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Abstract

In this paper, a novel polymerized fiber doped with metal–organic frameworks (UIO-66(Zr)-NH₂) was developed in a capillary by microwave irradiation. The fiber was successfully prepared using 4-vinyl pyridine as the monomer and ethylene glycol dimethacrylate as the cross-linker to reinforce its durability. The resultant fiber was applied to extract sulfonylurea herbicides from environmental soil and water samples before analysis with high-performance liquid chromatography. The optimization of different parameters affecting solid-phase microextraction procedure was carried out. Under the optimum experimental conditions, the extraction capability of the hybrid fiber was two times greater than that of the polymer fiber without UIO-66(Zr)-NH₂.The proposed hybrid fiber extraction method exhibited low detection limits (0.19–1.79 µg/l), good precision (<12.0%), and wide linear range (10–700 µg/l). Satisfactory recoveries of sulfonylurea herbicides were obtained with spiked soil and water samples in the range of 75.7–94.2 and 82.2–95.3%, respectively.

Keywords

solid-phase microextraction sulfonylurea herbicides the hybrid fiber high-performance liquid chromatography

Introduction

Sulfonylurea herbicides (SUHs) are widely applied in agricultural fields due to their high herbicidal activity and selectivity in the control of annual, perennial gramineous, and broad-leaved grasses (Brown, 1990). Although SUHs are used at much lower dosage rates than other herbicides, as long-term applied herbicides, they are considered as a potential environmental risk due to their high phytotoxicity, adverse influence on mammals and plants, especially crops, and indirect effect on the whole food web. Because of their low level existence and complicacy of sample matrices, enrichment and cleanup are crucial procedures for the analysis of SUHs in environmental samples. So far, a number of methods have been developed to determine the concentrations of SUHs in different matrices, such as crops, soil, water, and plants. These methods are mainly based on gas chromatography (GC) (Klaffenbach and Holland, 1993), liquid chromatography (LC) (Carabias-Martínez et al., 2004; Font et al., 1998; Lian et al., 1996), high-performance liquid chromatography (HPLC)-MS (Yan et al., 2005; Zhou et al., 2010), capillary electrophoresis (Hickes and Watrous, 1999; Krynitsky and Swineford, 1995), capillary LC (Gure et al., 2013, 2014), and immunoassay (Degelmann et al., 2006; Eremin et al., 2002). Several extraction techniques have been described in SUHs analysis. They include solid-phase extraction (Niu et al., 2009; Seccia and Albrizio, 2011), molecularly imprinted polymers (Yang et al., 2010; Zhu et al., 2002), matrix solid-phase dispersion (Liang et al., 2014; Rao et al., 2012; Zhang et al., 2014), dispersive liquid–liquid microextraction (Wu et al., 2010), microwave-assisted solvent extraction (Font et al., 1998), and the one-step QUEChERS-based approach (the quick, easy, cheap, effective, rugged, and safe method) (Kaczyński and Łozowicka, 2016). However, some of them are often time consuming, require large amounts of organic solvents, and involve complicated pretreatment steps and loss of analytes. Thus, the development of new and more effective enrichment materials to monitor trace SUHs from environmental samples is needed.

Solid-phase microextraction (SPME), developed in the early 1990s by Arthur and Pawliszyn (1990), is a novel sample preparation method that is fast, highly sensitive, and less solvent consumptive. When it is comparable with other conventional sample preparation methods and can be coupled with GC or HPLC. SPME has been successfully used in biological, food, and environmental analyses (Abolghasemi et al., 2013; Chen et al., 2013; Fang et al., 2013; Vuckovic et al., 2011). In the past two decades, great progress has been made in the thermal and chemical stability of prepared SPME materials as well as the sensitivity, selectivity, and reproducibility of specific analytes. In addition, substantial effort has been made to explore novel SPME coating materials (Bagheri et al., 2012; Feng et al., 2013; Hu et al., 2015; Tian et al., 2013).

Metal–organic frameworks (MOFs) (Li et al., 1999) are a new class of porous solid material self-assembled between metal ions and organic ligands. They have attracted considerable attention due to their high surface area, tailorable chemistry, and tunable and uniform nanostructured cavities. Their unique characteristics and structure make them very promising materials for different applications such as sorbents for sampling, SPME (Wu et al., 2014), magnetic solid-phase extraction (Bagheri et al., 2012; Shu-Hui and Xiu-Ping, 2012), stationary phases for chromatography (Ameloot et al., 2010; Yan et al., 2014), and stir bar sorptive extraction (Cong et al., 2014).

