The determination of organochlorine pesticides based on dynamic microwave-assisted extraction coupled with on-line solid-phase extraction of high-performance liquid chromatography

Analytica Chimica Acta 589 (2007) 239–246

The determination of organochlorine pesticides based on dynamic microwave-assisted extraction coupled with on-line solid-phase extraction of high-performance liquid chromatography Ligang Chen, Lan Ding ∗ , Haiyan Jin, Daqian Song, Huarong Zhang, Jiantao Li, Kun Zhang, Yutang Wang, Hanqi Zhang College of Chemistry, Jilin University, 2699 Qianjin Street, Changchun 130012, China Received 8 November 2006; received in revised form 26 February 2007; accepted 1 March 2007 Available online 12 March 2007

Abstract A rapid technique based on dynamic microwave-assisted extraction coupled with on-line solid-phase extraction of high-performance liquid chromatography (DMAE–SPE–HPLC) has been developed. A TM010 microwave resonance cavity built in the laboratory was applied to concentrate the microwave energy. The sample placed in the zone of microwave irradiation was extracted with 95% acetonitrile (ACN) aqueous solution which was driven by a peristaltic pump at a flow rate of 1.0 mL min−1 . The extraction can be completed in a recirculating system in 10 min. When a number of extraction cycles were completed, the extract (1 mL) was diluted on-line with water. Then the extract was loaded into an SPE column where the analytes were retained while the unretained matrix components were washed away. Subsequently, the analytes were automatically transferred from the SPE column to the analytical column and determined by UV detector at 238 nm. The technique was used for determination of organochlorine pesticides (OCPs) in grains, including wheat, rice, corn and bean. The limits of detection of OCPs are in the range of 19–37 ng g−1 . The recoveries obtained by analyzing the four spiked grain samples are in the range of 86–105%, whereas the relative standard deviation (R.S.D.) values are <8.7% ranging from 1.2 to 8.7%. Our method was demonstrated to be fast, accurate, and precise. In addition, only small quantities of solvent and sample were required. © 2007 Elsevier B.V. All rights reserved. Keywords: Dynamic microwave-assisted extraction; Solid-phase extraction; High-performance liquid chromatography (HPLC); On-line determination; Organochlorine pesticides; Grain

1. Introduction Organochlorine pesticides (OCPs) are an important group of environmental contaminants [1–3]. They have been particularly effective in the control of pests and diseases, but their resistance to degradation has resulted in their being universal contaminants in water, soil and foods. Conventional method for extraction of OCPs from solid samples is Soxhlet extraction, which requires large amounts of solvent, and is tedious and time consuming [4]. Improving extraction techniques to minimize waste solvents and to shorten the duration of analytical procedures is very important. Modern technologies including the use of new sources of energy have

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been described, such as microwave-assisted extraction (MAE) [5–7], supercritical fluid extraction (SFE) [8,9], pressurized liquid extraction (PLE) [10,11] and subcritical water extraction (SWE) [12]. Extraction of organic pollutants using microwave energy was first introduced in 1986 by Ganzler et al. [13]. Since that time, this extraction technique has been successfully applied to extract organic compounds, such as PAHs, pesticides, PCBs and phenols, from various solid and liquid matrices, such as sediments, soils, plant materials and water samples [14–18]. MAE was more effective in reducing the sample preparation time and solvent volumes substantially in comparison with conventional extraction techniques, because the heating during MAE is based on the direct effect of microwave on molecules by ionic conduction and dipole rotation [19]. Over the past few years, MAE has been used in a static mode for accelerating the sample preparation step [20,21]. Dynamic microwave-assisted extrac-

