life of some SPME fibres [15].

The effect of temperature on extraction efficiency waschecked at 25, 45, and 658C. Maximum peak areas wereobtained at 258C, as could be expected because highertemperatures favour evaporation of the analytes from thewater, thus reducing the efficiency of direct immersion-SPME.

Finally, the absorption equilibrium was studied by extract-ing a water sample spiked at 50 lg L– 1 for different times(between 5 and 120 min). Analyte mass absorbed ontothe fibre as a function of extraction time was fitted to anequation given by Ai [27] which has already been tested inour laboratory [15, 28–30]:

n = n0 (1 – e–at)

where n and n0 are the amounts of analyte absorbed at atime t and at equilibrium, respectively, and a is a param-eter that measures how fast the absorption equilibriumcan be reached in the SPME process. Fitting data for n0, aand teq (calculated as the time needed to extract 95% ofn0) are given in Table 1.

According to the fitted equations, an extraction time of30 min should limit the total amount absorbed to around60% with respect to the equilibrium situation (equilibriumtimes calculated were 76 and 93 min for MITC and 1,3-DCP, respectively), with an important reduction in anal-ysis time and, consequently, an increase of samplethroughput. However, even in the equilibrium situation,only about 2% of the initial amount of the analyte wasextracted, indicating poor efficiency in the DI-SPME forthe compounds studied (low distribution constants).Although in some cases extraction efficiency of SPME isbelow 10% (due to the fact that SPME is based on anequilibrium process and thus depends on the distributionconstants), the low value obtained for these compoundswill reduce the sensitivity of the method, and will limit itsapplicability to samples with concentrations in the low ppbrange.

The procedure developed was applied to the extraction ofMITC and the two isomers of 1,3-DCP in groundwater


Figure 1. Effect of SPME fibre type on extraction efficiencyof MITC and 1,3-DCP (3 mL water spiked at 50 lg L– 1,30 min extraction).

Table 1. Parameters resulting from mathematical fitting ofabsorbed amount versus extraction time (water samplesspiked at 50 lg L– 1; PA fibre).

n0 [ng] a teq [min](estimated)

MITC 2.6 0.039 76

1,3-DCP 2.6 0.032 93


samples spiked at medium-high concentration levels. Thelinearity of the method was tested using a series of aque-ous solutions (distilled water) in the concentration range10–500 lg L–1 (6 levels, two replicates for each level).The SPME procedure showed a linear behaviour in therange tested with r 2 values ranging between 0.995 and0.999. Precision (repeatability) of the method wasobtained by analysing six replicate water samples at threeconcentration levels each (10, 100, 500 lg L– 1). RSDvalues were below 15% at the higher concentration levels,and around 20% at the lowest level.

Figure 2 shows both ECD and NPD chromatograms of agroundwater sample spiked at 10 lg L– 1, and extractedby SPME. This technique in the DI mode is simple anddoes not require sample manipulation; however, in thisparticular case its limited sensitivity only allows its applica-tion at the ppb concentration level, with LOD of around0.1–0.5 lg L– 1.

Once the SPME technique had been tested for MITC and1,3-DCP determination in water, the next step was toextend its application to soil samples. In a first approach,spiked soil was prepared by adding 0.1 mL of a methano-lic standard mixture of pesticides to 0.5 g of soil and 3 mLof distilled water (mixed by a Vortex). Then it wasextracted by direct immersion of the PA fibre, using theprocedure developed for water samples. As the amountextracted was very low compared with the total amount ofpesticide present in the spiked sample, the use of differentorganic solvents was considered to help in releasing thepesticides from the soil matrix. Four organic solvents(methanol, acetone, hexane, and ethyl acetate) typicallyused in solvent extraction of pesticides in soil, weretested. In all experiments, the volume of organic solventadded was 300 lL. The best responses for 1,3-DCP wereobtained with methanol, while the effect of the solvent wasnot significant in the case of MITC. The effect of organicsolvent in the soil sample SPME extraction is twofoldbecause solvent addition should increase pesticiderelease from soil but also decrease the absorptivity ofSPME fibre by covering the main part of the available sur-face [31]. Different volumes of methanol (10, 100, 300,

and 500 lL) were added to the soil maintaining the massof pesticide constant (100 ng); the best results wereobtained on addition of 10 lL (i.e. the lowest volume usedfor spiking soil samples). Even under these conditions,LOD’s were not satisfactory and chromatograms pre-sented a high background noise. Therefore, we consid-ered HS-SPME as an alternative to DI-SPME. HS-SPMEhas been applied to the determination of semi-volatileorganophosphorus pesticides in soil by Ng et al. [32].Considering the physico-chemical characteristics of ana-lytes, as regards volatility, the headspace mode seems tobe an attractive alternative to direct immersion. Prelimin-ary results in the HS-SPMEmode, obtained on increasingthe extraction temperature to 508C, showed a consider-able increase in the chromatographic responses for both1,3-DCP andMITC.

A complete optimisation scheme was applied consideringthe main variables involved in HS-SPME and using thePA fibre over spiked soil samples (0.5 g of soil spiked with10 lL of methanol pesticide solution). First, three extrac-tion temperatures were considered (25, 50, and 908C).The best results were for 508C, as 258C was too low tovolatilise the analytes, while 908C produced excessivewater vapour that interfered with analyte adsorption onthe fibre.