More recently, MOF-based SPME fibers have garnered increasing interest. For example, fibers based on Cd(II)-MOF (Li et al., 2014), MOF-199 (Zhang et al., 2013), and MIL-101 (Xie et al., 2015) have been applied to analyze volatile polycyclic aromatic hydrocarbons and benzene homologs using headspace SPME. However, these explored SPME coatings suffer from poor solvent resistance, substrate stripping in solvents, and poor extraction performance for polar and less polar analytes. Thus, developing novel fiber materials with good solvent resistance and high affinity toward polar and less polar compounds is an important area of study. In addition, to the best of our knowledge, little has been reported on MOF hybrid fibers for the enrichment of SUHs in real samples.

Here, we synthesized UIO-66(Zr)-NH₂ (zirconium-based MOF) by solvothermal synthesis and rapidly prepared a novel SPME fiber consisting of UIO-66(Zr)-NH₂, 4-vinyl pyridine (4-VP), and ethylene glycol dimethacrylate using microwave radiation. Extraction conditions were investigated and extraction performance was evaluated. Coupled with HPLC, the new fiber was then employed to simultaneously determine the concentrations of four SUHs from different environmental samples to demonstrate its applicability.

Materials and methods

Instruments and equipment

HPLC (Agilent 1100, Inc, Waldbronn, Germany), equipped with an HP 1100 series pump system, a 20 μl quantitative loop (Rheodyne 7725i, Rheodyne, Inc., Cotati, CA, USA), a Dikma C₁₈ column (Diamonsil, 250 mm × 4.6 mm, I.D., 5 µm), and UV detector were used for all experiments. A TWCL-D magnetic stirrer was purchased from Henan Aibote Science and Technology Development Co., Ltd (Zhengzhou, Henan). An ultrasonicator (KQ-5200DE, Kunshan, China) was used for desorption of analytes. The polymerization experiments were conducted with a home microwave oven (Midea, China). The morphologies of fibers were determined on a Quanta 200 scanning electronic microscope (FEI, Hillsboro, Oregon, USA). Fused silica capillaries (I.D. 530 µm) were supplied by Yongnian Optic Fiber Plant (Handan, China).

Chemicals and reagents

Four SUHs, including metsulfuron-methyl (MSM), amidosulfuron (AS), thifensulfuron-methyl (TIM), and sulfosulfuron (SF), were purchased from the National Pesticide Engineering Research Center (Tianjin, China). Ethylene dimethacrylate (EDMA, 98%), azodiisobutyronitrile (AIBN), zirconium tetrachloride (ZrCl₄), N,N-dimethylformamide (DMF), 2-aminoterephthalic acid, and 4-VP were obtained from Aladdin Chemistry Co., Ltd (Shanghai, China). Methanol and acetonitrile of HPLC grade and acetic acid of analytical grade were purchased from Tianjin Kemiou Chemical Reagent Co. Ltd. (Tianjin, China). To remove inhibitors, the monomers were purified by distilling. All other chemicals were filtered through a 0.45 µm nylon filter before instrumental analysis. The analyte stock solutions were prepared by dissolving each analyte in acetonitrile at a concentration of 0.5 g/l, after which they were kept in a refrigerator at 4℃ until further use. The working standard solutions were prepared by diluting stock solution with ultrapure water.

Chromatographic evaluation

Chromatographic separations and detections were performed at room temperature. The mobile phase was composed of acetic acid–ultrapure water (0.1%, v/v) and acetonitrile (60/40, v/v), with a flow rate of 1.0 ml/min through the column. The UV detection wavelength was set at 235 nm. The injection volume was 20 μl.

Synthesis of UIO-66(Zr)-NH₂

UIO-66(Zr)-NH₂ was prepared according to previous research, with some modification (Chen et al., 2015). The mixture of DMF (10 ml) and acetic acid (7 ml) was first prepared, and then ZrCl₄ (0.9320 g) and 2-aminoterphthalic acid (0.7240 g) were added into this mixture. The solution was adequately stirred for 30 min at room temperature and then transferred to a Teflon-lined bomb, which was subsequently sealed and placed in an oven at 120℃ for 24 h. After cooling, the yellow precipitate was collected by centrifugation and then soaked with trichloromethane for 18 h to obtain fine crystals. The procedure was repeated three times. After centrifugation at 8000 r/min for 10 min at room temperature, the final product was evacuated in a vacuum at 120℃ for 24 h.

