WO2013068026A1

WO2013068026A1 - Development of a new coating for sorptive extraction by stir bars

Info

Publication number: WO2013068026A1
Application number: PCT/EP2011/069517
Authority: WO
Inventors: Noushin RASTKARI; Reza AHMADKHANIHA
Original assignee: Tehran University Of Medical Sciences (Tums), Faculty Of Environmental University Of Tehran
Priority date: 2011-11-07
Filing date: 2011-11-07
Publication date: 2013-05-16

Abstract

It is disclosed a method to make high efficacy stir bars for sorptive extraction. The stir bars consist of glass bars coated with different kinds of carbon nanotubes. A sol- gel technique was used to provide carbon nanotubes coatings. Scanning electron micrographs of the coatings revealed a homogenous coating surface. The coated stir bars were successfully used for the extraction of poly aromatic hydrocarbons and organophosphorus pesticides in water samples.

Description

DEVELOPEMENT OF A NEW COATING FOR SORPTIVE EXTRACTION BY STIR BARS

Field of Invention

The present invention relates to a stationary phase for stir bar sorptive extraction (SBSE), e.g., for preconcentrating trace organic materials from various matrices a method of making such a coated stir bar.

Background of the Invention

In recent years, stir bar sorptive extraction (SBSE) has been employed as a novel sample preparation technique based on the same principles as those of solid-phase micro extraction (SPME). The SBSE has been widely used for enrichment and sensitive determination of priority organic micro-pollutants in water samples, as well as in other matrices [1-7]. The amount of typical coating of commercial stir bars is substantially higher than that on a SPME fiber, usually with a maximum volume of 0.5 μΐ_^. The phase ratio of SBSE is about 50-250 times lower than that in other modes of SPME, resulting in better recovery and higher sample capacity [8-10].

In SBSE, the coating layer of extraction phase is vitally important to the performance of the device. One of the limits of SBSE is that PDMS is the only polymer at present commercially available as a coating for stir bar. Since the PDMS phase is a non- polar liquid phase, it is preferable that the polarity of target analyte be low. In other words relatively high polarity compounds are not well recovered by PDMS coating [11-16].

Adsorption of compounds depends on the physico-chemical properties of stationary phase and surface area. Increasing the surface area increases the sensitivity of analysis and lowers the limit of detection [17].

Carbon-based materials have excellent adsorption and sieving properties for a wide range of organic and inorganic species [18-24].

In the present invention by applying an inexpensive and effective sol-gel technique, a suitable coating of different kinds of CNTs on glassy stir bars is achieved [25].

The results of designed experiments showed that using the developed carbon nanotube coated stir bar is a powerful technique for extraction and analysis of organic compounds in aqueous matrices. The system can be used for fast quality control of food and fragrance samples and for trace analysis in environmental, food, pharmaceutical and biomedical samples.

Summary of the Invention

The present invention provides a new and useful stir bar coated with carbon nanotubes for extraction of target compounds. The present invention uses sol-gel chemistry to provide a simple and convenient method for the synthesis of advanced material systems and applying them as surface coatings. The sol-gel chemistry provides efficient incorporation of carbon nanotube components into the polymeric structure of surface coating [26].

Among the advantages of the use of sol-gel technology in connection with the present invention are; (a) low costs, (b) simplicity and (c) ability to achieve uniformity in the synthesis of organic-carbon nanotubes composites [25].

Another object of the present invention is to provide a method for preparation of nano stationary phase of SBSE with high surface area and high adsorption power of compounds such as pesticides and polycyclic aromatic hydrocarbons in water samples.

The developed carbon nanotubes coated stir bar showed better extraction yield, higher selectivity and larger linear range than that of commercial polymeric coated stir bars to different kinds of organic compounds.

The developed carbon nanotubes coated stir bar could be used without apparent damage and kept in dried air for long time without reducing of extraction ability.

The present invention provides an extraction method for pesticides, PAH and other target compounds which avoids the safety hazards of organic solvents and their eventual evaporation and potential toxicity [27].

The present invention provides a method for making high efficacy carbon nanotubes coated stir bars which extracts target compounds at trace levels and minimum time.

Additionally, the carbon nanotubes coated stir bars made according to the present invention can be effectively used in combination with high performance chromatography systems such as GC, GC/MS, HPLC and LC/MS.

