Multivariate optimization of the factors influencing the solid-phase microextraction of pyrethroid pesticides in water

Journal of Chromatography A, 1124 (2006) 148–156

Multivariate optimization of the factors influencing the solid-phase microextraction of pyrethroid pesticides in water Vanessa Casas a , Maria Llompart a,∗ , Carmen Garc´ıa-Jares a , Rafael Cela a , Thierry Dagnac b a

Departamento de Qu´ımica Anal´ıtica, Nutrici´on y Bromatolog´ıa, Facultad de Qu´ımica, Instituto de Investigaci´on y An´alisis Alimentario, Campus Sur, Universidad de Santiago de Compostela, E-15782 Santiago de Compostela, Spain b Centro de Investigaciones Agrarias de Mabegondo (CIAM), Food Safety and Organic Pollutants, Abegondo Apartado 10, E-15080 A Coru˜ na, Spain Available online 3 July 2006

Abstract A method based on solid-phase microextraction (SPME) and gas chromatography with micro-electron capture detection (GC-␮ECD) has been optimized for the analysis of pyrethroids in water samples. The influence of parameters such as temperature, fibre coating, salting-out effect and sampling mode on the extraction efficiency has been studied by means of a mix-level factorial design, which allowed the study of main effects as well as two factor interactions. Finally, a method based on direct SPME at 50 ◦ C, using polydimethylsiloxane fibre is proposed. The method showed good linearity (R2 > 0.995) and repeatability (RSD ≤ 16%) for all compounds, with detection limits ranging from 0.05 pg/mL for transfluthrin to 2.18 pg/mL for permethrin, and in general ≤1 pg/mL for most pyrethroids. Reliability was demonstrated through the evaluation of the recoveries in different water samples, such as tap water, groundwater, river water, runoff water, and wastewater. These studies demonstrated the validity of external standard calibration to quantify the target compounds in real samples, including a simple dilution step for the most complex matrices, which notoriously simplifies quantification by SPME. © 2006 Elsevier B.V. All rights reserved. Keywords: Multifactor optimization; Factorial design; Solid-phase microextraction; Gas chromatogaphy-␮ECD; Water analysis; Pyrethroids; Pesticides

1. Introduction In recent years, synthetic pyrethroid pesticides have become widely used to control insects in crop production and to kill household insects. They are derived from natural compounds, the pyrethrins [1] and they are very similar in structure to them. However, pyrethroids are more resistant in the environment and they are often more toxic to insects [2]. Even though effects to humans are still unclear, the US Environmental Protection Agency (EPA) has classified some of them (cypermethrin, permethrin, and biphenthrin) as possible human carcinogens [3–5]. As a result of their predominant usage for domestic and agricultural control of pests, pyrethroids are expected to be present in environment, which is cause of increasing environmental concern. Analytical approaches for monitoring pyrethroid pesticides in water are based on liquid–liquid extraction [2,6], solid-phase extraction [6–9], and stir bar sorptive extraction (SBSE) [10].

∗

Corresponding author. Tel.: +34 981563100x14225; fax: +34 981595012. E-mail address: [email protected] (M. Llompart).

0021-9673/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.chroma.2006.06.034

Recently, solid-phase microextraction (SPME) was applied to determine some pyrethroids in water [11–14]. This technique has also been applied to fruit and vegetables [15,16]. Gas chromatography is the determination technique usually used, coupled to mass spectrometry (GC–MS) [17] or with electron-capture detection (GC-ECD) [8,11]. Many pyrethroids possess one or more halogenated atoms in their structure, which can make ECD selective and sensitive enough to analyse them. Analytical methods for multi-residue pesticides have been proposed to simultaneously determine compounds belonging to different pesticide families by SPME, including pyrethroids [14,17] although, most of these studies do not include analytical validation data for pyrethroids. The broad purpose of these methods sometimes hampers the finding of the best extraction conditions for all compound classes, and thus sensitivity may be low for some of them. Due to the increasing use of pyrethroids and to their potential environmental and even human toxicity, fully optimization of sensitive analytical methods focused on this compound family is required. The European Union Council in Directive of 3 November 1998 on the quality of water intended for human consumption established a limit for individual pesticides of 0.1 ng/mL [18].