In order to promote the transfer of pesticides from the soilto the gas phase, the effect of adding distilled water wasstudied. The addition of water can release analytes fromthe matrix and it is often used to increase extraction effi-ciency [33]. Three volumes (0, 100, and 500 lL) wereadded. As expected the addition of water increased theextraction efficiency (best results for 100 lL) as it helps toremove analytes from the soil.

Finally, the effect of increasing the amount of soil was alsostudied using 0.5, 1, and 2 g (with a fixed sample concen-tration of 0.2 mg kg– 1) while maintaining the same propor-tions of MeOH and water. As expected, increasing soilsample led to a significant increase in response; thus 2 gof soil was selected. Higher quantities were not consid-ered as adequate stirring of the slurry was not feasible.

The linearity of the method was determined by preparinga calibration curve in soil matrix, by spiking blank soil sam-ples with methanolic solutions of pesticides at six concen-tration levels between 0.01 and 0.2 mg kg–1 and applyingHS-SPME to each of them (two replicates for each level).The SPME procedure showed linear behaviour in therange tested with r 2 values ranging between 0.994 and0.999.

Precision (repeatability) of the procedure was estimated(n = 5) at two concentration levels (0.02 and 0.2 mg kg– 1).The RSD values obtained were satisfactory (below 10%)(Table 2). The accuracy of the procedure was checked byanalysing spiked samples (0.02 and 0.2 mg kg– 1 concen-


Figure 2. Gas chromatograms corresponding to a distilledwater sample spiked at 10 lg L– 1 with 1,3-DCP and MITCand extracted by SPME.


tration levels), obtaining recoveries over 90% at the twoconcentration levels assayed.

As an example, Figure 3 shows the GC (ECD and NPD)chromatograms obtained after HS-SPME of a soil samplespiked at 0.02 mg kg–1.

A conventional simple procedure based on solvent extrac-tion as published by Gan et al. [3] (see Experimental) wasalso optimized and applied for comparison with the HS-SPME (Table 2). When comparing extraction times,amount of sample used, and LOD, it can be concludedthat HS-SPME is an attractive alternative for MITC and1,3-DCP determination in soil samples at very low con-centration levels (low ppb range). The high sensitivity ofthe HS-SPME approach allows LOD’s to be obtainedwhich are well below those of other methods proposed forfumigants in soil [2], and with even better precision.

3.2 Analysis of real-world soil samples

In order to assay the applicability of the SPME procedure,levels of MITC in soil samples after application ofmetham-Na have been studied. Metham-Na decomposesrapidly in soil to the volatile MITC (DT50 23 minutes to4 days) [34]. The SPME procedure developed wasapplied to several soil samples collected from experimen-tal green house plots treated with metham-Na (Raisan-50, 50% p/v metham-Na, application rate 750 kg activeingredient/Ha). Quantification of MITC in samples wascarried out by a three point level calibration curve obtainedby extracting spiked blank soil samples (0.05–0.1 mg kg–1) and analysing every sample in duplicate.

Data for MITC concentrations in real samples followed theexpected behaviour, with a decrease in concentration asa general trend but, logically, displaying some exceptionsdue to the fact that each sample is taken separately and isaffected by spatial inhomogeneity in the soil environment.MITC concentrations decreased proportionally to timeelapsed since active ingredient application (dissipationcurve), starting with a value of 42 mg kg– 1 immediatelyafter application and decreasing to less than 1 mg kg– 1

after only 1 day. MITC disappeared after 1 week, whichagrees with the DT50 values reported in the literature [34](Table 3).

As an example, Figure 4 shows a chromatogramobtained from two soil samples analyzed, one corre-sponding to a blank (collected before application) and theother obtained 1 day after application.


Table 2. Precision (n = 5) and limit of detection of HS-SPME and solvent extraction (SOLV) procedures for the determination ofsoil disinfectant residues in soil.

RSD [%]

0.2 mg kg– 1 0.02 mg kg– 1 LOD [mg kg– 1]

SPME SOLVa) SPME SOLVb) SPME SOLVb)

MITC 7 6 5 7 0.001 0.01

1,3-DCP 3 6 6 7 0.001 0.01

a) Values calculated using spiked samples at 0.1 mg kg– 1.b) Values calculated using spiked samples at 0.05 mg kg– 1.

Figure 3. Gas chromatograms obtained after SPME wasapplied to a soil sample spiked at 0.02 mg kg– 1 with 1,3-DCPand MITC.

Table 3. Summary of MITC concentrations (mg kg– 1) foundin soil samples analyzed by SPME after application ofmetham-Na.

Days after treatment Conc. [mg kg– 1]

Untreated n.d.

0 42

1 0.5

3 1.3

4 0.03

7 n.d.

n.d.: not detected.

Figure 4. Gas chromatograms obtained from soil samplesafter HS-SPME: (A) sample obtained one day after applica-tion of metham-Na (MITC 0.5 mg kg– 1); (B) sample obtainedbefore application of metham-Na.


4 Concluding remarksHS-SPME is a useful analytical tool for the qualitative andquantitative determination of MITC and 1,3-dichloropro-pene in soil. This technique is simple and does not requirethe use of organic solvents, and could be an excellentalternative to the more laborious and time consuming con-ventional methods based on solvent extraction. The wellknown difficulty of accurate quantification has been over-come by the use of matrix-matched calibration curves.

SPME in the headspace mode allows achievement ofsatisfactory LOD and cleaner chromatograms for volatileanalytes (specially when compared with solvent extrac-tion chromatograms), without the need for extensivesample treatment (extraction and clean-up procedures)as there are no interfering peaks near the analyte peaks.

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