The developed carbon nanotubes coated stir bar can be used with common stir plates. Detailed description of the invention

The present invention relates to a magnetic stir bar suitable for stir bar sorptive extraction, which magnetic stir bar is provided with a coating having a polymeric structure of siloxane polymer with incorporated therein carbon nanotubes having an average length within a range of 0.5 to 50 μιη.

In a preferred embodiment, the present coating has a polymeric structure of trimethoxysilane, poly (methylhydrosiloxane) or mixtures thereof.

In another preferred embodiment, the present carbon nanotubes comprise, or preferably are, OH-functionalized single walled carbon nanotubes, having an average diameter of 0.5 nm to 4 nm and an average length of 1 μιη to 10 μπι, preferably having a an average diameter of 0.7 nm to 2 nm and an average length of 3 μιη to 8 μιη.

In yet another preferred embodiment, the carbon nanotubes comprise single walled carbon nanotubes and/or multi walled carbon nanotubes.

In another preferred embodiment, the present single walled carbon nanotubes have an average diameter of 0.5 nm to 4 nm, preferably 1 nm to 4 nm and an average length of 0.5 μιη to 20 μπι, preferably 1 μιη to 10 μπι, and wherein the multi walled carbon nanotubes have an average diameter of 20 nm to 70 nm, preferably 30 nm to 60 nm, and an average length of 1 μιη to 40 μπι, preferably 5 μιη to 30 μιη.

In another preferred embodiment, the single walled carbon nanotubes have a specific surface area of 350 to 450 m²/gram single walled nanotube.

In another preferred embodiment, the present coating having a polymeric structure of siloxane polymer has a thickness in a range from 50 μπι to 175 μπι.

Further, the present invention relates, according to another aspect, to a method for manufacturing a magnetic stir bar provided with a coating having a polymeric structure of siloxane polymer with incorporated therein carbon nanotubes, which method comprises the steps of:

(i) treating the glass surface of a magnetic stir bar with an alkaline

solution to optimally expose silanol groups on said surface for providing a treated magnetic stir bar;

(ii) preparing a liquid mixture of siloxane polymer and carbon nanotubes for providing a coating mixture;

(iii) contacting the treated magnetic stir bar with the coating mixture for providing a magnetic stir bar with a coat of coating mixture; and (iv) subjecting the magnetic stir bar with a coat of coating mixture to a temperature in the range of 40°C to 80°C, for a time period sufficient to cure the coat of coating mixture, thereby providing the magnetic stir bar having a polymeric structure of siloxane polymer with incorporated therein carbon nanotubes;

wherein the step (iii) of contacting the treated magnetic stir bar with the coating mixture may be repeated for obtaining a thicker coating.

In a preferred embodiment, the present carbon nanotubes are carbon nanotubes as defined above.

In a preferred embodiment, the siloxane polymer is trimethoxysilane, poly (methylhydrosiloxane) or mixtures thereof.

In a preferred embodiment, the present step (iv) comprises subjecting the magnetic stir bar with a coat of coating mixture to a temperature in the range of 50°C to 70°C for a time period of 20 hours to 28 hours, preferably about 24 hours.

In a preferred embodiment, the present glass surface of the magnetic stir bar is treated with an alkaline solution, preferably a NaOH solution, for a time period of 7 to 9 hours.

Considering the advantageous characteristics of the present magnetic stir bar provided with a coating having a polymeric structure of siloxane polymer with

incorporated therein carbon nanotubes, the present invention relates according to a further aspect to the use of the present magnetic stir bar for extraction of pesticides. Further, the present invention relates to the use of the present magnetic stir bar for extraction of polycyclic aromatic hydrocarbons. Still further, the present invention relates to the use of the present magnetic stir bar for quality control of, and or trace analysis in, food, fragrance, pharmaceutical and/or biomedical samples.

Description of the figures

FIG. 1 A represents scanning electron micrographs (SEM) image of surface coating of a glassy magnetic stir bar coated with single walled carbon nanotubes-OH functionalized (OH-SWCNT) at magnifications of 15000.