V. Casas et al. / J. Chromatogr. A 1124 (2006) 148–156

Therefore, the main goal of this study is to fully optimize and validate a sensitive SPME-GC-␮ECD method to determine some widespread used pyrethroids (tefluthrin, transfluthrin, allethrin, tetramethrin, lambda cyhalothrin, cyphenothrin, permethrin, cyfluthrin, cypermethrin, and deltamethrin) in environmental water samples. Optimization of the experimental conditions could be carried out by varying one-factor-at-a-time. Nevertheless, this widespread methodology can lead to mistaken conclusions when the effect of a factor can be altered by other factors since interaction effects can be very important in the process of optima selection and might keep masked. Therefore, a multifactor optimization strategy has been selected in this work to simultaneously take into account the main factors and their interactions influencing the SPME process. The quality parameters have been studied and the method performance was established in terms of limits of detection (LODs), linearity and repeatability. Finally, the optimized method was applied to real water samples including tap water, groundwater, river water, runoff water, and wastewater. 2. Experimental 2.1. Reagents Cypermethrin (mixture of isomers) and deltamethrin were supplied by Supelco (Bellefonte, PA, USA). Cyphenothrin (mixture of cis and trans isomers), allethrin (mixture of stereo isomers), transfluthrin, tefluthrin, cyfluthrin (mixture of isomers), tetramethrin, permethrin (mixture of cis and trans isomers), ␭cyhalothrin were of Pestanal grade and were purchased from Riedel-de Ha¨en (Seelze, Germany). Organic solvents (acetone and n-hexane) were of pesticide grade and were obtained from Merck (Mollet del Vall´es, Barcelona, Spain). Sodium chloride was supplied by Merck and sodium thiosulphate 5-hydrate was obtained from Panreac (Castellar del Vall´es, Barcelona). Standard mixture stock solutions of about 10 mg/mL were prepared in acetone and hexane depending on the use; acetone to mix with aqueous samples and hexane to be directly injected into the GC. Working solutions were prepared by convenient dilution and stored in amber-coloured vials at −4 ◦ C before use. The fortified water samples used in this study were obtained by spiking them with the acetone standard solutions of pyrethroids. Commercially available 100 ␮m polydimethylsiloxane (PDMS), 65 ␮m polydimethylsiloxane-divinylbenzene (PDMSDVB), 85 ␮m polyacrylate (PA), 75 ␮m Carboxen-polydimethylsiloxane (CAR-PDMS), and 65 ␮m Carbowax-divinylbenzene (CW-DVB) fibres housed in manual SPME holders were obtained from Supelco. Real water samples analyzed were from different origin: tap water; ground water; river water; wastewater from urban sewage treatment plant and runoff water. Samples were filtered through glass fibre filters (Millipore, Madrid, Spain). 2.2. Experimental setup Aliquots of 8 mL water sample were placed in 10 mL glass vials. Acetone and sodium thiosulphate 5-hydrate where added