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tion (DMAE) has also been used in recent years, allowing the automation of the preliminary step of the analytical process [22–29]. An additional problem with the determination of pesticides is the necessary cleanup and concentration of the extracts prior to chromatographic analysis. The cleanup of complex sample matrixes is usually carried out by solid-phase extraction (SPE). The SPE presents several advantages: (i) it decreases the use of toxic solvents; (ii) the extraction efficiency is not hindered by the formation of emulsions, and (iii) it offers the possibility of automation [30,31]. It has been successfully applied to cleanup different sample matrices with different sorbents, such as florisil, C18, alumina, silica and carbon [32]. But off-line detection was used in these studies. On-line SPE has gained popularity in the past years as a useful technique for extracting a wide range of analytes from biological and environmental samples [33,34]. During on-line SPE, the sample is loaded into an SPE column where the analytes are retained while the unretained matrix components are washed away. Then, the analytes are automatically transferred from the SPE column to the analytical column for their chromatographic separation [35]. On-line SPE determination of organic compounds imposes special requirements on equipment, sorbents and solvents; a fast and quantitative recovery of analytes and their separation from interfering components; application of preconcentration column of minimum volume to avoid excess peak broadening; precise sample and wash solution dispensing [36]. Because the evaporation and reconstitution steps usually present in off-line SPE are eliminated, on-line SPE method is less labor intensive, leads to better sensitivity and lower detection limits than off-line SPE. Most determinations of OCPs have been based on gas chromatography (GC) with various detectors, such as flame ionization detector (FID) [37], mass spectrometric (MS) detector [38], and electron-capture detector (ECD) [39]. But during GC-injection, one of the OCPs, p,p -DDT, degrades due to a contamination of the injection port with high boiling residues by sample injections over a sequence of routine GC analyses [40]. The use of HPLC with UV detection has also been applied to determination of OCPs [41,42]. The microwave resonance cavity is a hollow cavity consisted of metals, such as silver, copper. When a microwave field is applied to the cavity, the microwave energy is more concentrated than with a microwave oven. The microwave energy can be transformed into heat and other kinds of energy with the cavity. There are various types of cavities. 3/4 wavelength cavity which was first used by Zelikoff et al. in 1952 [43], and TM010 cavity, which was first described by Beenakker in 1976 [44], are the common used cavities currently. The 3/4 wavelength cavity consists of two concentric metal tubes. The cavity built in the laboratory was used as the microwave coupling device to extract flavonoids from Radix et caulis acanthopnacis senticosi [45]. Higher extraction efficiency was obtained than that obtained by Soxhlet extraction. Subsequently, a TM010 microwave resonance cavity also built in the laboratory was coupled with a spectrophotometer for on-line determination of safflower yellow in Flos Carthami [46]. The method was

applied simultaneously to monitor the extraction process and determine the content of safflower yellow. Compared with off-line detection, the method may provide more rapid measurement and is more convenient for obtaining continuous measurements. In this paper, the DMAE was coupled with on-line solidphase extraction of HPLC for determination of OCPs in grains, including wheat, rice, corn and bean. HPLC can overcome the above mentioned obstacle in GC analysis and also can be coupled with on-line SPE easily. In this study, on-line operation was achieved by using a two-dimensional HPLC system with multiple switching. The extraction was performed in a recirculating system. When a number of extraction cycles were completed, the extract (1 mL) was diluted on-line and introduced into the SPE column. Subsequently, the analytes retained on the SPE column were driven to the analytical column with 75% ACN aqueous solution and determined by UV detector at 238 nm. The DMAE and on-line solid-phase extraction conditions were examined and optimized. 2. Experimental 2.1. Instruments A DMAE system coupled with on-line SPE–HPLC was assembled in our laboratory (Fig. 1). In this system, an Agilent 1100 two-dimensional liquid chromatograph (Palo Alto, CA, USA) was used, which was equipped with an automatic 10-port switching valve (V 2), a 7725i injection valve (V 3), a quaternary pump (Pump 3), a heated column compartment, a UV detector, a LC workstation and a Pinnacle 11 C18 column (250 mm × 4.6 mm i.d., 5 ␮m) used as analytical column (Restek, Bellefonte, PA, USA). A guard column packed with C18 sorbent (2.1 mm × 12.5 mm i.d., 5 ␮m) was used as SPE column for cleanup and concentration of the analytes. A TM010 microwave resonance cavity built in the laboratory was applied as the microwave coupling device and its schematic diagram was represented in the literature [46,47]. Two tuning screws in the cavity were used to tune the reflected power. A WGY-20 Microwave generator (Letter Swan Instrument Co. Changchun, China) with a maximum microwave output power of 100 W was used. The extraction vessel (60 mm long, i.d. 3 mm) was made out of PTFE (polytetrafluoroethylene). A FI-2100 peristaltic pump (Pump 1, Haiguang Instrument Co. Bingjing, China) and a P230 high pressure pump (Pump 2, Elite Analytical Instruments Co. Dalian, China) were used for completing DMAE and SPE, respectively. A six-port valve (V1) and a chemifold were used. 2.2. Reagents The pentachloronitrobenzene, p,p -DDE, p,p -DDT, o,p DDE, o,p -DDT and o,p -DDD were obtained from Supelco (Bellefonte, PA, USA). Stock solutions of the above analytes at 0.5 mg mL−1 were made in ACN. Working standard solutions containing each analyte ranging from 1.0 to 50.0 ng mL−1 were prepared by diluting 0.5 mg mL−1 stock solutions with ACN.

