Polydimethylsiloxane/metal-organic frameworks coated stir bar sorptive extraction coupled to gas chromatography-flame photometric detection for the determination of organophosphorus pesticides in environmental water samples

doi:10.1016/j.talanta.2016.05.001

Talanta

Volumes 156–157, 15 August 2016, Pages 126-133

https://doi.org/10.1016/j.talanta.2016.05.001 Get rights and content

Highlights

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PDMS/MOFs-coatings were explored for SBSE of OPPs with high extraction efficiency.
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A method of PDMS/MOFs-SBSE-LD-GC-FPD was developed for environmental water analysis.
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The proposed method presented fast extraction/desorption kinetics for OPPs analysis.

Abstract

In this work, the metal-organic frameworks (MOFs), MIL-101-Cr-NH₂ was synthesized via a direct hydrothermal method, and a polydimethylsiloxane (PDMS)/MIL-101-Cr-NH₂ coated stir bar was prepared by sol-gel technique. Good reproducibility was obtained for the preparation of PDMS/MIL-101-Cr-NH₂ coated stir bar with the relative standard deviations (RSDs) ranging from 3.7 to 5.2% (n=7) in one batch, and from 5.4 to 9.2% (n=7) among different batches. With the high surface area and rich benzene ring structure of MIL-101-Cr-NH₂, the prepared PDMS/MIL-101-Cr-NH₂ coated stir bar presented higher extraction efficiency for target organophosphorus pesticides (OPPs, including phorate, diazinon, malathion, fenthion, quinalphos and ethion) over PDMS coated stir bar. Based on it, a new method of PDMS/MIL-101-Cr-NH₂ coated stir bar sorptive extraction (SBSE) coupled to gas chromatography-flame photometric detection (GC-FPD) was proposed for the determination of six OPPs in environmental water samples. The operation parameters affecting the extraction efficiency of SBSE, including extraction time, stirring rate, desorption time and ionic strength, were investigated. Under the optimal conditions, the limits of detection (S/N=3) were found to be in the range of 0.043–0.085 μg L⁻¹ for the six target OPPs, and the linear range was 0.5–100 μg L⁻¹ for malathion and 0.2–100 μg L⁻¹ for other five OPPs. The RSDs of the proposed method evaluated at 1 µg L⁻¹ for each OPP were in the range of 5.9–8.7% (intra-day, n=7) and 6.1–10.7% (inter-day, n=5), respectively. The enrichment factors were varied from 110 to 151-fold (theoretical enrichment factor was 200-fold). The proposed method was applied to the analysis of OPPs in East Lake and pond water samples with recoveries in the range of 89.3–115% and 80.0–113% for the spiked East Lake and pond water samples, respectively.

Graphical abstract

Introduction

Organophosphorus pesticides (OPPs) are widely used for agricultural practices nowadays because of its high efficiency and low environmental persistence. However, acetyl-cholinesterase can be inactivated when OPPs enter human body through skin and respiratory system, causing several serious symptoms such as pulmonary edema and respiratory failure. Due to their high toxicity to human body, European Union (EU) sets a maximum level of 0.5 μg L⁻¹ for total OPPs and 0.1 μg L⁻¹ for a single compound in drinking water. Therefore, the development of sensitive, efficient and rapid methods for monitoring OPPs in environmental samples is expected urgently.

The widely used methods for the determination of OPPs are based on chromatographic techniques, such as gas chromatography (GC) [1], [2], [3] and high-performance liquid chromatography (HPLC) [4], [5]. For some OPPs with low thermal stability and insufficient volatility, HPLC is a good alternative due to its merits of good resolution, high reproducibility and easy automation. For the other majority of OPPs, GC equipped with various detectors, especially flame photometric detector (FPD), nitrogen phosphorus detector (NPD) and mass spectrometry (MS), are the most widely used one because of its high sensitivity and resolution. Among them, FPD is very sensitive and selective for OPPs analysis. Comparatively, the performance of NPD depends on the Rb bead which is consumable and should be replaced frequently; FPD is more stable and convenient. Compared with MS detector, FPD is much cheaper and more easily available. However, direct determination of OPPs in environmental samples is usually difficult because of their low concentration and complicated sample matrix. Therefore, a suitable sample preparation is usually required before GC analysis.