FIG. IB represents scanning electron micrographs (SEM) image of surface coating of a glassy magnetic stir bar coated with single walled carbon nanotubes

(SWCNT) at magnifications of 15000. FIG. 1C represents scanning electron micrographs (SEM) image of surface coating of a glassy magnetic stir bar coated with Multi-walled carbon nanotubes

(MWCNT) at magnifications of 15000.

FIG. 2 UPLC chromatogram for polycyclic aromatic hydrocarbons (PAHs) spiked into a sample of analytical grade water at the Stir bar sorptive extraction (SBSE) optimized and validated conditions. Peaks identification: (1) fluoranthene, (2) benz [a] anthracene, (3) benz [b] fluoranthene, (4) benz [k] fluoranthene, (5) benz [a] pyrene, (6) dibenz [ah] anthracene, (7) benz [ghi] perylene and (8) indeno [ 1, 2, 3 - cd\ pyrene.

Chromatography conditions: analytical column, 25 cm x 4.6 mm i.d. stainless steel analytical column packed with 5 μπι Supelcosil LC-PAH; elution conditions, 32 min linear gradient elution from 80:20 acetonitrile/water to 97:03 acetonitrile/water followed by 3 min linear gradient to 100% acetonitrile; Flow rate 1 mL min^"1; Elution temperature 33 °C ; fluorescence detection, 10 min ( X_ex at 284 nm- X_em at 464 nm) followed by 6 min ( X_ex at

274 nm - X_em at 414 nm), followed by 6 min ( X_ex at 300 nm - X_em at 446 nm), followed by 6 min ( X_ex at 296 nm - X_em at 406 nm), followed by 9 min ( X_ex at 300 nm - X_em at 470 nm). Gain was 1000 and slit, 40 nm.

FIG. 3 Ion chromatogram (SFM acquisition) of the pesticides extracted from blank water sample spiked at 50 ng L^"1. (1) dichlorvos, (2) diazinon, (3) malathion, (4) edifenphos. Chromatography conditions: The analyses were carried out using a HP-5 MS column (30 m x 0.25 mm i.d. x 0.1 μπι film thickness). The column was kept at 80 °C for 1 min, ramped at 20 °C min^"1 to 140 °C then at 4 °C min^"1 to 200 °C, then at 8 °C min^"1 to 300 °C, which was held for 5 min. Helium was used as the carrier gas with an initial column flow rate of 1.6 mL min^"1 in constant pressure mode. The present invention is further elucidated in the following non limiting examples.

Examples:

Example 1

Determination of polycyclic aromatic hydrocarbons (PAHs) in water samples

Materials and Apparatus The eight PAHs studied; benz [b] fluoranthene (B[b]F, 98%), benz [k]

fluoranthene (B[£]F, 98%), benz [a] pyrene (B[a]P, 97%), benz [ghi] perylene (B[ghi]P, 98%), indeno [ 1, 2, 3 - cd] pyrene ( I [1, 2, 3 - cd] P, 98%), fluoranthene (FL, 99%), benz [a] anthracene (B [a] A, 98%) and dibenz [ah] anthracene (DB [ah] A, 97%) were purchased from Aldrich ( St. Louis, MO, USA) and Supelco (Be!lefonte, PA, USA).

The first five are indicators of drinking water quality, whereas it is important to monitor the rest in environmental (wild animals, soil, particulate matter in air, etc.) and food (oil, fried food, etc.) [28].

Acetonitrile (ACN), water, methanol and methylene chloride of HPLC grade were supplied by Merck (Darmstadt, Germany).

Methyl trimethoxysilane (MTMOS 97%), trifluoroacetic acid (TFA 99%) and poly (methylhydrosiloxane) (PMHS) were purchased from Aldrich (St. Louis, MO, USA).

[0022] The SWCNTs synthesized by chemical vapor deposition process used as the material coating are 1-2 nm in diameter and 1-10 μπι in length (SWCNT-1, Nanoshel, Panchkula, India). The specific surface area and thermal conductivity of the SWCNTs are 350-450 m² g^"1 and 3000 ± 450 Wm^"1 k^"1, respectively. Multi-walled carbon nanotubes (MWCNTs) with range of external diameter 30-60 nm and length 5-30 μπι and single walled carbon nanotubes-OH functionalized (OH-SWCNT-1) with range of diameter 0.7-2 nm and length 3-8 μπι were purchased from the same manufacturer. Before use, SWCNTs, MWCNTs and OH-SWCNTs were dried at 200 °C for 2 h and then stored in a desiccator.