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Fig. 1. The set up of on-line DMAE–SPE–HPLC. The arrows indicate the direction of the flow depending on the valve position. Two ports joined by a solid back line are connected and two joined by a hollow line are not, when the valve is at position A; two ports joined by a hollow line are connected and two joined by a solid back line are not, when the valve is at position B.

These solutions were used for the construction of calibration curves and the preparation of spiked grain samples. HPLC grade ACN was obtained from Fisher Corporation, USA. Pure water was obtained by the Milli-Q water purification system (Millipore Corporation, USA). 2.3. Preparation of samples The powder of grain samples, including wheat, rice, corn and bean, were purchased from Wal-Mart located in Changchun, China. Five grams of samples powder and a certain amount of standard stock solution of OCPs were mixed thoroughly. After an equilibrium time of 24 h, the solvent was eliminated with a gentle stream of nitrogen. The concentration of each analyte in the spiked sample is 400 ng g−1 . Spiked grain samples were left to stand for at least a week. Blank samples were prepared in the same way but pure solvent not containing OCPs was added.

2.4. Dynamic microwave-assisted extraction coupled with on-line solid-phase extraction high-performance liquid chromatography The procedure of on-line DMAE–SPE–HPLC was represented in Table 1. A 50.0 mg sample was weighed accurately and placed between two small plugs of glass fiber in an extraction vessel. Then the extraction vessel was put in microwave resonance cavity. Four millilitres extraction solvent was added in the solvent reservoir. Pump 1 was activated and the extraction solvent was passed through the extraction vessel. When the extraction vessel was properly filled with solvent, microwave heating was started and the two tuning screws were adjusted to get the lowest reflected power. The extraction solvent was recirculated. The total volume of the tube in the recirculating extraction system was 1 mL. The time required for one cycle was about 1 min when the flow rate of extraction solvent was set at 1 mL min−1 . When the extraction was completed, the extract was