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in proportion of 2% (v/v) and 0.02% (w/v), respectively; then, the vials were sealed with a headspace aluminium cap furnished with a PTFE-faced septum. Samples were frozen, and defrost 20 min before extraction. Then, samples were let to equilibrate at the working temperature for 3 min and the SPME fibre was then exposed to the headspace over the sample or immersed into the sample for 20 min. All samples were magnetically stirred. Once finished the exposition period, the fibre was immediately inserted into the GC injection port and the chromatographic analysis was performed. The maximum recommended desorption temperature for each fibre was selected; therefore, desorption temperature was 260 ◦ C for CW-DVB, 270 ◦ C for PDMS-DVB, 280 ◦ C for PDMS, 290 ◦ C for CAR-PDMS, and 300 ◦ C for PA. Desorption time was set at 3 min. 2.3. Gas chromatographic analysis A Hewlett-Packard 6890 GC system equipped with 63 Ni electron micro-electron capture detection (␮ECD) was used. Pyrethroids were separated in a HP-5 (30 m × 0.32 mm, 0.25 ␮m film thickness) column, using nitrogen as carrier gas at an initial pressure of 12 psi. The GC oven temperature program was initially 60 ◦ C, hold 2 min, rate 20 ◦ C/min to 230 ◦ C, rate 5 ◦ C/min to 270 ◦ C with 5 min hold; rate 5 ◦ C/min to 290 ◦ C. Injector temperature was between 260 ◦ C and 300 ◦ C depending on the fibre used. Injector was operated in the splitless mode and programmed to return to the split mode after 2 min from the beginning of a run. Detector temperature was set at 300 ◦ C. 3. Results and discussion 3.1. Multivariate optimization of the SPME process First experiments were conducted to optimize the chromatographic separation of the target analytes. Fig. 1 shows the chromatogram of a standard mixture of pyrethroids. It can be noticed that some of the target pyrethroids, such as cyphenothrin, cyfluthrin and cypermethrin, gave isomeric peak clusters. In the present study, the total area of the isomers for each compound was taken into account for quantification. Some initial experiments were carried out with the purpose of selecting the factors and the factor levels to be considered in the multivariate experimental approach. In these experiments, five fibres were tested (PDMS, PDMS-DVB, CAR-PDMS, PA, CW-DVB) and only one of them, CAR-PDMS, was not suited for pyrethroid extraction. Therefore, the other four fibres were included in the experimental design. Regarding the extraction temperature, some experiments were performed at 25 ◦ C and 100 ◦ C in immersion mode. Since the results obtained at 25 ◦ C were considerably lower than those obtained at 100 ◦ C, this first temperature (25 ◦ C) was discarded for further optimization of the SPME process. Some preliminary experiments were also carried out to study the possibilities of headspace sampling mode. Results showed that all target compounds could be efficiently extracted from the headspace. Therefore, the influence of the sampling mode, direct sampling (D-SPME) and headspace sam-

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Fig. 1. GC-␮ECD chromatogram corresponding to a standard mixture of pyrethroids (20 ng/mL).

pling (HS-SPME), will be studied by the experimental design approach. A multivariate strategy of optimization was carried out to assess the influence of main factors on the microextraction process in order to select the optimal working conditions. The

factors included in the factorial design were: fibre coating (A), at four levels (PDMS, PDMS-DVB, PA, CW-DVB); temperature (B), at two levels (50 ◦ C, 100 ◦ C); sampling mode (C), at two levels (HS-SPME, D-SPME); and salt addition (NaCl) (D) at two levels (0%, 30%).

Table 1 ANOVA results showing the significance of main effects and interactions Factors

