A molecularly imprinted polymer as the sorptive phase immobilized in a rotating disk extraction device for the determination of diclofenac and mefenamic acid in wastewater
Graphical abstract
Introduction
Despite the increasing selectivity and sensitivity achieved by current analytical techniques, sample preparation remains a critical issue in a chemical measurement process, particularly in complex samples in which the analytes are very dilute and a number of unknown interferences are present. In this context, the extraction and cleanup steps are of paramount importance in the sample preparation.
Extraction techniques have been the focus of intensive research over the last two decades, with advances in automation, miniaturization, and simplification driving this evolution [1], [2]. The need to develop analytical processes that can replace toxic reagents and polluting solvents as well as minimizing the consumption of energy, reagents, and samples to reduce waste generation is an equally important driver of novel extraction techniques [3].
Solid-phase extraction (SPE) is currently the most widely used replacement for liquid–liquid extraction (LLE) for both the enrichment of organic pollutants in water samples [4], [5] and cleanup purposes. According to recent reviews [6], [7], [8], an important vein of research in analytical chemistry has focused on the development of SPE systems that are combined with chromatography in both the thermal and solvent desorption modes. SPE, together with minimizing the use of solvents, has a number of additional advantages with respect to LLE, such as the more complete extraction of the analyte, more efficient separation of interferences from the analytes, no emulsion formation, easier collection of the analytes, more convenient manual procedures, removal of particulates, and ability to be more easily automated [5].
The selection of the SPE sorbent has a direct relationship with the analytical selectivity of the method. Selectivity will be achieved when significant differences occur between the analyte-solid phase interactions with respect to the interference-solid phase interactions. In this regard, when an organic molecule is highly hydrophobic, its interaction with an apolar support, for example C18, will be the basis for the removal of any polar interferences. However, if the analyte is capable of forming hydrogen bonds, besides containing apolar groups, it would be advisable to use a polymeric support containing N-vinylpyrrolidone and divinylbenzene (commercially available as Oasis HLB).
Although an Oasis HLB phase provides a hydrophilic-lipophilic balance for a good matching interaction with amphiphilic molecules, these interactions are not fully specific for the analyte alone and interactions with interfering molecules can also occur, reducing the active sites available to the analytes. A powerful and reproducible manner to achieve greater selectivity in SPE is the use of molecularly imprinted polymers (MIPs) as the solid phase (molecularly imprinted solid phase extraction, MISPE) [9].
MIPs are obtained by the copolymerization of mono- and poly-functional monomers in the presence of a template. After polymerization, the template molecules are removed from the polymeric network, leaving selective sites for other molecules that are complementary in size, shape and functionality to the template. The resulting MIPs are stable in wide pH and temperature ranges and in different solvents [10], [11].
MISPE is the most advanced technical application of MIPs; however, non-exhaustive sorptive microextraction techniques, such as solid phase microextraction (SPME) and stir bar sorptive extraction (SBSE), have also implemented the use of MIPs as the sorptive material achieving the selective extraction of analytes from real samples [12], [13]. The methods developed for the synthesis of MIP fibers for SPME or MIP stir bars for SBSE are rather simple and robust; therefore, their use will be extended to analytical laboratories in the coming years [13].
In 2009, our research group developed a new sample preparation technique called rotating disk sorptive extraction (RDSE), which is an alternative to the current microextraction/cleanup techniques and provides a number of advantages [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27]. The extraction device used in RDSE exhibits an extraction phase with a high surface area-to-volume ratio, and it can be stirred at much higher velocities than the stir bar used in SBSE without damaging the phase because the extraction phase is only in contact with the liquid sample. Thus, higher rotating velocities facilitate analyte mass transfer to the sorptive surface.
Two configurations of the extraction device have been proposed for RDSE, providing a high versatility because any sorptive material used in either SPE or SBSE can be immobilized on the rotating disk. In addition, RDSE provides some advantages over SPE, especially that it allows the recirculation of the sample through the extraction phase and thus maximizes its sorptive capacity (in SPE the sorption occurs while the sample passes unidirectionally through the solid support). Furthermore, in RDSE, the interface is continuously renewed during the extraction process, which minimizes the involved cleanup steps for complex samples, which are required with SPE. Other important characteristics of RDSE are related to the shape design of the extraction device, which allows an easier automation of the extraction process [21], direct spectroscopic measurements in the extraction phase [17], [18], [19], [22], and feasibility of its use in bioavailability studies [25].
A proof-of-concept application of the RDSE associated with a MIP sorptive material is presented for the determination of some NSAIDs in water samples. These drugs, which are widely used and can be acquired without a medical prescription, are emerging pollutants [23] currently found in waste and natural waters. A number of determinations of NSAIDs based on the use of MISPE have been reported, with most of them using the same analyte as the template molecule and 2-vinylpyridine or 4-vinylpyridine as the monomer in the synthesis of the MIP [28], [29], [30], [31], [32], [33]. The MIP that we prepared in the present case is synthesized using diphenylamine (DPA) as the template and the monomer VI together with the cross-linker DVB (Fig. 1). As seen in Fig. 2, diphenylamine is part of the molecules of diclofenac, mefenamic acid and paracetamol, and consequently, a better match between the MIP and these molecules is expected compared with other NSAIDs.
Section snippets
Reagents
Water from a Millipore Milli-Q Plus water system (Billerica, MA) was used throughout the experiment. All nonsteroidal anti-inflammatory drugs (ketoprofen, ibuprofen, naproxen, diclofenac, acetylsalicylic acid, and mefenamic acid) and the internal standard (meclofenamic acid) were purchased from Sigma–Aldrich (Milwaukee, WI, USA). Paracetamol and the syringe standard (hexachlorobenzene) were obtained from Dr. Ehrenstorfer (Augsburg, Germany). The standard stock solutions of the analytes (50 mg L
MIP and NIP characterization
The yield obtained for MIP was 43.8%; meanwhile, the yield for the NIP was higher, reaching a value of 51.5%, indicating that the template molecule DPA slightly inhibited the polymerization of the radical. This yield is referred only to fraction of particle size, 100–180 μm. This fraction is obtained after the whole processes, washing, and removal of the template, grinding and sieving the polymers. The FT-IR spectra for the MIP and NIP are very similar to each to other, consistent with the fact
Conclusion
The determination of diclofenac and mefenamic acid in water samples using RDSE containing MIP as the sorbent phase was feasible because the method presented extraction efficiencies between 99 and 100% with RSDs of less than 6%. Furthermore, the method based on the MIP was able to extract paracetamol with a significantly better efficiency with respect to the NIP because of its molecular similarity to the template used to synthesize the MIP.
Similar concentrations were found when the proposed
Acknowledgments
The authors would like to thank FONDECYT (grants 1140716 and 1100775) for financial support. One of the authors (VM) would like to thank CONICYT for her doctoral fellowship (21110232).
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