All HPLC measurement were taken using a thermo separation products (TSP) P2000 binary pump, equipped with a TSP AS 1000 auto sampler, a TSP SCMIOOO vacuum membrane degasser and a Jasco FP-1520 fluorescence detector. The chromatographic data were collected and processed using the Chrom-Card software. The optimized instrumental parameters for the chromatographic analysis of PAHs were as follows: injection loop, 50 μί; analytical column, a 25 cm x 4.6 mm i.d. stainless steel analytical column packed with 5 μπι Supelcosil LC-PAH (Supelco); elution conditions, 32 min linear gradient elution from 80:20 acetonitrile/water to 97:03 acetonitrile/water followed by 3 min linear gradient to 100%) acetonitrile. Flow rate was 1 mL min^"1 throughout. Elution temperature was maintained at 33 °C by means of a column heater with the intention of avoiding changes in

PAHs retention times; fluorescence detection, 10 min X_ex at 284 nm and X_em at 464 nm, followed by 6 min X_ex at 274 nm and X_em at 414 nm, followed by 6 min X_ex at 300 nm and X_em at 446 nm, followed by 6 min X_ex at 296 nm and X_em at 406 nm, and followed by

9 min X_ex at 300 nm and X_em at 470 nm. Gain was 1000 and slit, 40 nm.

A thermal gravity (TG) analyzer (Netzsch TG-209; Bavaria, Germany) was used to investigate the thermal stability of the coating.

Preparation of carbon nanotubes coated stir bar:

A glass tube (10 mm in length, 1mm in diameter) with an iron bar sealed in it was used as the support of coating. Before coating, the glass bar was dipped in 1 mol L^"1 NaOH solution for 8 h to expose the maximum number of silanol groups on the surface, cleaned with water, and then placed in 0.1 mol L^"1 HC1 solution for 2 h to neutralize the excess NaOH, cleaned again and dried at 150 °C for 3 h.

Preparation of the sol mixture: A 50 mg amount of different kinds of CNTs including OH-SWCNTs, SWCNTs and MWCNTs was added to 100 μΐ, of methylene chloride. Then 200 μΐ, MTMOS, 50 μΐ, distilled water and 100 μΐ, PMHS were added.

The mixture was agitated thoroughly by sonication for 30 min. Then 50 μΙ_, TFA 95% was added to the resulting mixture with ultrasonic agitation for 10 min [25].

The treated glass bar was immersed vertically into the sol mixture and held for 3 min in order that a sol-gel coating was formed on the surface of the glass. This coating process was repeated several times in the fresh sol mixture until the needed coating thickness was obtained. Subsequently, the coated stir bar was removed and placed in an oven at 60 °C for 24 h.

After removal from the oven the stir bar was cooled to room temperature and placed in a desiccator.

Characterization of CNTs coated stir bars:

The surface characteristics of the coated stir bars were investigated by scanning electron micrograph (SEM) technique (Fig. 1).

The micro structural details revealed in the images clearly show that the sol-gel coating was homogeneous with a rough surface. The film thickness was measured by the cross-section of the scanning electron microscopic image of the stir bars and was estimated from 50 μπι to 175 μπι attributing to different coating times. The CNTs are insoluble in any organic solvent because of the pure carbon element and their stable structure, which limits its usage in the sol-gel preparation. To improve the solubility of CNTs in organic solution, the OH-functionalized SWCNTs were used [29]. Here the functionalized SWCNTs contribute at two levels during sol-gel fabrication. Firstly, it improves the solubility of CNTs. Using non-functionalized CNTs, less stable homogenous solution was obtained. In addition, in the gelification step, SiOR chains formed in polymerization process and contribute to the formation of the matrix structure [30]. As can be seen from Fig la, lb and lc, the coating of OH-functionalized SWCNTs possesses more homogeneous rough surface with considerable porosity and CNTs condensation which result in larger surface area and higher extractive capacity than the other CNTs coating, so for providing the CNTs coated stir bar and performing the performance evaluation, the OH-functionalized SWCNTs (OH-SWCNT-1) were chosen as coating material.

