Fabric phase sorptive extraction followed by ultra-performance liquid chromatography-tandem mass spectrometry for the determination of fungicides and insecticides in wine
Introduction
Production of vinification grapes is the agriculture activity involving the highest application rate of organic fungicides, defined as mass of active ingredient per hectare [1]. At the same time, the use of insecticides has also been increased to control pests, which either directly damage vines or serve as vectors for virus affecting vine plants [2]. A fraction of the pesticides remaining in the harvested grapes is not removed during must fermentation; thus, they persist in the elaborated wines. Presence of these residues in wine poses potential health risks to consumers and therefore it must be closely monitored. However, in the European Union (EU), the maximum residue levels (MRLs) of pesticides in wine are far to be regulated. Instead, the recommendation of the International Organization of Vine and Wine, OIV (10% of MRLs for vinification grapes) is generally accepted [3]. Therefore, the monitoring of fungicides and insecticides in commercial wines becomes an overwhelming analytical challenge and a valuable tool to understand the frequency and intensity of human exposure, to develop future regulations, and to verify the quality standards of ecologic labelled wines.
Although some studies describe wine analysis by direct injection of the filtered sample in the chromatographic system [4,5], a sample preparation step is usually required to remove unwanted components, to increase the concentration of pesticides and/or to make the wine matrix compatible with the chromatographic instrument (in case of gas chromatography (GC) based methods). Solid-phase extraction (SPE) [[6], [7], [8], [9], [10]] and QuEChERS [[11], [12], [13], [14]] are the most common sample preparation methodologies to extract and/or to concentrate multiclass pesticides from wine. Despite the widespread use of the above techniques in food control laboratories, there is an increasing concern about the voluminous consumption of organic solvents during sample preparation, a trend to move towards the so-called Green Analytical Chemistry (GAC) supported extraction methodologies; and also a continuous search to reduce resources dedicated to sample preparation. In this sense, microextraction techniques have undergone a great deal of transformations over the last years.
The pioneer and the most popular of these formats is the solid-phase microextraction (SPME) technique [15]. Its combination with GC methods can be considered totally solvent free and it has been applied in different occasions for the determination of pesticides in wine [16,17]. On the other hand, for non-GC amenable compounds, the success of SPME followed by LC analysis is limited [18,19]. Other drawbacks of SPME are the cost of the commercial sorbents (polymer coated fibers) that need to be reused, their limited variety, and the small volume of sorbent incorporated in SPME fibers that results in poor detection/quantification sensitivity, particularly when combined with solvent desorption. Stir-bar sorptive extraction (SBSE) presents the advantage of a larger amount of sorbent; nevertheless, the number of available coatings is even more limited compared to SPME [20]. In order to overcome this drawback, SBSE has recently incorporated a solvent-assisted approach to enhance the recovery of polar pesticides from wine [21].
Regarding liquid-phase microextraction (LPME) approaches, dispersive liquid-liquid extraction (DLLME) has been used for pesticides determination in wine, either as extraction-concentration technique [22], or in combination with other techniques such as SPE [6]. DLLME shows very fast mass transfer kinetics and high extraction yields in comparison with other microextraction techniques, such as SPME. However, in DLLME separation of phases is a critical issue, in particular when applied to complex matrices as in the case of wines. Also, most extractants are not directly compatible with reversed-phase LC separation.
Fabric phase sorptive extraction (FPSE), developed by Kabir and Furton [23], is a recent microextraction technique with several appealing features. The first one is the coating approach, which is not based on physical adhesion of a thin layer of the polymer on the substrate surface but on sol-gel coating technology [24]. Conventional surface coating technology creates a thick/thin layer of an organic polymer on the surface of the fiber substrate which is subsequently immobilized by a free radical cross-linking reaction. The physical adhesion of the polymer coating created in this manner displays many weaknesses including poor thermal and solvent stability and limits the application substantially. On the other hand, sol-gel coating technology used in FPSE chemically binds the organic polymer with the substrate surface. Due to the strong covalent bonding between the substrate and the sol-gel derived sorbent coating, coated fabrics can be exposed to any organic solvent for analytes elution after the extraction and to a pH range from 1 to 12, without any loss of microextraction performance.
A second advantageous feature of FPSE is the physical format of the microextraction device [25], which incorporates a substantially larger sorbent amount than SPME fibers. FPSE uses chemically stable permeable fabrics (i.e. cotton, polyester or fiberglass) as the substrate to host different polymeric sorbents via sol-gel process. FPSE simultaneously utilizes an inorganic precursor, a fabric substrate and an organic polymer that collectively determine the overall polarity and selectivity of the FPSE media. As such, unlike conventional microextraction techniques including SPME or SBSE, the polarity and selectivity of the FPSE media can be easily customized based on the polarity and other physico-chemical characteristics of the target analyte(s).
Over the last years, FPSE has been successfully employed in a number of unique applications including the concentration of estrogens [26], cytostatic drug residues [27], non-steroidal anti-inflammatory drugs [28], triazine herbicides [29] and UV-stabilizers [30] in water; to the extraction of amphenicols [31] and sulfonamides [32] from raw milk, and to the determination of additives in food packaging materials [33], among others.
The aim of this work is to assess the suitability of the FPSE technique for the extraction of a relevant number of pesticides (mainly fungicides), belonging to different chemical classes, from wine samples. Extracted compounds are selectively determined by ultra-performance liquid chromatography (UPLC) with tandem mass spectrometry (MS/MS) detection. The effects of the extraction set up, the sorbent coating, the ionic strength of the sample and the extraction time in the responses of selected analytes are carefully investigated. Solid-phase extraction (SPE) was used as an auxiliary technique in order to determine the absolute extraction efficiency of FPSE.
Section snippets
Material and chemicals
All substrates, chemicals, reagents, and solvents used in the current project were of highest quality. Substrate cotton fabric (100% cellulose) was purchased from Jo-Ann Fabric (Miami, FL, USA). Organic polymers: poly(tetrahydrofuran), poly(ethylene glycol), poly(caprolactone triol), poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol); solvents: acetone and dichloromethane; sol-gel precursor methyltrimethoxysilane (MTMS), and sol-gel catalyst trifluoroacetic acid were
Selection of FPSE substrate and sorbent chemistry
Due to the wide dispersion of log Kow values (from 1.65 to 4.96) of the pesticides involved in the current study and aqueous nature of wine sample matrix, hydrophilic cotton fabric (100% cellulose) was the rational choice among many available substrate candidates. This substrate was coated with five organic polymers possessing different polarities: Carbowax 20 M (highly polar containing poly(ethylene glycol), H[OCH2CH2]nOH as the building block); poly(ethylene glycol)-block-poly(propylene
Conclusions
For the first time, FPSE followed by UPLC-ESI-MS/MS has been successfully optimized and validated for the determination of a broad group of fungicides and three insecticides in wine samples. The set-up of the FPSE process and the type of coating were the variables exerting the most important effects in the repeatability and the yield of the extraction. The proposed methodology provides accurate concentration values in samples spiked at different concentration levels and a reduced consumption of
Acknowledgements
This study has been supported by the Spanish Government, Xunta de Galicia and E.U. FEDER funds (projects CTQ2015-68660-P and GRC-ED431C). L.P-M acknowledges a FPU contract to the Spanish Ministry of Education.
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