Elsevier

Talanta

Volume 154, 1 July 2016, Pages 23-30
Talanta

Zirconium (IV)-based metal organic framework (UIO-67) as efficient sorbent in dispersive solid phase extraction of plant growth regulator from fruits coupled with HPLC fluorescence detection

https://doi.org/10.1016/j.talanta.2016.03.038Get rights and content

Highlights

  • A high efficient method for analyzing plant growth regulators with good linearity, low LODs.

  • The UIO-67 was used for adsorbent of DSPE for enrichment of 8 plant growth regulators.

  • The consumption of sorbent is low and extraction efficiency was greatly improved.

Abstract

A stable zirconium (Ⅳ)-based metal organic frameworks (UIO-67) material possessing good chemical, thermal and water stability was synthesized and applied as a sorbent for the dispersive solid phase extraction (DSPE) of 8 plant growth regulators (PGRs) in fruit samples. Fluorescence labeling combined with high performance liquid chromatography fluorescence detection (HPLC-FLD), was used to quantify the target analytes. Characterization of the UIO-67 material was performed by X-ray diffraction(XRD) and scanning electron microscopy (SEM). The experimental parameters, such as amount of UIO-67, type and volume of eluting solvent, adsorption and desorption time, were optimized. Under the optimized conditions, good linearity was observed in the range of 10–1000 pmol/mL with R2>0.9989. The limits of detection and limits of quantification were in the range of 0.21–0.57 ng/mL and 0.81–1.91 ng/mL, respectively. The intra-day and inter-day precisions (based on the relative standard deviation, n=3) of the PGR derivatives were under 3.1% and 5.3% respectively and the accuracies of the method for the PGRs were in the range from 89.3% to 102.3%. The developed method was successfully applied to analyze PGRs residues in fruit samples. The proposed method is proved to be simple, environment-friendly and inexpensive and it is feasible to directly use UIO-67 as sorbent to extract targets by varying conditions.

Introduction

Plant growth regulators (PGRs), either produced naturally by the plant or artificially synthesized, play a crucial role in coordinating growth and development in a whole range of developmental processes of plants including cell division and differentiation, enlargement, organogenesis and germination, as well as adjusting to numerous external environmental stimuli [1]. PGRs are widely used in agriculture to increase plant growth and reproduction including crops as well as fruit trees and vegetables. However, the use of PGRs in agriculture has also raised some questions, for example, the presence in fruits and vegetables resulted in potential threaten to humans and animals, including precocity, impaired reproduction, carcinogenicity, acute toxicity and neurotoxicity [2]. And legislature have made the maximum residue limits (MRL) for health protection, for example, the U.S. Environmental Protection Agency and the European Union(EU) have set up a MRL value of 100 μg/kg for naphthylacetic acid (NAA) in pome fruits and a higher value (1000 μg/kg) for NAA in apples and pears [3], and the EU has set a MRL of 5 mg/kg for gibberellin A3 (GA3) in grapes [3], [4]. So maintaining the lowest PGR residue concentration in commodities is very important in market monitoring and international trade [5]. Given these concerns, many dealers are currently labeling their products as free-pollutant vegetables and fruits in order to highlight the safety of their products. Therefore, the development of sensitive and efficient analytical methods for determining the PGRs in foods is of the most importance and urgency.

However, due to the complex biological matrix and trace levels of PGRs in fruits and vegetables, the determination of PGRs residues in samples is challenging. And more importantly, sample preparation affects nearly all the subsequent experiment steps and is critical for the unequivocal identification, especially those present at trace or ultra-trace levels in complex matrices. In order to decrease the matrix effect and protect the analytical instruments in the long run, various pretreatment techniques have been employed for the purification of PGRs in different samples, such as solid phase extraction (SPE) [6], [7], classical liquid–liquid extraction (LLE) [8], [9], solid phase microextraction (SPME) [10], [11] and dispersive liquid–liquid microextraction (DLLME) [12], [13]. However, some operations came with certain limitations including time consuming and tedious, and analytes loss, which usually leads to poor accuracy and low recovery. SPME was a solvent-free sample pretreatment but the fragile SPME fibers have a limited lifetime. Compared with LLE and SPE, DLLME has gained considerable applicability in analysis of PGRs with advantages of being economical, effective, and solvent-minimized. Even so, it is still important to develop simple and reliable purification procedures for the accurate determination of PGRs.