Extracted compounds can be thermally or solvent desorbed from the stir bar, which requires the stir bar possesses good thermal or solvent stability. The thermal property of the OH-SWCNTs coating was assessed by its TG curve. The mass loss step begins at 337 °C.

The stability of the sol-gel coating in different polar solvent was also tested. After the stir bar was immersed in methanol and acetonitrile for 48 h, respectively, and then ultrasonic agitated for 60 min, the extraction peak area of the analytes did not significantly change. The thermal stability and solvent stability are due to the strong chemical binding formed between the stationary phase and the glass substrate by sol-gel technology. Good solvent- resistance of the OH-SWCNTs coated stir bar was especially advantageous to extraction of polar compounds because in most cases they should be solvent desorbed and analyzed by HPLC [12].

Stir bar sorptive extraction (SBSE), pre-treatment, extraction and desorption:

For PAH analysis, new or used stir bars were conditioned as follows: the stir bars were placed into a vial containing lmL of methanol, and treated for 5 min with sonication. Then the solvent was rejected and the procedure repeated three times. The stir bars were dried in a desiccator at room temperature and heated for 90 min at 300 °C with a nitrogen stream of about 100 mL min^"1. To extract and enrich the PAHs from water samples, a 15 mL water sample spiked at 50 ng L^"1 of mixed standard solution of PAHs was placed in a 20 mL glass vial and then extracted by an OH-SWCNT coated stir bar at 50 °C for 60 min at a stirring speed of 1000 rpm. After extraction, the stir bar was removed with clean tweezers and dried with a lint-free tissue. Then the stir bar was placed into an insert (250 μΐ. glass flat bottom) of a 2 mL vial. The insert was filled with 200 μL ACN. Desorption of the PAHs was performed with an ultrasonic device for 15 min. The device is equipped with a circulation cooler, enabling the temperature of the bath to be kept constant (25 °C). After desorption the stir bar was removed by a magnetic rod and the vial with the 250 μL insert was placed into the autosampler of the LC instrument used.

Optimization of the SBSE conditions for the OH-SWCNTs coating

The experiments comprised the optimization of the extraction time and temperature, the optimization of desorption time, temperature and the number of desorption steps, the selection of the most convenient extraction solvent and the control of the carryover.

The adequate SBSE parameters for the OH-SWCNTs coating were determined by "one factor at a time" (OF AT) method [31]. In this approach all the parameters are kept constant except one and the remaining one is changed to find the optimum condition.

To evaluate the effect of different parameters affecting the stir bar sorptive extraction yield, analytical grade water was spiked with a concentration of 50 ng L^"1 of all PAHs investigated. Each sample was analyzed in triplicate, at different experimental conditions. Results

To evaluate the effect of extraction temperature, the spiked samples were analyzed at 25 °C, 50 °C and 75 °C. Results showed that the extraction efficiency increased with temperature by a factor of 2-3 comparing 50 °C and 75 °C with 25 °C and this happened especially with the PAHs of higher molecular weight. This fact could be explained because temperature enhances the extraction kinetics of the analytes. No significant differences were observed between 50 °C and 75 °C. Therefore, the extraction temperature of 50 °C was considered as the optimum condition in the following experiments. To find the optimum desorption temperature some similar experiments were performed within the temperature range of 10-50 °C and based on the results 25 °C was chosen as optimum desorption temperature.

To find the optimum adsorption time, the extraction time was varied within 6- 300 min. Because after a time of 60min, equilibrium was almost reached, the adsorption time was set at 60min.

Desorption time was varied between 5 and 30 min. It was found that the peak areas increased from 5 to 15 min desorption time but remained nearly constant for 20 and 30 min of desorption time. As a result, the remaining investigations were performed with setting desorption time at 15 min.

The recoveries were determined for desorption with pure ACN and methanol as well as with ACN-water mixtures (4: 1, 1 : 1, 1 :4) and methanol-water mixtures (4: 1, 1 : 1, 1 :4). The best results were obtained by using pure acetonitrile as extraction solvent.

The smallest volume of desorption solvent was chosen as 200 μΐ., which enables the stir bar to be completely immersed.