Interactions A

B

C

D

AB

AC

AD

BC

BD

CD

Transfluthrin

F-value p-ratio

0.91 0.46

15.60 0.00

42.81 0.00

3.12 0.10

1.00 0.42

0.45 0.72

1.14 0.37

12.68 0.00

16.48 0.00

19.84 0.00

Allethrin

F-value p-ratio

1.02 0.41

8.96 0.01

0.24 0.63

7.18 0.02

0.36 0.78

0.66 0.59

1.90 0.18

30.21 0.00

5.14 0.04

49.62 0.00

Tetramethrin

F-value p-ratio

0.48 0.70

3.30 0.09

3.05 0.10

4.38 0.05

0.74 0.55

0.70 0.57

1.27 0.32

12.04 0.00

0.74 0.41

0.84 0.37

Lambda cyhalothrin

F-value p-ratio

1.64 0.23

0.11 0.74

0.48 0.50

0.50 0.49

0.59 0.63

0.32 0.81

0.55 0.66

7.59 0.02

0.78 0.39

3.05 0.10

Cyphenothrin

F-value p-ratio

1.68 0.22

0.40 0.54

2.46 0.14

1.55 0.23

1.14 0.37

1.04 0.41

0.75 0.54

3.35 0.09

1.38 0.25

3.84 0.07

Permethrin

F-value p-ratio

2.83 0.08

0.10 0.75

0.90 0.36

0.00 0.96

0.42 0.74

0.02 1.00

0.19 0.90

7.67 0.02

1.04 0.33

4.72 0.05

Cyfluthrin

F-value p-ratio

2.36 0.12

0.16 0.69

4.27 0.06

0.02 0.89

0.33 0.80

0.18 0.91

0.31 0.82

6.30 0.03

0.51 0.49

4.11 0.06

Cypermethrin

F-value p-ratio

2.55 0.10

0.03 0.86

5.12 0.04

0.11 0.74

0.34 0.80

0.20 0.89

0.27 0.85

5.13 0.04

0.30 0.59

4.95 0.04

Deltamethrin

F-value p-ratio

1.65 0.23

0.08 0.78

3.14 0.10

0.08 0.78

0.65 0.59

0.69 0.57

0.46 0.72

1.81 0.20

0.26 0.62

0.65 0.43

Italized numbers are used to denote a significant effect at 90% confidence level.

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A multifactor categorical design {4 × 2 × 2 × 2}, which involved 32 runs, was selected [19]. This type of experimental design consists of all level combinations of two or more factors, where the user sets the number of levels. The experimental results obtained are analyzed using the analysis of variance (ANOVA). The experiments were carried out with 8 mL aliquots of Milli-Q water spiked at 5 ng/mL with each target analyte. Samples were magnetically stirred and the sampling time was set at 20 min to allow maximum throughput, since the chromatographic run time was 27.5 min. The analysis of variance (ANOVA) measures whether a factor contributes significantly to the variance of the response. The results of ANOVA are shown in Table 1, where F-ratios and p-

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values are given. The F-ratios measure the contribution of each factor (or interaction) to the variance of the response, while pvalues test the statistical significance of each of the factors and interactions. If p-value is ≤0.10, then the factor has statistical significance at the 90% confidence level. As can be seen in the table, all main factors were significant for the extraction of some of the target compounds. As such, fibre coating (A) was significant for permethrin and cypermethrin; temperature (B) was significant for transfluthrin, allethrin and tetramethrin; sampling mode (C) for transfluthrin, tetramethrin, cyfluthrin, cypermethrin and deltamethrin; and salt addition (D) for transfluthrin, allethrin and tetramethrin. The influence of these factors in the response can be seen in Fig. 2, which includes

Fig. 2. Mean plots of the main factors studied in multifactor categorical design: (a) fibre; (b) temperature; (c) sampling mode; (d) salt addition.

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Fig. 3. Interaction plots for some selected pyrethroids: (a) temperature-sampling mode; (b) sampling mode-salt addition; (c) temperature-salt addition.

the mean plots obtained for the four main factors. Only examples of some selected compounds are shown to underline general behaviours. As can be seen in the mean plots for the fibre factor (Fig. 2a), the mean results obtained for the four fibres are quite homogeneous for all the target compounds. It should be noticed that two fibres, PA and PDMS, exhibited the maxima responses in all cases. Therefore, these two coatings seem to be the most favourable ones for pyrethroid extraction. Good efficiencies obtained with the PDMS phase could be expected from the physico-chemical properties of pyrethroids. They are characterized by high octanol–water distribution coefficients, and low solubility in water [15]. Furthermore, pyrethroid affinity for PDMS coating led to quantitative results obtained very recently by SBSE [10]. On the other hand, PA is the most polar of the five coatings tested due to the presence of polar groups and, initially, it is recommended for the extraction of polar compounds. Nevertheless, the non-polar hydrocarbon structure of this polymer gives it the possibility of satisfactorily extracting less polar compounds. In fact, various authors have reported the efficiency of PA fibres for the extraction of low-polar species, such as polycyclic aromatic hydrocarbons (PAHs) and polybrominated diphenyl ethers (PBDEs) [20–22]. Furthermore, this coating is recommended to analyse esters (pyrethroids are esters of the pyrethric acid) [23].