Fiber preparation

A 530 µm (I.D.) fused silica capillary was successively processed with 1 mol/l NaOH, ultrapure water and 1 mol/l HCl, ultrapure water for 30 min, respectively. After being dried by nitrogen gas, the pretreated capillary was cut into 9 cm long pieces. Then, 42 μl of 4-VP and 355 μl of EDMA were dissolved in the mixed solvent (1 ml of methanol and 100 μl of toluene), followed by ultrasonication for 20 min to make a homogeneous solution. UIO-66(Zr)-NH₂ and AIBN (initiator) were then added to the above solution, which was again ultrasonicated for 20 min to ensure full mixing and dissolution, followed by nitrogen treatment to remove oxygen. The obtained solution was injected into the fused silica capillary, which was then sealed at both ends with rubbers and placed in a home microwave oven for 9 min at 800 W to fabricate a homogeneous network. After the protecting polyimide layer was scraped with a pencil knife, the silica wall was etched off with 3 mol/l NH₄HF solution. Afterward, all prepared fibers were immersed in methanol to remove unreacted compounds. For comparison, a polymer fiber was prepared without UIO-66(Zr)-NH₂ using the same preparation procedure as above.

SPME procedure

Prior to use, the prepared fibers were placed into hollow 2 cm long capillaries with a 1 cm long exposed tip, which was dipped in 4 ml standard solution equipped with a 8 mm × 3 mm Teflon magnetic stirring bar at a constant stirring speed of 400 r/min on a magnetic stirrer. Desorption was then carried out with 200 μl of acetic acid/methanol (1/9, v/v) in a 300 μl vial by ultrasonication for 30 min. Then, 20 μl of the filtrated solution was injected for HPLC analysis.

Sample preparation

Water samples were collected from the local eastern lake at the Henan Institute of Science and Technology, Xinxiang, China. After filtration through a 0.45 µm nylon membrane, the sample pH was adjusted to 3.0 using 1 mol/l HCl. Soil samples were obtained from farmland outside Xinxiang, Henan Province, China. After being air-dried for five days, the soil samples were passed through a 0.45 mm sieve. Dried soil (10 g) was then extracted through ultrasonication with 20 ml acetone/ultrapure water (3:1 v/v) for 30 min. The extract was obtained by centrifugation at 6000 r/min for 10 min at room temperature. The extraction procedure was repeated three times, and acetone was subsequently evaporated at 40℃ under vacuum. The final solution was modified to 20 ml with ultrapure water and then adjusted to a pH of 3.0 using 1 mol/l HCl, with 4 ml of the solution used for SPME. For analytical performance assessment, a certain amount of each compound was added to the samples to spiked levels of 10, 50, and 200 µg/l in the lake water, and 0.02, 0.10, and 0.40 mg/kg in the soil, respectively.

Results and discussion

Characterization of the prepared fibers

The morphological properties of the prepared fibers were investigated using a scanning electron micrograph (SEM). Figure 1 shows the SEM images of the fibers after removing the “capillary coat” by NH₄HF₂. The prepared fiber possessed a continuous 3D network structure and no cracks were seen on the smooth surface of the SEM images (Figure 1(a) and (b)). In the cross-section (Figure 1(c)) of the polymer fiber without UIO-66(Zr)-66, the surface displayed similar spherical particles and an irregular porous structure. Compared with the polymer fiber, the hybrid fiber image (Figure 1(d)) revealed a uniformly porous size and structure. The obvious differences between the hybrid and polymer fibers mainly resulted from the addition of UIO-66(Zr)-66. Both fibers were flexible and rigid enough to use in repeated magnetic stirring.

Figure 1.

SEM of the hybrid fiber and the polymer fiber: (a) side section of the hybrid fiber; 160 × ; (b) cross-section of the hybrid fiber, 200 × ; (c) cross-section of the polymer fiber, 5000 × ; and (d) cross-section of the hybrid fiber, 5000 × . SEM: scanning electron micrograph.

Figure 2.

Effect of different pH values on the extraction performance.

Optimization of the hybrid fiber extraction conditions

The experimental parameters, including the extraction and desorption conditions, were investigated by HPLC peak area representing the amount of the analytes. In order to reduce the usage of organic solvents, water was chosen as an extraction solvent in this study. During the whole experiment, the concentrations of the four SUHs in ultrapure water were 400 µg/l for the SPME procedures. Several factors affecting the extraction efficiency of the four SUHs were optimized using the hybrid fiber.