It is worth mentioning that the dispersive solid phase extraction(DSPE) is deserved more and more attention nowadays from an environment-friendly point of view because of the less consumption of organic solvents. DSPE is a procedure which involves single extraction of sample with solid adsorbent and followed by a rapid clean-up process. The solid sorbents are usually graphitized carbon black(GCB), primary-secondary amine(PSA), octadecylsilane(C18), silica gel(Si) and florisil [14], [15], [16], [17]. Recently, metal–organic frameworks(MOFs) has been widely employed as a class of organic-inorganic hybrid super-molecular materials which can be self-assembly by metal (oxide) cation with organic electron donor linkers [18], [19], [20]. MOFs have been studied extensively, and due to the wide variety of organic linkers, they may be tailored to many applications such as gas storage and separation [18], [21], [22], [23]. In addition, MOFs are also utilized as DSPE sorbent due to their porous structure and stability recently. MIL-101 as sorbent could not only concentrate organochlorine pesticides in water samples but also pesticides in high fatty matrices [24], [25]. ZIF-8 was also employed as sorbent for the pretreatment of acidic drugs and PAHs in water samples [26], [27]. Chen et al. utilized water-stable MIL-53 as SPME fiber for the extraction of PAHs in water samples [28]. Liu et al. [29] made use of magnetic ZIF-8 as sorbent for the determination of phthalate esters in environmental water. Priscilla Rocío-Bautista et al. used HKUST-1 as efficient sorbent to extract parabens [30]. The UIO-series(University of Oslo) MOF are a new zirconium-based building brick possessing high surface area, good chemical resistance and unprecedented thermo-stability [31]. And the UiO-66 had successfully employed to extract phenols in water samples by solid-phase microextraction [32]. So it is significant and necessary to explore MOFs as novel sorbents for the DSPE of analytes.

Here, UIO-67 was synthesized by the solvothermal synthesis. It is the first time that the UIO-67 was used for adsorbent of DSPE for the preconcentration of 8 PGRs in fruit samples including Indole-3-acetic acid(IAA), Indole-3-propionic acid(IPA), Gibberellic acid(GA3), indole-3-butyric acid(IBA), 1-naphthalene acetic acid(1-NAA), 2-naphthalene acetic acid(2-NAA), p-chlorophenoxyacetic acid(CPA), 2,4,5-trichlorophenoxyacetic acid(2,4,5-T). And the extracted PGRs were labeled with a sensitive derivatization reagent 1-(9H-carbazol-9-yl) propan-2-yl-methanesulfonate(CPMS) [33] synthesized successfully in our research group and measured by high performance liquid chromatography fluorescence detection(HPLC-FLD). Parameters affecting the sample preparation performance were optimized in detail. This method not only provided a new adsorbent for DSPE process, but also a useful extraction of polar compounds in different complex samples.

Section snippets

Materials and chemicals

Eight standards including GA3, IAA, IPA, IBA, CPA, 1-NAA, 2-NAA, 2,4,5-T were purchased from Sigma-Aldrich (Sigma-Aldrich Company, USA). HPLC grade acetonitrile(ACN) was purchased from Yucheng Chemical Reagent Co.(Shandong Province, China). Pure distilled water was purchased from Watson (Guangzhou, China). Zirconium tetrachloride(ZrCl4) and 4,4-biphenyl-dicarboxylic acid (H2BPDC) were obtained from Aladdin Chemistry Co. Ltd.(Shanghai, China). N,N‘-dimethylformamide (DMF) and potassium carbonate

Stabilities of reagent (cpms) and its derivatives

The stability of CPMS reagent was evaluated by the analysis of stored CPMS solution. When CPMS solution was stored at 4 °C for one month, there are no significant changes of the peak area for the derivative of PGRs. The stability of corresponding PGRs derivatives was also studied. The solution of CPMS-PGRs derivatives (1.0×10−4 mol/L) were kept at 4 °C for at least three weeks and analyzed repeatedly by HPLC, the RSDs of peak areas for the labeled PGRs were less than 2%. It was indicated that

Conclusions

In this study, Zr(Ⅳ)-based MOF material (UIO-67) was employed as the sorbent of DSPE to simultaneously extract and concentrate the 8 PGRs for the first time. And a method based on DSPE-HPLC-FLD combined with fluorescence labeling was used to analysis fruit samples. Under optimized conditions, the proposed method showed good analytical performances with low detection limits, high recoveries and short extraction times. The developed method was proved to have excellent advantages such as simple

Acknowledgments

This work was supported by National Natural Science Foundation of China (Nos. 427 21505084, 21275089, 214750753130159521475074) and Natural 428 Science Foundation of Shandong Province (Nos. ZR2014BM029ZR2013BQ019).

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