Conventional techniques such as liquid-liquid extraction (LLE) have been applied for the analysis of OPPs. However, LLE is time consuming and harmful to human health due to the large volume of toxic organic solvents used. Therefore, many environmentally friendly and miniaturized pretreatment techniques without/with low organic solvent consumption, such as liquid phase microextraction (LPME) [6], [7], [8], solid phase microextraction (SPME) [9], [10], [11] and stir bar sorptive extraction (SBSE) [12], [13], [14], [15], [16], [17], [18], have been employed for the determination of OPPs in recent years. SBSE, as a novel environmentally friendly microextraction technique, was developed in 1999 by Baltussen and co-workers [19]. Compared with SPME, a larger volume and surface area of coating used in SBSE brings a higher extraction efficiency of target analytes and better reproducibility. Due to its merits of simplicity, rapidity, strong ability of sample clean-up and high extraction efficiency, SBSE has been successfully applied in the analysis of food, environmental and biological samples [20].

Up to now, only three commercial coatings, PDMS, PA (polyacrylate) and EG-silicone, are available for SBSE. PDMS coating is the most used stir bar coating for OPPs analysis while PA and EG-silicone coatings are suitable for polar analytes. However, the extraction kinetics for the extraction of OPPs by PDMS coating is relatively slow. Bonet-Domingo et al. [15] applied commercial PDMS stir bar to the analysis of forty-six semi-volatile compounds including several OPPs and the extraction time was over 14 h. Liu et al. [16] prepared a home-made PDMS coated stir bar for the determination for OPPs in cucumber and potato and it cost more than 4 h for the whole extraction process. Therefore, the development of novel coatings with high extraction efficiency and fast extraction kinetics has become one of the most important research directions in SBSE. Guan et al. [17] prepared a poly(phthalazine ether sulfone ketone) coated stir bar for the analysis of organochlorine pesticides (OCPs) and OPPs. This home-made stir bar showed better performance for the interest analytes with relatively higher polarity than PDMS coated stir bar, but it still costs nearly 4 h to reach the extraction equilibriums for OPPs. In order to solve this problem, Hu et al. [18] prepared a PDMS/polythiophene (PTH) coated stir bar by the sol-gel technique. This PDMS/PTH coated stir bar could extract five target OPPs from water samples with the extraction efficiencies ranging from 56.7 to 80.9%, and the extraction equilibrium could be reached in 25 min.

Due to diverse structures and tunable porosity, metal-organic frameworks (MOFs) have attracted considerable attention and have been widely applied in catalysis [21], gas storage/capture [22], separation [23], drug delivery [24] and sensors [25] in recent years. The large diversity of structure and pore size, high surface area, good adsorption affinity and selective penetration has also made MOFs attractive as good sorbents for the analysis of different compounds such as gaseous benzene homologs [26], polycyclic aromatic hydrocarbons (PAHs) [27], [28], [29], [30], polychlorinated biphenyls (PCBs) [31], [32], estrogens [33] and pesticides [34], [35], [36]. Zhang et al. [35] prepared a hybrid material of MOF-199 and graphite oxide (GO) as a SPME fiber coating for the determination of several OCPs. The dense carbon layers of GOs making it a good supporting material to favor the dispersive forces of MOF-199. It cost just 40 min for the extraction of target OCPs and the composite coating showed better efficiency for OCPs than PDMS and PDMS/DVB coatings under the optimal extraction time. Zhang et al. [36] synthesized a magnetic MIL-101-Fe hybrid material and applied it for the analysis of several OPPs in human hair and urine samples. This hybrid material combined the good properties of MIL-101-Fe with the ferromagnetic properties of Fe₃O₄ and exhibited a good extraction and desorption kinetics for the target OPPs. But the LODs of this method for some target OPPs were relatively high due to the low enrichment factors. To the best of our knowledge, the research works on the application of MOFs as the SBSE coatings are relatively scarce, only PDMS/IRMOF-3 [33] and PDMS/MIL-53-Al-NH₂ [29] have been explored as SBSE coatings by Hu et al. for the analysis of estrogens and PAHs, respectively.