Effect of pH

To investigate the effect of pH on extraction performance, hydrochloric acid was used to adjust the sample pH. The experiment was conducted for an extraction and desorption time of 30 min under desorption solvent of methanol/acetic acid (9:1, v/v). Figure 2 showed that the highest extraction capacity was found at pH 3.0 for SF and MSM, pH 5.0 for TIM, and pH 4.0 for AS. However, adsorption decreased when pH increased. This was due to the SUHs being present in their neutral molecular state, which was beneficial for the analytes to completely transfer from the extraction phase to the fiber. A pH value of 3.0 was able to meet the analytical requirements and was therefore applied as the optimum value for subsequent studies.

Effect of ionic strength

The effect of ionic strength on extraction capacity was investigated by adding sodium chloride (0–25 w/v, %) as the salting-out agent (Figure 3) to the sample solution. In this study, the peak areas of all analytes decreased with the increase in sodium chloride concentration. It is possible that the salt blocked pore passage and hindered mass transfer. Hence, no sodium chloride was added throughout the whole experiment.

Figure 3.

Effect of sodium chloride on the extraction performance.

Optimization of extraction time

As SPME is an equilibrium process, extraction time is a crucial factor affecting extraction efficiency. By varying the extraction time from 10 to 90 min (Figure 4), the results showed that extraction equilibrium of the four target analytes occurred at 60 min. Therefore, 60 min was selected as the optimum extraction time.

Figure 4.

The evolution of peak area with extraction time for four SUHs on the hybrid fiber. SUH: sulfonylurea herbicide.

Selection of desorption solvent

The desorption solvent was optimized to obtain higher desorption efficiency. Methanol, methanol–acetic acid (9:1, v/v), acetonitrile, acetonitrile–acetic acid (9:1, v/v), and mobile phase solution were selected for study. The best result was achieved with methanol–acetic acid (9:1, v/v), and hence it was applied as the optimum desorption solvent for the following experiments.

Optimization of desorption time

For the effective desorption of the adsorbed analytes from the fiber, desorption time was optimized in the range of 10–70 min with methanol–acetic acid (9:1, v/v) as the desorption solvent. Desorption equilibrium (Figure 5) was reached after ∼30 min and was therefore considered the optimal desorption time.

Figure 5.

The profile of desorption time of four SUHs. SUH: sulfonylurea herbicide.

Comparison of hybrid and polymer fibers

To evaluate the specific effect of the MOFs on the extraction performance of the fiber, two fibers were synthesized in the absence and presence of MOFs, respectively. The extraction performance of the hybrid fiber for the four target analytes was two times greater than that of the polymer fiber (Figure 6). The excellent performance of the hybrid fiber likely resulted from the combined effects of the unique porous structure of UIO-66(Zr)-NH₂, the hydrogen bond interaction between the oxygen atom of the sulfonylurea bridge and the hydrogen atom of –NH₂ in the MOFs, and the weak π–π interactions between the aromatic and heterocyclic rings.

Figure 6.

Comparison of the extraction performances of four SUHs at the same concentration of 400 µg/l by the hybrid fiber (with MOFs) and the polymer fiber (without MOFs). MOF: Metal–organic framework; SUH: sulfonylurea herbicide.

Quantitative analysis

To validate the quantification method of the target analytes in the water and soil samples, the analytical evaluation parameters were tested under the optimized conditions. Calibration curves (Table 1) were determined by applying the hybrid fiber to aqueous solutions spiked with different concentrations of SUHs. The R² values were 0.9907–0.9958 in the concentration ranging from 10 to 700 µg/l for the four analytes at S/N = 3. The limits of detection ranged from 0.19 to 1.79 µg/l. The relative standard deviations (RSDs) for the four target analytes using one fiber, based on three replicate extractions of 400 µg/l standard solution containing four SUHs, were from 4.6 to 8.5%. The fiber-to-fiber reproducibilities were lower than 5.1% by the five fibers fabricated in the same way.

Table 1.

Analytical performance for HPLC determination of four SUHs with the UIO-66(Zr)-NH₂ SPME fiber.

Compound	Regression equation	R²	Linear range (µg/l)	Detection limit (µg/l)	RSD
Compound	Regression equation	R²	Linear range (µg/l)	Detection limit (µg/l)	One bar (%, n = 3)	Bar to bar (%, n = 5)
TIM	y = 0.1212x + 1.6702	0.9958	10–700	1.45	6.2	5.1
MSM	y = 0.1738x + 0.8179	0.9907	10–700	1.32	5.8	5.3
AS	y = 0.1244x + 0.1249	0.9926	10–700	1.79	4.6	2.1
SF	y = 0.3178x + 0.5788	0.9936	10–700	0.19	8.5	4.0

HPLC: high-performance liquid chromatography; RSD: relative standard deviation; SPME: solid-phase microextraction; SUH: sulfonylurea herbicide.