The dependence of the extraction yields on the number of desorption steps was studied by five consecutive desorptions of 15 min each. The majority of the desorbed compounds was found in the first extract, and a smaller part in the second extract, while in extracts three and four the PAH concentrations were below the detection limits. Table 1 lists the recoveries of the PAHs for 50 ng L^"1 in water obtained by the stir bar sorptive extraction. The total recoveries (sum of the first and second desorption) are between 88.4 (benzo (a) pyrene) and 96.3% (dibenz [ah] anthracene).

Because between 80 and 90 % of the desorbed sample amount was found in the first extract, and to simplify the desorption procedure, all further investigations were performed with only one desorption step. Ultrasonic treatment was used to accelerate desorption of the compounds from the stir bars.

Under optimized conditions, the HPLC chromatogram showed in Fig. 2 was obtained.

The calibration curves parameters listed in table 1 were obtained under the optimized condition. Linearity of the calibration curves was determined in the range of 0.5-100 ng L^"1 for all the PAHs. The linearity of the method was established by plotting analyte areas of each compound vs. the analyte concentration. Coefficient of estimation (r²) ranged from 0.97 to 0.99. Limit of detection (LOD) was calculated as three times the baseline noise (S/N=3) after 5 successive extractions of blank samples. According to the ICH (International Conference on Harmonization of Technical Requirements for

Analytical Methods) criteria for analytical method validation, limit of quantification (LOQ) for each analyte was determined as the lowest concentration on the calibration curve with a precision of less than 20 % coefficient of variation (CV%) and an accuracy of 80 to 120 %. For all target analytes, LOD and LOQ were determined as 0.50 and 1.50 ng L^"1 respectively, which indicates sensitivity of the method. The precision of the method was evaluated in terms of repeatability (or interday precision) through calculating the analyte concentrations in quality control samples (each six replicates) on three consecutive days. Interday precision values for the analytes were always <12% (Table 2). Coating reproducibility was evaluated with QC samples (50 ng L^"1) through SBSE. Batch produced five stir bars were used for the evaluation of the reproducibility between stir bars. As shown in table 2, the reproducibility between the OH-SWCNT coated stir bars for extraction of PAHs was acceptable (RSD <12 %). The results proved the feasibility of the coating method proposed in this work. Expression of the intraday precision is based on the relative standard deviation (RSD %) of determined responses of six replicates of quality control (QC) samples, which were reported in table 1. The estimated recoveries are also shown in table 1. To determine the recovery, mean peak area of each analyte was determined for a blank water sample spiked with the PAHs and compared with that of standard solution (in acetonitrile) at the same concentration. All these results indicate the feasibility and reliability of the developed method for determining PAHs in water samples.

Example 2

Determination of organophosphorus pesticides in water sample Materials and Apparatus:

The four organophosphorus pesticides (dichlorvos, diazinon, malathion, edifenphos) were purchased from Aldrich (St. Louis, MO, USA) and Supelco (Bellefonte, PA, USA). The purity of all the standards was always higher than 98%. A working solution containing all compounds studied at a concentration of 50 μg L^"1 in methanol was prepared.

The extractions were carried out by using OH-SWCNTs coated stir bars which were described in previous sections. Prior to use, the stir bars were conditioned in an empty thermal desorption tube at 300 °C for 240 min with a helium flow of 50 mL min^"1.

For the preparation of the calibration curves and development of the assay, different amounts of target analytes standards were added to analytical grade water sample (blank matrix) to make the final concentration range from 0.5 to 1000 ng L^"1. The method was optimized by using blank water sample spiked with the organophosphorus pesticides standards at 50 ng L^"1 concentration as quality control sample (QC). The optimized condition was determined by "one factor at a time method" as described in section

"Example 1". SBSE sampling was optimized for sample solution pH, salting-out, methanol addition, adsorption time and temperature, desorption time and temperature.

20 mL of quality control sample were transferred to a 25 mL screw cap vial. 6 g of NaCl and a stir bar were added and the vial was capped with a screw cap. SBSE was

simultaneously performed at 35 °C for 60 min while stirring at 1500 rpm. After extraction, the stir bar was removed with clean tweezers, dipped briefly in analytical grade water, dried with a lint-free tissue and placed in a glass liner of a thermal desorption system. The glass liner was then placed in the thermal desorption unit. No further sample preparation was necessary.