Table 1 The procedure of on-line DMAE–SPE–HPLC analysis DMAE

Sampling

Active SPE column

Equilibrate SPE column

Sample loading

Establishing the baseline of the detector

Analyte transfer and determination

Time (min) Valve 1 position Valve 2 position Valve 3 position

10 A A B

2 B A B

3 A B B

4 A B B

4 A B B

7 A A B

30 A B A

Activated or stopped Flow rate (mL min−1 ) Solvent

Activated 1.0 95% ACN

Activated 1.0 Extract

Stopped

Stopped

Stopped

Stopped

Stopped

Pump 1

Activated or stopped Flow rate (mL min−1 ) Solvent

Stopped

Stopped

Stopped

Stopped

Activated 0.4 100% ACN

Stopped

Stopped

Pump 2

Activated or stopped Flow rate (mL min−1 ) Solvent

Stopped

Stopped

Activated 1.0 100% ACN

Activated 1.0 20% ACN

Activated 1.6 Pure water

Activated 1.0 75% ACN

Activated 1.0 75% ACN

Pump 3

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pumped into a 1 mL sample loop. The SPE column was conditioned in sequence with 3 mL ACN and 4 mL 20% ACN aqueous solution before sample loading. Subsequently, 1 mL extract collected in the sample loop was diluted on-line with pure water, and then introduced into the SPE column for cleanup and analyte concentration. The baseline of the detector was established by the chromatographic mobile phase of 75% ACN aqueous solution. Finally, the analytes trapped on the SPE column were eluted into the analytical column with the mobile phase at a flow rate of 1.0 mL min−1 and the eluate was monitored at 238 nm. DMAE can be performed by extracting the next sample during the previous chromatographic separation. The overall analysis time can be shortened in this way. 3. Results and discussion 3.1. Operation mode of DMAE–SPE–HPLC Several different operation modes for performing on-line SPE–HPLC have been proposed in the literature [33,34]. The most common of these was constructed from an autosampler, two high pressure pumps and one six-port switching valve and was used for direct analysis of liquid samples including water, plasma and urine [36,48]. During on-line SPE–HPLC, the sample was loaded into an SPE column where the analytes were retained while the unretained matrix components were washed away. Then, the analytes were automatically transferred from the SPE column to the analytical column for chromatographic separation. In this work, the DMAE coupled with on-line SPE–HPLC was used for direct analysis of solid sample. Prior to SPE, the extract had to be diluted on-line to match the water/organic solvent composition necessary for trapping analytes on the head of the SPE column. The on-line dilution of extract can be preformed by a 10-port switching valve. Initially, the extract from extraction vessel was diluted directly, and then introduced into the SPE column. But leakage of extraction vessel occurred due to the increased back pressure on the SPE column. Moreover, bubbles were generated in the DMAE which would affect the result of trapping analytes on the SPE column. Therefore, the extract (1 mL) was collected in the sample loop and introduced into the SPE column subsequently. In this way, the limitation brought by direct introduction into the SPE column was overcome.

Fig. 2. The effect of ACN concentration in extraction solvent on the recoveries of OCPs in spiked wheat samples (n = 3). o,p -DDD (1); p,p -DDT (2); o,p -DDT (3); pentachloronitrobenzene (4); p,p -DDE (5) and o,p -DDE (6).

ever, these extraction solvents do not fit for the following on-line analysis. For keeping consistency of the solvent used in the whole on-line analysis, ACN was selected as extraction solvent in which the OCPs are readily dissolved. Usually, the adding of water in the extraction solvent was useful for increasing MAE efficiency. Mixtures of ACN–water were tested for extraction of OCPs (Fig. 2). Recoveries of OCPs were very low when low concentration of ACN was used as extraction solvent. The highest recoveries were obtained when the ACN concentration was 95 and 100%, but the extract was cleaner at 95% than at 100%. 95% ACN aqueous solution was selected as extraction solvent in this work. 3.2.2. The microwave output power A high extraction temperature is optimal for accelerating the extraction process and reducing its reaction time. Temperature of the extraction medium increased with increasing microwave output power. The effect of microwave output power on the recoveries of OCPs in spiked wheat samples is represented in Fig. 3. The experimental results demonstrate that recoveries are greatly affected by the microwave output power. At 20 W recoveries are in the range of 21–33%, increasing to 57–67% at 40 W, 82–95% at 60 W, 92–103% at 80 W and 92–101% at 100 W. In this work, following extractions were performed at 80 W.

3.2. Operation parameters of DMAE Several parameters affecting the performance of the DMAE were evaluated. All experiments were performed using spiked wheat samples. A 50 mg sample was selected for all experiments. The extraction solvent volume was kept at 4 mL. Other parameters, such as extraction solvent, microwave output power, extraction time and extraction solvent flow rate were investigated. 3.2.1. The extraction solvent Traditionally, the extraction of OCPs was performed with hexane, acetone, dichloromethane or ethyl acetate [3,4]. How-

3.2.3. The extraction time The effect of the extraction time on recoveries of OCPs was investigated. From Fig. 4, we can find that the extraction can be completed in 4 min for p,p -DDE and o,p -DDE, 6 min for 1,o,p -DDD and pentachloronitrobenzene and 8 min for p,p DDT and o,p -DDT. The subsequent extractions were performed in 10 min. 3.2.4. The extraction solvent flow-rate The effect of the extraction solvent flow-rate on recoveries of OCPs was investigated (Table 2). The extraction solvent flowrate in the tested range has no significant effect on the recoveries

L. Chen et al. / Analytica Chimica Acta 589 (2007) 239–246

Fig. 3. The effect of microwave output power (W) on the recoveries of OCPs in spiked wheat samples (n = 3). o,p -DDD (1); p,p -DDT (2); o,p -DDT (3); pentachloronitrobenzene (4); p,p -DDE (5) and o,p -DDE (6).