The mean plots for the temperature factor are presented in Fig. 2b. Two different behaviours were observed: for the most volatile pyrethroids (tetramethrin and allethrin) higher responses were obtained at 50 ◦ C and for the rest of analytes mean responses were equivalent at the two temperatures. Regarding sampling mode (Fig. 2c), direct sampling seems to be more favourable for all compounds excluding the most volatile one. This behaviour can be explained according to the relative volatility. Transfluthrin is characterized by the highest vapour pressure among the pyrethroids studied in this work, with a value of 4 × 10−5 Torr. In contrast, vapour pressures for compounds with high molecular mass such as cypermethrin, cyfluthrin, and deltamethrin, are about 10−10 Torr. Finally, Fig. 2d shows the effect of salt addition on extraction efficiency. The addition of NaCl showed a favourable effect on the extraction of only three compounds, allethrin, transfluthrin, and tetramethrin. For the remaining compounds, this factor did not affect or even decreased the response. After this first interpretation of the experimental design results, the overall most favourable conditions seem to be: PA or PDMS fibre, at 50 ◦ C, with direct sampling and with 0% NaCl addition. Nevertheless, some two-order factors (interactions) must be fully analyzed. The interactions temperature-

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sampling mode (BC), temperature-salt addition (BD) and sampling mode-salt addition (CD) were significant for all compounds, except deltamethrin. Charts showing the simultaneous two-factor effects (BC, CD, and BD) are presented in Fig. 3 for some selected pyrethroids. The interaction between temperature and sampling mode (Fig. 3a) is clearly highlighted. Using D-SPME, the highest responses were obtained at 50 ◦ C, while by HS-SPME mode, maxima responses were obtained at 100 ◦ C. This trend was observed for all pyrethroids. The combined effect of sampling mode and NaCl addition (CD) is shown in Fig. 3b. It can be seen that the NaCl addition to the sample led to the best responses only when the extraction was performed in the headspace mode and this behaviour was also observed for all pyrethroids. Fig. 3c shows the interaction between the extraction temperature and NaCl addition; when operating at 50 ◦ C better results were obtained without salt addition for most of the compounds, except deltamethrin. For the first eluting compounds (transfluthrin, allethrin and tetramethrin), only a slight difference between 50 ◦ C and 100 ◦ C was found when salt was added. However, the differences between temperature levels became important when salt was not added. For the remainder compounds, responses at 100 ◦ C were higher by adding 30% NaCl, while at 50 ◦ C, responses improved without salt addition. Therefore, the experimental conditions giving the highest responses were 50 ◦ C without salt addition. Taking into account both main and second order effects, the most favourable extraction conditions indicated above are confirmed: PA or PDMS fibre, 50 ◦ C, D-SPME and 0% NaCl. The multifactor categorical design carried out is a complete design. The high number of experiences involved allows analyzing them using other kinds of factorial designs. Therefore, a screening design of two levels (24 involving 16 experiences) was selected in order to corroborate the optimal conditions achieved by the multifactor categorical design. Since the multifactor categorical design showed PDMS and PA fibres as the most suitable ones for the SPME of the target pyrethroids, only these two fibres have been used in this second design. Fig. 4 shows the main effect plots for some of the target compounds (transfluthrin, cyphenothrin and deltamethrin). The main effects are drawn with a line between the lower and the higher levels for the corresponding factors. The length of the lines is proportional to the magnitude of each factor effect and the sign of the slope indicates the factor level producing the highest response. Factor A (fibre) was not significant for any compound, being PDMS the most efficient for the extraction of transfluthrin, lambda cyhalothrin, cyphenothrin, cyfluthrin, and cypermethrin, while tetramethrin was better extracted with the PA fibre. For some other target compounds, such as allethrin, permethrin, and cypermethrin, both fibres gave good and comparable results. Factor B (temperature) had a negative effect on the response obtained for transfluthrin, allethrin, tetramethrin, and cyphenothrin (Fig. 4a and b). For compounds with high molecular mass, like deltamethrin (see Fig. 4c), the difference between the two temperature levels is lower. Therefore, 50 ◦ C gives better responses for most compounds. The sampling mode (factor C) is