The coated stir bars were thermally desorbed using a commercial thermal desorption unit TDS-2 (Gerstel GmbH, Miilheim a/d Ruhr, Germany) connected to a programmed-temperature vaporization (PTV) injector CIS-4 (Gerstel GmbH) by a heated transfer line. The PTV injector was installed in an Agilent 6890 GC-5973 MS system (Agilent Technologies, Palo Alto, CA, USA). The CIS-4 PTV injector was used to cryofocus the analytes prior to transfer onto the analytical column. Liquid nitrogen was used to cool the CIS-4 down to -100 °C during thermal desorption. SBSE desorption was performed at 20 °C for 0.5 min, then programmed to increase at 60 °C min^"1 to 300 °C, which was held for 5 min under a flow of 50 mL min^"1 helium. The injector temperature was held at -100 °C for 0.5 min, then programmed to increase at 12 °C s^"1 to 300 °C, which was held for 10 min. The analyses were carried out using a HP-5 MS column (30 m x 0.25 mm i.d. x 0.1 μπι film thickness). The column was kept at 80 °C for 1 min, ramped at 20 °C min^"1 to 140 °C then at 4 °C min^"1 to 200 °C, then at 8 °C min^"1 to 300 °C, which was held for 5 min. Helium was used as the carrier gas with an initial column flow rate of

1.6 mL min^"1 in constant pressure mode [32].

The MS was operated in the EI mode and with a scan range of m/z 50 to 550 at 2.94 scan s^" ¹ to create a selected ion monitoring (SIM) method. The SIM method was used for quantitation. Two ions were monitored for each pesticide as qualifier and target ion. A representative ion chromatogram from SIM acquisition for the pesticides extracted from the fortified blank water sample is shown in Fig. 3. Results

Desorption temperature, desorption time and desorption flow (helium flow) and cryo focusing temperature in the PTV injector were evaluated to achieve the best global analyte transfer from the stir bars to the column by comparing the peak areas obtained under the same conditions. The factors affecting the analyte extraction (pH of sample solution, salting out effect, addition of methanol, volume of samples, equilibrium- time profile) were evaluated to achieve the best global analytical conditions. Based on the results the extraction efficiencies of 4 pesticides were hardly affected by the changes in pH between 5.5 and 7.5. The extraction efficiencies were also estimated upon the addition of NaCl without pH adjustment. The results indicated that the extraction efficiencies of all the pesticides were improved by NaCl addition. The best results were obtained by 30 % NaCl addition. Finally, the extraction efficiencies of the pesticides were examined upon methanol addition (0-30 %) without pH adjustment and salt addition. Decreased recoveries were observed for all pesticides. Therefore, 30% NaCl addition was finally selected for further study. The other optimized conditions were as mentioned in the "Materials and Apparatus" section.

To validate the developed SBSE method, we evaluated linearity at seven concentration levels between 5 and 1000 ng L^"1 in analytical grade water. For each level, triplicate analyses were performed. For all compounds, good linearity was achieved with coefficient of estimation (r²) above 0.96. The LODs were calculated at a signal-to-noise ratio of three. For all compounds, low LOD in the range of 0.6 - 2.4 ng L^"1 was obtained.

Absolute recovery was also assessed by replicate analyses (n=6) of fortified blank water sample at 50 ng L^"1. Each recovery was calculated by comparing peak areas with those of a direct analysis of a standard solution for calibration curves, which was spiked on quartz wool placed in an empty thermal desorption liner. The recovery was in the range of 80-88 % with low relative standard deviation (RSD) in the range of 8.2-9.5%. Validation of the method is listed in table 3. Table 1. Recovery, intraday precision and calibration curve parameters of the developed SBSE method for determining PAHs in water samples.

Table 2. Method precision for target PAHs at different concentrations (n = 6) in QC samples.

Nominal Mean of calculated RSD%

Target RSD % of calculated RE % of calculated

concentration concentration between stir compound concentration concentration³

(ngL^"1) (ngL^"1) bars

B[6]F 50 46 10.4 8 9.6

B[£]F 50 48 9.5 4 10.3

B[aP 50 45 9.4 10 9.2

B\gfa ^> 50 45 10.3 10 8.7 l[l, 2, 3-cd ? 50 46 11.2 8 10.3

FL 50 47 8.5 6 11.2

B [a] A 50 46 9.2 8 9.4

ΌΒ [α 50 47 8.6 6 9.2 Table 3.