243

Fig. 4. The effect of extraction time on the recoveries of OCPs in spiked wheat samples (n = 3). o,p -DDD (1); p,p -DDT (2); o,p -DDT (3); pentachloronitrobenzene (4); p,p -DDE (5) and o,p -DDE (6).

of OCPs. In this work, 1.0 mL min−1 of extraction solvent flow rate was selected. 3.3. Optimization of SPE 3.3.1. The sorbent of SPE column The cleanup step and preconcentration were applied to diminish matrix effect. Sorbents, such as florisil, C18, alumina, silica and carbon have been successfully applied to cleanup and concentration of OCPs in different sample matrices [32]. In on-line SPE–HPLC system, it is critical that the sorbent used in the SPE column be identical with the material packed in the analytical column in order to prevent broadening of the peak [36]. C18 was used as the material packed in both the analytical and SPE column in this work. 3.3.2. The solvent for sample loading Prior to SPE, the extract must be diluted to match the solvent for being trapped on the SPE column. The sample solution containing 5 ng mL−1 of every analyte in different concentration of ACN was introduced into the SPE column at a flow rate of 2 mL min−1 . As can be seen from Fig. 5, when the ACN concentration was equal to or lower than 20%, the recovery values ranging from 95 to 103% were obtained. However, more co-extracted matter retained on the SPE column when 10% ACN aqueous solution was used as solvent. When ACN con-

Fig. 5. The effect of ACN concentration in sample loading solvent on recoveries of OCPs (n = 3). o,p -DDD (1); p,p -DDT (2); o,p -DDT (3); pentachloronitrobenzene (4); p,p -DDE (5) and o,p -DDE (6).

centration was 30%, the recovery of o,p -DDD decreased. The recoveries of the six OCPs were all decreased when 40% ACN aqueous solution was used as solvent. In this work, the flow rate of extract and pure water were kept at 0.4 mL min−1 and 1.6 mL min−1 , respectively, to match the water/organic solvent composition.

Table 2 The effect of the extraction solvent flow-rate on recoveries of OCPs from spiked wheat samples Extraction solvent flow-rate (mL min−1 )

Recoveries of OCPs o,p -DDD

p,p -DDT

o,p -DDT

Pentachloronitrobenzene

p,p -DDE

o,p -DDE

0.6 0.8 1.0 1.2 1.4

96 (2.8) 94 (3.7) 97 (1.4) 93 (7.1) 95 (5.6)

98 (3.1) 101 (1.4) 96 (0.8) 97 (4.6) 103 (2.9)

101 (6.4) 99 (5.4) 102 (3.7) 97 (2.7) 99 (3.7)

95 (4.7) 93 (6.0) 96 (4.1) 94 (3.4) 94 (1.9)

89 (4.1) 91 (2.7) 90 (6.5) 95 (1.7) 93 (4.9)

90 (1.5) 92 (5.6) 91 (4.2) 90 (4.0) 94 (1.8)

The values in the parentheses represent R.S.D. % (n = 3).

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Fig. 6. The effect of sample flow rate (mL min−1 ) on recoveries of OCPs (n = 3). o,p -DDD (1); p,p -DDT (2); o,p -DDT (3); pentachloronitrobenzene (4); p,p DDE (5) and o,p -DDE (6).

3.3.3. The flow rate of sample loading The flow rate of the sample loading through the SPE column was tested in order to obtain the highest signal in the minimum time (Fig. 6). Both maximum and constant signal was obtained within a range of flow rates between 1.0 and 2.5 mL min−1 . When the flow rate was higher than 2.5 mL min−1 , the signal decreased gradually. The reason is that there was insufficient contact time between the OCPs and sorbent at the higher flow rate. A flow rate of 2.0 mL min−1 was selected in this work. 3.4. HPLC analysis The analysis of the extract of grain samples was carried out by HPLC with UV detection. The mobile phase used for the separation of the six OCPs was 75% ACN aqueous solution. The flow rate was 1.0 mL min−1 and the eluate was monitored at 238 nm. The chromatograms obtained for the blank and spiked wheat sample are shown in Fig. 7. The retention times of the six OCPs are represented in Table 3. Under the described operational conditions, satisfactory separation of the analytes can be completed in 30 min. 3.5. Evaluation of on-line DMAE–SPE–HPLC All validation for the procedures was performed using spiked wheat samples.