Real sample analysis

The proposed method was validated by determining the concentrations of the four SUHs in two types of environmental samples (lake water and soil), respectively. The results are given in Table 2. The recoveries for the four SUHs at spiked levels of 10, 50, and 200 µg/l in five replicates were 82.2–95.3% for lake water and 75.7–94.2% at 0.02, 0.10, and 0.40 mg/kg for soil. The RSDs in the corresponding spiked samples (n = 5) were 3.6–9.9 and 2.9–11.3%, respectively. Figures 7 and 8 show typical HPLC chromatograms for the lake water and soil, respectively. The samples before (Figures 7(a) and 8(a)) and after (Figures 7(b) and 8(b)) pretreatment by the hybrid fiber were analyzed by HPLC, with no SUHs detected compared with the spiked water and soil. The HPLC baseline (Figure 7(b)) after pretreatment became flatter. However, the blank and 200 µg/l spiked samples (Figures 7(c) and 8(c)) without pretreatment had some interfering peaks. At the same time, the peak heights (Figures 7(e) and 8(e)) of the four SUHs (peak orders: 1.TIM; 2.MSM; 3.AS; 4.SF) in the spiked samples after pretreatment with the hybrid fiber were obviously higher than those (Figures 7(d) and 8(d)) of the polymer fiber. This was attributed to the different enrichment and cleanup abilities of the hybrid and polymer fibers.

Table 2.

Analytical results for the determination of SUHs in environmental samples.

Compound	Lake water						Soil
	10 µg/l		50 µg/l		200 µg/l		0.02 mg/kg		0.10 mg/kg		0.40 mg/l
	R (%)	RSD (%)	R (%)	RSD (%)	R (%)	RSD (%)	R (%)	RSD (%)	R (%)	RSD (%)	R (%)	RSD (%)
TIM	93.1	8.9	91.8	3.8	88.0	4.8	92.4	9.5	83.7	10.7	82.6	8.5
MSM	95.2	4.9	92.6	9.2	93.4	5.2	94.2	8.9	86.6	8.3	86.1	6.6
AS	91.5	9.9	88.5	8.9	82.2	5.3	86.2	10.9	83.8	2.9	75.7	5.0
SF	94.2	7.6	95.3	4.7	93.7	3.6	91.4	11.3	78.2	5.3	86.3	7.0

AS: amidosulfuron; MSM: metsulfuron-methyl; RSD: relative standard deviation; SF: sulfosulfuron; SUH: sulfonylurea herbicide; TIM: thifensulfuron-methyl.

Figure 7.

Chromatograms of lake water samples. (a) Blank lake water sample without pretreatment, (b) lake water sample after pretreatment with the hybrid fiber, (c) spiked lake water sample (200 µg/l) without pretreatment, (d) spiked lake water sample after pretreatment with the polymer fiber, and (e) spiked lake water sample after pretreatment with the hybrid fiber.

Figure 8.

Chromatograms of soil samples. (a) Blank soil sample, (b) blank soil sample after pretreatment with the hybrid fiber, (c) spiked sample (200 µg/l) without pretreatment, (d) spiked soil sample after pretreatment with the polymer fiber with the 4-vp-co-EGDMA polymer, and (e) spiked soil sample after pretreatment with the hybrid fiber. EGDMA: ethylene glycol dimethacrylate.

Conclusions

A novel hybrid fiber doped with UIO-66(Zr)-NH₂ was prepared using a common fused silica capillary and home microwave oven. The new fiber had high adsorption capacity for SUHs under optimized conditions. In view of its preparation simplicity, rapidness, and high absorption of SUHs, the developed method could be potentially applied for sample pretreatment in diverse environmental samples.

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

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A novel method for the determination of trace sulfonylurea herbicides by introducing a hybrid stationary phase to common capillary

Abstract

Keywords

Introduction

Materials and methods

Instruments and equipment

Chemicals and reagents

Chromatographic evaluation

Synthesis of UIO-66(Zr)-NH2

Fiber preparation

SPME procedure

Sample preparation

Results and discussion

Characterization of the prepared fibers

Optimization of the hybrid fiber extraction conditions

Effect of pH

Effect of ionic strength

Optimization of extraction time

Selection of desorption solvent

Optimization of desorption time

Comparison of hybrid and polymer fibers

Quantitative analysis

Real sample analysis

Conclusions

Footnotes

Declaration of Conflicting Interests

Funding

References

Synthesis of UIO-66(Zr)-NH₂