The aim of this work was to develop a new method of PDMS/MIL-101-Cr-NH₂-SBSE-GC-FPD for the determination of OPPs in environmental water samples. For this purpose, MIL-101-Cr-NH₂ was synthesized by a direct hydrothermal method, and a novel PDMS/MIL-101-Cr-NH₂ coated stir bar was prepared by sol-gel technique. The parameters affecting extraction of target OPPs by PDMS/MIL-101-Cr-NH₂-SBSE, including extraction time, stirring rate and salt concentration, were investigated. The analytical performance of the developed method was validated, and the method was applied to the analysis of OPPs in East Lake and pond water samples to demonstrate its applicability.

Section snippets

Reagents and standards

Hydroxyl-terminated polydimethylsiloxane (OH-PDMS) was purchased from Sigma-Aldrich (Milwaukee, WI, USA). Methyltrimethoxysilane (MTMS) and poly (methylhydrosiloxane) (PMHS) were purchased from the Chemical Plant of Wuhan University (Wuhan, China). Trichloromethane, trifluoroacetic acid (TFA), dichloromethane, acetone, sodium chloride, sodium hydroxide and chromic nitrate hydrate were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). 2-Aminoterephthalic acid was purchased

Characterization of MIL-101-Cr-NH₂

Fig. S2 shows the XRD pattern of the synthesized MIL-101-Cr-NH₂. As can be seen, a typical structure with a similar pattern to the simulated curve of MIL-101 was presented. The synthesized MIL-101-Cr-NH₂ was characterized by TG analysis and the results are shown in Fig. S3. The weight loss appeared at below 100 °C was attributed to the removal of physically adsorbed water and the weight loss at around 280 °C was possibly caused by the collapse of the framework. In other words, the prepared

Conclusions

In this work, MIL-101-Cr-NH₂ was synthesized by a direct hydrothermal method, and PDMS/MIL-101-Cr-NH₂ coated stir bars were prepared by sol-gel technique for the sorptive extraction of the target OPPs. The prepared PDMS/MIL-101-Cr-NH₂ coated stir bar is featured with low cost, good reproducibility, mechanical property and high extraction efficiencies for target OPPs. Based on it, a new method of PDMS/MIL-101-Cr-NH₂-SBSE-GC-FPD was proposed for the determination of six target OPPs in

Acknowledgments

Financial support from the National Natural Science Foundation of China (20775057) and the Science Fund for Creative Research Groups of NSFC (Nos. 20621502, 20921062) are gratefully acknowledged.

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Polydimethylsiloxane/metal-organic frameworks coated stir bar sorptive extraction coupled to gas chromatography-flame photometric detection for the determination of organophosphorus pesticides in environmental water samples

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Reagents and standards

Characterization of MIL-101-Cr-NH2

Conclusions

Acknowledgments

Anal. Chim. Acta

J. Chromatogr. A

Food Chem.

Talanta

Anal. Chim. Acta

Talanta

J. Chromatogr. A

J. Chromatogr. A

Anal. Chim. Acta

J. Chromatogr. A

J. Chromatogr. A

J. Chromatogr. A

J. Chromatogr. A

J. Chromatogr. A

J. Chromatogr. A

Anal. Chim. Acta

J. Chromatogr. A

Talanta

J. Chromatogr. A

Talanta

Characterization of MIL-101-Cr-NH₂