Pesticides studied and corresponding coefficient of estimations, selected ions for quantification, linearity, limit of detection (LOD), limit of quantification (LOQ) and recovery obtained for the developed SBSE analysis of spiked blank water sample.

Linear range of

CoefBcientof LOD(ng LOQ Recovery RSD o. Compounds m/z calibration curve (ng

estimations^ L-¹) (ngL^"1) (%) (%)

L-¹)

1 dichlorvos 185 0.98 5-1000 2.4 8 80 9.5

2 diazinon 304 0.99 5-1000 0.6 2 88 8.3

3 malathion 173 0.97 5-1000 1.3 4 81 8.2

4 edifenphos 310 0.98 5-1000 1.6 5 84 9.3

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Claims

1. A magnetic stir bar suitable for stir bar sorptive extraction, which magnetic stir bar is provided with a coating having a polymeric structure of siloxane polymer with incorporated therein carbon nanotubes having an average length within a range of 0.5 to 50 μιη.

2. Magnetic stir bar according to claim 1, wherein the coating has a polymeric structure of trimethoxysilane, poly (methylhydrosiloxane) or mixtures thereof.

3. Magnetic stir bar according to claim 1 or claim 2, wherein the carbon nanotubes comprise OH-functionalized single walled carbon nanotubes, having an average diameter of 0.5 nm to 4 nm and an average length of 1 μιη to 10 μπι, preferably having a an average diameter of 0.7 nm to 2 nm and an average length of 3 μιη to 8 μιη.

4. Magnetic stir bar according to claim 1 or claim 2, wherein the carbon nanotubes comprise single walled carbon nanotubes and/or multi walled carbon nanotubes.

5. Magnetic stir bar according to claim 4, wherein the single walled carbon nanotubes have an average diameter of 0.5 nm to 4 nm and an average length of 0.5 μιη to 20 μπι and wherein the multi walled carbon nanotubes have an average diameter of 20 nm to 70 nm and an average length of 1 μιη to 40 μιη.

6. Magnetic stir bar according to any of the claims 3 to 5, wherein the single walled carbon nanotubes have a specific surface area of 350 to 450 m²/gram single walled nanotube.

7. Magnetic stir bar according to any of the claims 1 to 6, wherein the coating having a polymeric structure of siloxane polymer has a thickness in a range from 50 μπι to 175 μπι.

8. Method for manufacturing a magnetic stir bar provided with a coating having a polymeric structure of siloxane polymer with incorporated therein carbon nanotubes, which method comprises the steps of:

(i) treating the glass surface of a magnetic stir bar with an alkaline

(iii) contacting the treated magnetic stir bar with the coating mixture for providing a magnetic stir bar with a coat of coating mixture; and

(iv) subjecting the magnetic stir bar with a coat of coating mixture to a temperature in the range of 40°C to 80°C, for a time period sufficient to cure the coat of coating mixture, thereby providing the magnetic stir bar having a polymeric structure of siloxane polymer with incorporated therein carbon nanotubes;

9. Method according to claim 8, wherein the carbon nanotubes are carbon nanotubes as defined in any of the claims 1 to 7.

10. Method according to claim 8 or claim 9, wherein the siloxane polymer is trimethoxysilane, poly (methylhydrosiloxane) or mixtures thereof.

1 1. Method according to any of the claims 8 to 10, wherein step (iv) comprises subjecting the magnetic stir bar with a coat of coating mixture to a temperature in the range of 50°C to 70°C for a time period of 20 hours to 28 hours, preferably about 24 hours.

12. Use of a magnetic stir bar according to any of the claims 1 to 7, for extraction of pesticides.

13. Use of a magnetic stir bar according to any of the claims 1 to 7, for extraction of polycyclic aromatic hydrocarbons.

14. Use of a magnetic stir bar according to any of the claims 1 to 7, for quality of, and or trace analysis in, food, fragrance, pharmaceutical or biomedical samples.