Fig. 7. The chromatograms of the blank wheat sample (a); standard of OCPs (b); and spiked wheat sample (c). o,p -DDD (1); p,p -DDT (2); o,p -DDT (3); pentachloronitrobenzene (4); p,p -DDE (5) and o,p -DDE (6).

The linearity of OCPs was determined using spiked wheat samples fortified in the range of 80–4000 ng g−1 (Table 3). The correlation coefficients (r) ranging from 0.995 to 0.999 are obtained for all the analytes. Limit of detection (LOD) values, determined as the minimum amount of each analyte in the sample matrix providing a signal to noise ratio of at least 3, are in the range of 19–37 ng g−1 (Table 3). To assess accuracy and repeatability, extractions were carried out at three fortification levels, 80, 1000 and 4000 ng g−1 .

Table 3 Validation of on-line DMAE-SPE-HPLC Analytes

Retention time (min)

Linear range (ng g−1 )

Correlation coefficient

LOD (ng g−1 )

R.S.D. (%) (n = 6)

o,p -DDD

13.2 16.9 17.6 19.9 24.2 25.1

80–4000 80–4000 80–4000 80–4000 80–4000 80–4000

0.996 0.998 0.998 0.999 0.997 0.995

26 34 32 37 19 29

4.5 3.9 6.4 7.1 5.6 5.7

p,p -DDT o,p -DDT Pentachloronitrobenzene p,p -DDE o,p -DDE

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Table 4 Recoveries of OCPs with three different fortification levels Level 1 (80 ng g−1 )

Analytes

o,p -DDD

p,p -DDT o,p -DDT Pentachloronitrobenzene p,p -DDE o,p -DDE

Level 2 (1000 ng g−1 )

Level 3 (4000 ng g−1 )

Recoveries (%)

R.S.D. (%)

Recoveries (%)

R.S.D. (%)

Recoveries (%)

R.S.D (%)

96 98 97 95 93 91

5.6 4.8 6.2 3.9 5.4 7.5

97 101 99 97 89 92

3.4 2.9 2.7 4.6 5.1 3.7

93 99 103 95 92 92

0.8 1.4 2.7 5.2 3.7 3.6

n = 6. Table 5 Recoveries of OCPs from different spiked grain samples Analytes

o,p -DDD

p,p -DDT o,p -DDT Pentachloronitrobenzene p,p -DDE o,p -DDE

Wheat

Rice

Corn

Bean

Recoveries (%)

R.S.D (%)

Recoveries (%)

R.S.D (%)

Recoveries (%)

R.S.D (%)

Recoveries (%)

R.S.D (%)

93 98 99 94 91 89

3.2 5.7 4.8 4.6 5.1 2.7

91 95 95 94 89 86

2.9 4.1 1.2 3.9 1.9 5.3

95 102 99 92 93 91

7.4 5.9 3.1 2.5 4.1 1.7

99 105 103 97 96 94

4.4 4.6 2.9 3.7 5.4 2.1

n = 6.

Results are presented in Table 4. In all three fortification levels, recoveries were in the range of 89–103%, whereas the R.S.D. values were <7.5%, ranging from 0.8 to 7.5%. 3.6. Analysis of real grain samples The proposed method was applied to analyzing different real grain samples (wheat, rice, corn and bean) collected from different markets located in Changchun, China. None of the target analytes were detected in these samples under the experimental conditions described. The recovery test was carried out by adding the analytes to each uncontaminated grain samples. The recoveries obtained (Table 5) are in the range of 86–105%, whereas the R.S.D. values are <8.7%, ranging from 1.2 to 8.7%. 4. Conclusions The rapid technique, dynamic microwave-assisted extraction coupled with on-line SPE–HPLC has been developed to determine six OCPs in four grain samples. For DMAE based on a TM010 microwave resonance cavity, the microwave output power needed to achieve efficient extraction was much reduced. The proposed method offers several advantages, and many of the problems associated with more traditional approaches have been avoided. The method is typically faster and less solvent is needed. The reliability and repeatability of the analysis are improved and the risks of sample loss and contamination are decreased as well, since the analysis and sample extraction take place in a closed, automated system. The limits of detection of OCPs obtained are in the range of 19–37 ng g−1 . It can therefore be considered that this method is promising and may be a good alternative to the traditional techniques.

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