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Fig. 4. Main effect plots for some selected pyrethroids: (a) transfluthrin; (b) cyphenothrin; (c) deltamethrin.

significant for almost all the compounds, with a strong positive effect for the first eluting compounds (transfluthrin and allethrin, the analytes with smallest molecular size). It turns in negative effect for the remaining pyrethroids (see Fig. 4). HS-SPME was then best suited for the most volatile analytes, while the other pyrethroids gave better responses with D-SPME. Finally, the addition of salt was positive for the extraction of some target compounds such as allethrin, permethrin, lambda-cyhalothrin and deltamethrin, although the slope of the corresponding lines shows this effect was not very important (see Fig. 4c). In the remaining cases, the salt addition did not affect or affected negatively the responses, following the patterns showed in Fig. 4a and b. In an attempt to propose a general method for the analysis of pyrethroids in water, considering that PDMS was the most suitable fibre for most target analytes, the extraction conditions selected were PDMS coating, D-SPME, 50 ◦ C, and no salt addition.

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Table 2 Linearity, repeatability, and limits of detection and quantification of the proposed method Compounds

Tefluthrin Transfluthrin Allethrin Tetramethrin Lambda cyhalothrin Cyphenothrin Permethrin Cyfluthrin Cypermethrin Deltamethrin

Correlation coefficients (R2 )

0.9998 0.9998 0.9972 0.9982 0.9992 0.9987 0.9987 0.9983 0.9984 0.9955

Repeatability (%RSD) 0.25 ng/mL (n = 4)

2 ng/mL (n = 5)

6.2 6.0 12.6 3.0 5.6 3.9 6.9 4.3 5.6 10.2

16.3 12.9 9.8 15.2 9.7 11.9 6.0 11.4 15.7 12.8

3.2. Performance study and application of the SPME-µECD method To evaluate the method linearity, a calibration study was performed selecting the optimal conditions indicated above. In this study, tefluthrin was included. The calibration range was from 0.02 to 10 ng/mL, taking into account eight levels and two replicates by level. The method exhibited a direct proportional relationship between the extracted amount of pyrethroids and their initial concentration in the sample, with correlation coefficients (R2 ) ranging from 0.9955 to 0.9998 for all target analytes (Table 2).

LOD (pg/mL)

LOQ (pg/mL)

0.25 0.05 0.43 0.32 0.33 1.06 2.18 0.85 1.07 0.81

0.83 0.16 1.44 1.08 1.11 3.54 7.27 2.85 3.58 2.70

Precision of the experimental procedure was assessed at two concentration levels: 0.25 and 2 ng/mL. Results showed good repeatability, with relative standard deviation (RSD) ranging from 3.0 to 16% (see Table 2). This repeatability is similar to the one achieved in other recent studies using SPE [9] and SBSE [10]. Limits of detection, LODs (signal-to-noise ratio of 3) and limits of quantification, LOQs (signal-to-noise ratio of 10) are also presented in Table 2. LODs ranged from 0.05 pg/mL for transfluthrin to 2.18 pg/mL for permethrin, being lower than 1 pg/mL in most cases. These LODs were in general one or two order of magnitude lower than those previously described for pyrethroid

Fig. 5. SPME-GC-␮ECD chromatogram corresponding to a spiked river water sample (2 ng/mL). Extraction conditions: PDMS, 50 ◦ C, DSPME, 0% NaCl.

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Table 3 Extraction recoveries (%) in real water samples Compounds

Tefluthrin Transfluthrin Allethrin Tetramethrin Lambda cyhalothrin Cyphenothrin Permethrin Cyfluthrin Cypermethrin Deltamethrin

% Recovery Tap

Groundwater

River

Wastewater

Runoff

Runoff diluted

– 110 109 96.1 104 90.3 94.2 88.7 86.5 90.3

95.0 103 116 83.7 106 111 92.0 92.1 91.1 81.1

136 92.7 76.7 134 110 101 90.5 94.0 91.6 91.7

74.8 69.7 88.0 32.9 76.7 77.7 73.6 74.2 64.6 73.3

60.5 54.8 63.3 44.3 74.7 40.5 74.6 84.4 79.2 112

77.2 78.3 125 99.9 71.2 79.7 92.0 87.1 91.8 93.2

determination in water using SPME [11,13,24] or SPE [8,9], and similar to those very recently published using the novel and highly sensitive extraction technique of SBSE coupled with MS detection [10]. The reliability of the proposed method was checked by analysing real water samples including tap water, groundwater, river water, runoff water, and wastewater. None of these selected samples showed initial detectable concentration of the target compounds. They were then spiked with them at 2 ng/mL, and analysed with the optimized procedure. The chromatogram corresponding to a sample of spiked river water is shown in Fig. 5. No matrix effect was observed in tap water, groundwater, and river water, and quantitative recoveries were obtained when these samples were quantified by external calibration (Table 3). However, for most of the studied compounds, matrix effect has been shown in runoff water and wastewater especially for tetramethrin in wastewater, and for transfluthrin, tetramethrin, and cyphenothrin in runoff water, with recovery values lower than 60%. If matrix effects are observed in a sample, the external calibration method is not valid and the standard addition protocol must be applied for each sample. Nevertheless, if the analytical method is sensitive enough, dilution of these complex samples could offer a good alternative to reduce or even eliminate matrix effect [25]. This approach was applied to the current problematic samples, which were diluted with Milli-Q water (1:10) before analysis. Since the detection limits for the method developed in this study were in general equal or lower than 1 pg/mL, this dilution level would still allow achieving good detection limits. Recovery values obtained for the diluted runoff water samples are given in Table 3. These recoveries are now highly improved for most compounds (generally higher than 80%) thanks to the sample dilution. These results confirm that the dilution step constitutes a relevant approach for matrix effect reduction. One additional advantage of sample dilution is the prevention of contamination of the SPME fibres and of the GC equipment for the analysis of “dirty” samples. The method was finally applied to the monitoring of water samples of different origin. Some of the pyrethroids studied in this work were detected in wastewater and runoff water, partic-

ularly allethrin, permethrin and lambda cyhalothrin, at the low ng/mL level. 4. Conclusions A method based on SPME coupled to GC-␮ECD detection has been optimized for the determination of pyrethroid pesticides in water samples. The extraction conditions were selected after a fully multivariate optimization process. The influence of parameters such as temperature, fibre coating, salting-out effect and sampling mode on the efficiency of the extraction has been studied using factorial designs. The optimal conditions implied the use of PDMS fibres, D-SPME, and 50 ◦ C with an exposition time of only 20 min. One of the best attainments of the proposed method is its sensitivity, with LODs ≤ 1 pg/mL for most compounds. These LODs are only comparable to those obtained by SBSE, technique that uses much more extracting phase than SPME fibres. Additionally, the method performance study demonstrated good linearity, recoveries and repeatability. The method could be directly applied to determine the target compounds in tap water, groundwater and river water and, after dilution, in runoff and wastewater avoiding matrix effect. Acknowledgements This research was supported by the projects CTQ200500425/BQU from CICYT, Spanish Commission for Research and Development (Ministerio de Ciencia y Tecnolog´ıa) and PGIDIT05RAG50302PR from Xunta de Galicia. V.C. would like to acknowledge Fundaci´on Gil D´avila for her Pre-doctoral grant. References [1] C. Cox, J. Pestic. Reform 22 (2002) 14. [2] Agency for Toxic Substances and Disease Registry (ATSDR), Toxicological Profile for Pyrethrins and Pyrethroids, US Department of Health and Human Services, Public Health Service, Atlanta (US), September 2003, http://www.atsdr.cdc.gov/. [3] C. Cox, J. Pestic. Reform 16 (1996) 15. [4] C. Cox, J. Pestic. Reform 18 (1998) 14.

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